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dissertation entitled

INTEGRATING DISEASE PREDICTORS AND A REDUCED-
RISK FUNGICIDE AND EVALUATING DISEASE
THRESHOLDS FOR LATE BLIGHT MANAGEMENT IN
CELERY

presented by

Ryan Scott Bounds

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INTEGRATING DISEASE PREDICTORS AND A REDUCED-RISK FUNGICIDE
AND EVALUATING DISEASE THRESHOLDS FOR LATE BLIGHT
MANAGEMENT IN CELERY
By

Ryan Scott Bounds

A DISSERTATION

Submitted to
Michigan State University
in partial fulﬁllment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY
Department of Plant Pathology

2006

ABSTRACT
INTEGRATING DISEASE PREDICTORS AND A REDUCED-RISK FUNGICIDE
AND EVALUATING DISEASE THRESHOLDS FOR LATE BLIGHT
MANAGEMENT IN CELERY
By
Ryan Scott Bounds

Late blight, incited by the fungus Septoria apiicola, is the most important foliar
disease of celery in Michigan and results in necrotic lesions on leaves and petioles that
can reduce yields by up to 80%. Most celery growers begin applying the fungicide
chlorothalonil one to three weeks after transplanting and reapply it at 7- to 10—day
intervals to protect the crop, but some of these applications may not be needed early in
the season or when environmental conditions do not favor disease development. The
purpose of this research was to evaluate late blight management programs that reduce
fungicide use and incorporate a new reduced-risk fungicide, azoxystrobin, which poses a
lower risk to non-target organisms and the environment than chlorothalonil.

Disease predictors were evaluated, using a standard and a reduced-risk fungicide
program, for managing late blight on ‘Dutchess’ celery in 2003 to 2005. The TOM-
CAST lO-DSV predictor provided disease control that was comparable to the weekly
application program during each year. It required up to ﬁve fewer sprays, while reducing
fungicide costs up to $215/ha, compared to the weekly program. Other disease
predictors, such as the Septoria predictor, Cercospora predictor, and TOM-CAST 15-
DSV, also provided control similar to the weekly program, but their efﬁcacy was less
consistent than TOM-CAST lO-DSV and was often dependent on the fungicide program.

The fungicide programs frequently provided similar control, but azoxystrobin alternated

with chlorothalonil was more effective than chlorothalonil alone when disease pressure
was high in 2003.

Weekly sprays of azoxystrobin alternated with chlorothalonil were initiated early
(one week aﬁer transplanting), preventively (four weeks after transplanting), or when
disease symptom were detected at a trace, 5%, or 10% level on ‘Dutchess’ celery in 2003
and 2004. Preventive applications required three fewer sprays, reduced fungicide costs
by up to $134/ha, and provided Similar control compared to the early fungicide program.
Delaying the initial application until any level of disease developed subsequently resulted
in unacceptable levels of disease at harvest and cannot be recommended in Michigan due
to the risk of extensive yield loss.

A reduced-risk fungicide program was initiated preventively and reapplied
weekly or according to the Septoria predictor or TOM-CAST lO-DSV and was compared
with a weekly management program initiated early at a research farm and a commercial
ﬁeld in 2004 and 2005. Combining the use of preventive initial applications with disease
predictors reduced the number of sprays by two to six while providing Similar disease
control compared with the weekly fungicide program initiated early. At the commercial
ﬁeld, management programs that required fewer Sprays were effective in managing early
blight (incited by Cercospora apii) and crater rot (incited by Rhizoctom’a solani).

This research identiﬁed effective disease management programs that require
fewer fungicide sprays than the standard, calendar-based approach, while incorporating a
reduced-risk fungicide. Reducing fungicide use in celery will likely have societal and

environmental impacts that were not addressed in this study.

ACKNOWLEDGEMENTS

I am thankful for the support and guidance of my major professor, Dr. Mary
Hausbeck, throughout my graduate education at Michigan State University. I appreciate
the advice of my committee members Drs. Joe Vargas, Jan Byme, Mathieu Ngouajio, and
Larry Olsen. I thank the numerous MSU faculty, employees, and members of the
Hausbeck team for helping with various aspects of this research project. A special thanks
to Brian Cortright for ﬁeld assistance, Sheila Linderman for technical editing, and Drs.
Catarina Sande and Amanda Gevens for helpful discussion.

I express my sincere gratitude to the Michigan vegetable growers that I had the
privilege to work with: Ron, Dale, and Jeﬂ‘ Eding; Bruce and Eric Schreur; Bruce
Klamer; Rich Schreur; and Doug VanDyk.

Finally, to my wife and family: thanks for all of your encouragement and for

listening to me talk about celery.

iv

TABLE OF CONTENTS

LIST OF TABLES ...................................................................................................... vii

LIST OF FIGURES ...................................................................................................... x

LITERATURE REVIEW
Introduction ........................................................................................................ 2
Late Blight Epidemiology and Management ..................................................... 5
Early Blight Epidemiology and Management .................................................. 14
Disease Predictive Systems for Septoria apiicola and Cercospora apii ......... 19
Crater Rot Epidemiology and Management .................................................... 22
Bacterial Leaf Spot Epidemiology and Management ...................................... 26
Literature Cited ................................................................................................ 30

CHAPTER 1. Comparing Disease Predictors and Fungicide Programs for
Late Blight Management in Celery

Abstract ............................................................................................................ 39
Introduction ...................................................................................................... 40
Materials and Methods ..................................................................................... 42
Results ........................................................................ - ...................................... 47
Discussion ........................................................................................................ 59
Acknowledgements .......................................................................................... 64
Literature Cited ................................................................................................ 65

CHAPTER 11. Evaluation of a Reduced-Risk F ungicide Program Applied
According to Disease Thresholds for Late Blight
Management in Celery

Abstract ............................................................................................................ 71
Introduction ...................................................................................................... 72
Materials and Methods ..................................................................................... 74
Results .............................................................................................................. 78
Discussion ........................................................................................................ 82
Acknowledgements .......................................................................................... 85
Literature Cited ................................................................................................ 87

CHAPTER III. Coupling Delayed Initial Fungicide Applications and Disease
Predictors for Late Blight, Early Blight, and Crater Rot
Management in Michigan Celery Production

Abstract ............................................................................................................ 91
Introduction ...................................................................................................... 92
Materials and Methods ..................................................................................... 95
Results .............................................................................................................. 99
Discussion ...................................................................................................... 104
Acknowledgements ........................................................................................ 107
Literature Cited .............................................................................................. 109

APPENDIX A.

APPENDIX B.

Summary of Disease Predictor Data and Fungicide
Application Dates in 2003-2005 .................................................. 1 13

Field Experiments Conducted to Evaluate F ungicide Efficacy,
Disease Predictors, and/or Delayed Initial Applications for
Managing Late Blight, Early Blight, Crater Rot, and/or

Bacterial Leaf Spot of Celery in 2003-2005 ................................ 122

vi

LIST OF TABLES

CHAPTER I.
Table 1. Effects of application programs and ﬁmgicides on leaf and
petiole blight severity resulting from Septoria apiicola on ‘Dutchess’
celery during 2003 to 2005 .............................................................................. 50

Table 2. Effects of application programs and fungicides on yield loss
resulting from Septoria apiicola on ‘Dutchess’ celery during
2003 to 2005 .................................................................................................... 52

Table 3. Interaction of application programs and fungicides on control
of Septoria apiicola on ‘Dutchess’ celery in 2003 and 2005 ........................... 54

Table 4. Frequency of applications as required by the 7-day interval and
disease predictors for control of Septoria apiicola during 2003 to 2005 ........ 57

Table 5. Number of fungicide applications and cost of fungicides applied
according to different application programs for control of
Septoria apiicola in 2003-2005 ........................... . ........................................... 58

CHAPTER II.
Table 1. Late blight severity and yield of ‘Dutchess’ celery plants not
treated or treated with fungicides initiated at different timings for control
of Septoria apiicola in 2004 and 2005 ............................................................. 79

Table 2. Number of fungicide applications and cost of fungicides applied
when initiated at different timings for control of Septoria apiicola
in 2003 and 2004 .............................................................................................. 81

CHAPTER 111.
Table 1. Late blight severity and yield loss of ‘Dutchess’ celery plants
not treated or treated with fungicides for control of Septoria apiicola
at the research farm in 2004 and 2005 ........................................................... 101

Table 2. Early blight severity and yield loss of ‘Dutchess’ celery plants
not treated or treated with fungicides for control of Cercospora apii
and Rhizoctonia solam’ at the commercial ﬁeld in 2004 and 2005 ................ 101

Table 3. Frequency of fungicide applications required by different

management programs for control of Septoria apiicola at the
research farm and commercial ﬁeld in 2004 and 2005 .................................. 103

vii

Table 4. Number of fungicide applications and cost of fungicides applied
according to different management programs for control of

Septoria apiicola at the research farm and at the commercial ﬁeld

in 2004 and 2005 ............................................................................................ 103

APPENDIX A.
Table A1. Summary of the effects of late blight on disease and yield
assessments on ‘Dutchess’ celery plants not treated or treated every
seven days or according to disease predictors with chlorothalonil
(1.7 kg a.i./ha) or azoxystrobin (0.17 kg a.i.lha) alternated (alt.)
with chlorothalonil for management of Septoria apiicola at the
MSU Muck Soils Research Farm in 2003 ..................................................... 114

Table A2. Summary of the effects of late blight on disease and yield
assessments on ‘Dutchess’ celery plants not treated or treated every

seven days or according to disease predictors with chlorothalonil

(1.7 kg a.i./ha) or azoxystrobin (0.17 kg a.i./ha) alternated (alt.)

with chlorothalonil for management of Septoria apiicola at the

MSU Muck Soils Research Farm in 2004 ..................................................... 115

Table A3. Summary of the effects of late blight on disease and yield
assessments on ‘Dutchess’ celery plants not treated or treated every

seven days or according to disease predictors with chlorothalonil

(1.7 kg a.i.lha) or azoxystrobin (0.17 kg a.i.lha) alternated (alt.)

with chlorothalonil for management of Septoria apiicola at the

MSU Muck Soils Research Farm in 2005 ..................................................... 116

Table A4. Dates of fungicide applications when applied weekly or
according to disease predictors to manage Septoria apiicola on
‘Dutchess’ celery at the MSU Muck Soils Research Farm in 2003 .............. 117

Table A5. Dates of fungicide applications when applied weekly or
according to disease predictors to manage Septoria apiicola on
‘Dutchess’ celery at the MSU Muck Soils Research Farm in 2004 .............. 118

Table A6. Dates of fungicide applications when applied weekly or
according to disease predictors to manage Septoria apiicola on
‘Dutchess’ celery at the MSU Muck Soils Research Farm in 2005 .............. 119

Table A7. Dates of ﬁmgicide applications when applied early

(2 weeks after transplanting) and reapplied weekly or applied

preventively (5 weeks after transplanting) and reapplied weekly or

according to disease predictors to manage Septoria apiicola on

‘Dutchess’ celery at the MSU Muck Soils Research Farm in 2004

and 2005 ......................................................................................................... 120

viii

Table A8. Dates of fungicide applications when applied early

(1 week after transplanting) and reapplied weekly or applied

preventively (4 weeks after transplanting) and reapplied weekly

or according to disease predictors to manage Septoria apiicola

on ‘Dutchess’ celery at the commercial ﬁeld in 2004 and 2005 .................... 121

ix

LIST OF FIGURES

CHAPTER I.
Fig. 1. Leaf blight severity of untreated ‘Dutchess’ celery plants
when adjacent buffer row plants were inoculated with Septoria
apiicola at approximately six and seven weeks after transplanting

in 2003-2005 .................................................................................................... 49

LITERATURE REVIEW

Introduction

Celery (Apium graveolens L. var. dulce (Mill.) Pers.) is a cool season biennial
plant in the Umbelliferae family. It is produced as an annual crop and consumed fresh or
in processed foods. Nearly all of the celery produced in the United States is grown in
California, Michigan, and Florida. Michigan growers produce approximately 8% of the
celery raised in the United States (92). The Florida acreage is not currently included in
the USDA statistics because there is only one large celery operation in the state. Celery
grown in Michigan is available for about a 4-month period (early July through early
November), while California and Florida celery is available for 12- and 6-month periods,
respectively.

Michigan has a long history of celery production dating back to the 18903 when
approximately 350 growers farmed roughly 4,000 acres in Kalamazoo County (D. F rens,
personal communication). In the 19805, Sixty growers raised celery on 3,400 acres. The
majority of celery production in Michigan currently occurs in Allegan, Kent, Muskegon,
Newaygo, Ottawa, and Van Buren counties, although there is limited production
elsewhere in the state. In 2005, the celery crop destined for fresh market and processing
was produced on 2,200 acres by 21 growers and was valued at nearly $18.2 million (92).
Celery has a higher per acre value than several other important vegetable crops in
Michigan. With an average value of $8,273/A, celery is more than eight times the value
of fresh market and processing asparagus, six times the value of processing carrot, and
three times the value of processing tomato (92). About one third of the celery grown in
Michigan is processed for frozen food, soup, or juice. The value of celery for processing

is slightly lower than the value of celery destined for the fresh market; however, higher

yields in processing celery (41 tons/A) typically result in Similar returns when compared
to fresh market production where yields are lower (32 tons/A) (D. Frens, personal
communication).

All celery in Michigan is seeded into ﬂats and raised in greenhouses for a period
of six to eight weeks prior to transplanting in the ﬁeld (97). This ensures uniform plant
Size and spacing in the ﬁeld since celery seed does not uniformly germinate (3). Also,
greenhouse transplants typically produce plants with greater root weight compared with
plants originating from seed sown directly in the ﬁeld (91). Greenhouse seedling ﬂats
usually contain 266 cells, and individual cells are approximately 0.5 inches in diameter.
Pelletized seeds are planted with a precision seeder into ﬂats containing a
vermiculite/ﬁne peat soil mix. Optimal seed germination occurs between 50 and 70°F,
but alternating day and night temperatures of 70 and 60°F seem to improve the
percentage and uniformity of germination (97). Flats are usually irrigated twice each day
and fertilized at regular intervals during inigation events. Once the seedlings are well
established, they are mowed (clipped) to a height of three to four inches to facilitate ease
of handling and to ensure uniform growth in the ﬁeld. Mowing does not signiﬁcantly
reduce yield or increase the variation in plant size at harvest unless there is a high
percentage of small, weak seedlings prior to mowing (94).

Almost all of the celery grown in Michigan is raised on muck soils with soil pH
above 5.5 (97). These soils have high moisture holding capacity and high cation
exchange capacity, which are well suited for celery growth. Transplanting begins in mid-
April and continues almost daily until late-July (3). Transplants set in the ﬁeld earlier are

covered with plastic tunnels to avoid frost and promote early growth. A two- or four-

row, self-propelled transplanter is used to plant celery in the ﬁeld (97). Despite the use of
sprinkler irrigation just after transplanting, some growers loose 3% of the plants if they
are not set properly (2).

Planting populations and the days to harvest differ based on the end use of the
plants. Early-planted celery is normally planted at a high population and harvested for
small celery hearts. A common cultivar used for celery hearts is ‘Ventura’. Celery
destined for the fresh market is harvested about 80 to 90 days after transplanting.
Common cultivars used for fresh market production include ‘Dutchess’ and ‘Green Bay’.
Processing cultivars include ‘Sabroso’ and ‘Dutchess’ and may be harvested up to 95
days after transplanting to obtain high yield. Except for some celery hearts, nearly all of
the celery produced in Michigan is mechanically harvested and transported to packing
Sheds (3). The plants are trimmed, washed, and packaged prior to Shipping (3). Any
diseased tissue on the plants must be manually removed. This is a time-consuming
process that can reduce the size and quality grade of the plants.

In Michigan, three fungal diseases attack the marketable portion of celery plants
every year and include late blight, early blight, and crater rot (3). When leﬁ untreated,
these diseases have the potential to reduce yield (16,17,19) and additional trimming
needed to remove diseased tissues may render the plants unmarketable. Bacterial leaf
spot also occurs every year in Michigan (3) and is primarily a cosmetic disease of the
leaves which seldom reduces yields (5 8). Consumers demand blemish-free celery, and
growers are aware of the potential losses resulting from inadequate disease management.

Therefore, celery growers rely on cultural disease management tactics and foliar

fungicides, which are applied shortly after transplanting and at frequent intervals

throughout the season, to ensure a marketable crop.

Late Blight Epidemiology and Management

Late blight of celery is caused by the imperfect fungus Septoria apiicola Speg.
and is considered the most important foliar disease of celery in Michigan (51). California
(4,41 ,67) and Florida (9,47) celery production is also affected by late blight. The disease
was ﬁrst reported in the United States in 1891 (24). S. apiicola only infects celery and
celeriac (Apium graveolens var. rapaceum) (25). In 1932, Cochran (25) reported distinct
species of Septoria causing small-spot symptoms (S. apii-graveolentis Dorogin) and
large-spot symptoms (S. apii Chester) of late blight. Later conﬁrmation proved that one
Species, S. apiicola, is the cause of celery late blight (30,88). Attempts to discover the
sexual (teleomorphic) stage of the fungus have been unsuccessﬁil (25).

Late blight is identiﬁed by the appearance of small, tan to brown colored,
irregularly shaped lesions on leaves and petioles (34,51). Lesions are initially detected
on the older, outer leaves (26) which are sometimes surrounded by a diffuse chlorotic
margin (30). Extensive blighting of the foliage occurs when lesions coalesce, and
severely blighted tissues wither and senesce (24,26). Pycnidia of S. apiicola are black
fruiting structures of the fungus that are embedded in nearly all infected tissues, either
separate from one another or fused (30). They are 55 to 190 pm in diameter, generally
globose, and have round to irregularly Shaped ostioles that are less than half the diameter
of the pycnidium (30). Pycnidia can be seen without the aid of magniﬁcation and

resemble ﬂecks of ground black pepper. Each pycnidium contains an average of 3,675

conidia, and an average of 56 pycnidia are observed in an individual lesion (55). Conidia
of S. apiicola are hyline, 1.3 to 1.8 pm wide and 22 to 51 um long, ﬂexuous or straight,
septate (typically 33 septa), and tapered at one end and blunt at the other (30). Conidia
were sometimes referred to as “pycnidiospores” in earlier literature (88,89).

Donovan et a1. (28) used scanning electron microscopy (SEM) to examine the
infection process of S. apiicola on excised celery leaves. Conidia germinate under moist
conditions, and germ tubes arise from one or several cells of each conidium. Hyphae do
not grow directly toward stomata but branch and grow randomly across the leaf surface.
Host penetration occurs either directly through the epidermis or via stomata at ﬁve to
seven days after inoculation. Hyphal growth is conﬁned to intercellular spaces 21 days
after inoculation, and intracellular hyphal growth is observed 27 days aﬁer inoculation.
At 27 days after inoculation, fungal colonization is extensive and leaves are necrotic (28).
However, the fungus is not observed in phloem or xylem cells, and infection does not
affect stomatal or epidermal cells until pycnidia are observed (29). Pycnidia develop just
beneath the leaf surface and break through the leaf surface as they enlarge and near
maturity (28). Conidia remain attached to the inner surface of the pycnidium until a
wetting event occurs and they are extruded through the ostiole (28).

Conidia are disseminated by splashing water (9), including the movement of
conidia-laden water droplets by farm laborers (56) and machinery. Berger (9) did not
collect any S. apiicola conidia on spore trap Slides when commercial ﬁelds with extensive
late blight damage were monitored. S. apiicola conidia germinate on water agar after 8
hours of incubation at 21 or 25°C but do not germinate until 12 hours on celery leaves or

in water (50). A higher percentage of conidia germinate on celery leaves at 25°C than at

21°C for the ﬁrst 24 hours, but the percentages are Similar for both temperatures at 36
hours after inoculation. Aﬂer 36 hours of incubation at 21 or 25°C, the highest percent
germination is observed on water agar (>95%), followed by celery leaves (80%) and
water (40 to 60%). At 21°C, few lesions (<2) develop on leaves exposed to 24 hours of
continuous or interrupted (12 hours dry) leaf wetness at 21 days after inoculation.
Numerous lesions (>13) developed on leaves exposed to 36 hours of continuous leaf
wetness or 48 hours of continuous or interrupted leaf wetness at 21 days after inoculation.
No lesions developed on leaves exposed to <12 hours of leaf wetness, and only 0.3
lesions developed on leaves exposed to 12 hours of leaf wetness. At 25°C, lesions only
developed after leaves were exposed to 324 hours of continuous or interrupted leaf
wetness, and lesion numbers increased with longer periods of leaf wetness (50).

Mathieu and Kushalappa (59) investigated the infection of celery by S. apiicola at
various temperatures and leaf wetness durations. Celery plants were inoculated and
remained wet for periods of 12, 24, 36, 72, or 96 hours at temperatures of 10, 15, 20, 25,
or 30°C. Successful infection occurred at all temperatures and leaf wetness durations
tested, and lesions developed 12 to 24 days after inoculation. The greatest number of
lesions was observed at 25°C when leaves remained wet for 72 hours after inoculation.
Moderate infection occurred when plants were exposed to 12 hours of leaf wetness at 20
or 25°C, a leaf wetness and temperature scenario more likely to occur under ﬁeld
conditions (59).

Under ﬁeld conditions, Sheridan (87) obtained successful infection when the
mean relative humidity (RH) was 90% and mean temperature was 14°C on the two days

following inoculation. F iﬁeen hours of consecutive rainfall were also recorded during

the periods of high humidity when infection was successful. No infection resulted during
drier conditions when the mean RH was <90% and when no rainfall was recorded on the
two days following inoculation (87).

Septoria apiicola is seed borne and can be the primary source of inoculum for late
blight epidemics (5,9,3 7,46,60,61,86). Pycnidia are observed on the seed surface, and
mycelium is observed in the pericarp and testa but not in the embryo or endosperm (86).
Upon wetting, conidia are extruded from pycnidia on the seed coat which oﬂen adhere to
expanding cotyledons (9,60). Several methods to eradicate S. apiicola from celery seed
have been proposed including the use of aged seed which is two to three years old (46),
hot water treatment (5,46), and fungicide treatment (60,61). Aging seed does not provide
complete eradication since Berger (9) obtained infection from one lot of 3 year old seed.
Hot water treatments at 50°C for 25 minutes may reduce seed germination in older seed
lots (5,61) and may not entirely eradicate S. apiicola (61). Complete S. apiicola
eradication is reportedly achieved when seeds are soaked in a fungicide suspension of
0.2% thiram for 24 hours at 30°C (60,61).

Berger (9) investigated the role of seed-borne inoculum and weather parameters
in association with an epidemic of late blight in the Florida Everglades region in 1970.
Diseased seedlings were observed in 4 out of 19 celery seed lots when examined under
controlled conditions. Many of the infected seed lots were used in the 1970 growing
season and provided the initial source of S. apiicola inoculum for the epidemic. Weather
conditions were particularly favorable for the development of the epidemic due to record-
setting rainfall amounts. Celery leaves remained wet for >24 hours on several occasions,

providing conditions that were highly favorable for infection and spore dissemination (9).

Septoria apiicola is capable of persisting on infested celery debris in the soil and
can be the primary source of inoculum for the subsequent celery crops (45) planted up to
nine months after infested debris is incorporated into the soil (62). Infection does not
occur in new plantings where infested celery debris was incorporated 21 months prior to
planting. Maude (62) assumed that the lack of disease in the celery crop planted 21
months after the incorporation of infested debris was due to the rate of host tissue
decaying in the soil. High numbers of viable spores were recovered when infested debris
was left on the soil surface compared with the number of spores recovered from debris
incorporated into the soil (62). Sheridan (86) found viable S. apiicola spores on four-
month old infested leaf debris in the soil. Viable spores were also recovered from
infested debris after 10 months of storage under ambient conditions in a laboratory (86).
A 2-year crop rotation is recommended to avoid infection resulting from diseased celery
debris in the soil (62).

The majority of Michigan celery growers do not practice crop rotation, although
some use a l-year rotation (48) with onion (Allium cepa), radish (Raphanus sativus), or
lettuce (Lactuca sativa). Due to the limited availability of organic soil and the high value
of celery, many Michigan growers are unable to follow the recommended 2-year crop
rotation because planting lower-value rotational crops would reduce farm income.

Genetic resistance to S. apiicola is not currently available in commercial celery
cultivars. All commercial celery cultivars tested by Edwards et a1. (29) were classiﬁed as
moderately to highly susceptible to S. apiicola. Parsley (Petroselinum crispum) and
some wild celery lines are resistant, but crossing these plants with commercial celery

cultivars has not provided a stable level of resistance (29). Lacy and Honma (52)

reported highly variable S. apiicola resistance among F2 progeny from celery plants
originally crossed with parsley, although a later report indicated these plants were
moderately to highly susceptible (29). The genes and mechanisms controlling resistance
to S. apiicola in celery are not well understood (29).

In addition to the use of disease-free seed and adequate crop rotation, celery
growers are advised to implement other cultural control measures to avoid late blight
epidemics. If possible, growers should avoid the use of Sprinkler irrigation to minimize
the duration of leaf wetness and to reduce the risk of spreading the pathogen (41). In
Michigan, the use of Sprinkler irrigation is common, so growers should schedule
irrigation late in the afternoon or at night to avoid prolonged periods of leaf wetness.
Growers are advised to minimize ﬁeld operations, such as the movement of people or
machinery, when the leaves are wet to avoid dispersing the pathogen in the same ﬁeld
and to other ﬁelds (26,34,41,56). Proper fertilization is also recommended since it may
reduce late blight severity (65). Most of the aforementioned cultural control tactics
recommended for use in the ﬁeld also applies, to some extent, in greenhouse transplant
production, although sufﬁcient research in this area is lacking. Celery seedlings should
be irrigated at times that allow the leaves to dry quickly. Proper ventilation and air
circulation are tactics used to lower RH and shorten the duration of leaf wetness.
Mowing or any mechanical disturbance of the plants should be done when the leaves are
dry to avoid spreading the pathogen.

F oliar fungicides are an important component in late blight management programs
(53). A fungicide application is recommended prior to disease appearance Since S.

apiicola has a relatively long latent period (48). Benomyl, a systemic and curative (i.e.,

10

disease control after infection but before symptoms are visible) fungicide which is no
longer registered for use on celery, applied at 14-day intervals was more effective than
protectant fungicides applied at 7-day intervals when all fungicides were initially applied
three days after disease symptoms (48). The yield of celery plants treated with protectant
fungicides did not differ from the yield of celery plants leﬁ untreated. Although the
benomyl treatment provided superior disease control and higher yields relative to

- protectant fungicide treatments, it resulted in disease symptoms on nearly 10% of the leaf
area (48). Paulus et al. (72) obtained “satisfactory” control of S. apiicola with a 7-day
application interval of protectant and systemic fungicides initially applied when disease
symptoms were present on leaves and petioles, although the speciﬁc level of disease was
not Speciﬁed. The same fungicides applied at 14-day intervals did not provide adequate
control when initiated at the same timing. In a separate study, benomyl provided
signiﬁcantly better disease control relative to chlorothalonil when both fungicides were
initially applied when disease symptoms were ﬁrst observed on leaves and reapplied at
l4-day intervals (72). Although these studies did not directly compare preventive
applications with post-symptom applications, they identify the potential risk associated
with delaying the initial fungicide application until late blight symptoms are observed
(48,72).

Mudita and Kushalappa (69) examined the effect of initial disease incidence
levels on late blight progression and created a model to explain disease thresholds in
relation to yield loss. Ultimately, they sought to determine a disease threshold that could
be used to initiate fungicide treatment. Diseased transplants were randomly placed in

experimental plots to obtain initial blight incidence levels of 0, 2, 4, 8, and 16% diseased

11

transplants per plot. All plots were treated with chlorothalonil every 7 to 10 days,
starting three weeks after transplanting. Yield loss and disease incidence and severity
increased with increasing initial blight incidence levels. Most plants in the 0% blight
incidence treatment had lesions on the outer leaves, despite repeated fungicide
applications and the apparent lack of initial inoculum in these plots. The model predicted
yield loss at the 0% blight threshold, so it was not possible to determine an action
threshold for initiating fungicide treatment. Despite their results, the authors recommend
initiating fungicide treatment when late blight is ﬁrst observed in commercial ﬁelds (69).

Michigan celery growers are advised to apply fungicides after transplanting and
repeat every 7 to 10 days for the duration of the season (14). Five main active ingredients
are currently registered for control of late blight in Michigan and include chlorothalonil
(e.g., Bravo Ultrex 82.5WDG, Syngenta Crop Protection, Inc.), azoxystrobin (e.g.,
Quadris 2.08F, Syngenta Crop Protection, Inc.), triﬂoxystrobin (e.g., Flint 50WG, Bayer
CropScience), propiconazole (e.g., Tilt 3.6EC, Syngenta Crop Protection, Inc.), and
copper-based products (e.g., Kocide 2000 53.8DF, E. I. du Pont de Nemours and
Company) (14).

Most growers use the protectant fungicide chlorothalonil to manage late blight
because it is effective (19,48,72) and relatively inexpensive. Chlorothalonil disrupts
multiple enzyme pathways in fungal cells (31) thereby inhibiting hyphal growth and/or
spore germination. Attempts to improve the control of S. apiicola by mixing certain
adjuvants with chlorothalonil have been unsuccessful (49). Adjuvants either provided
positive or negative effects when applied with other (currently unregistered) fungicides

for control of S. apiicola (1) which indicates adjuvant-fungicide combinations must be

12

tested on a case-by-case basis. Copper-based compounds are protectants which have
activity against both fungi and bacteria. Coppers exhibit multi-Site activity by inhibiting
multiple enzyme pathways (27). These products are routinely applied in combination
with other fungicides, but typically do not provide satisfactory control of S. apiicola
when applied alone (20,33) unless disease pressure is extremely low (63).

Newer fungicide chemistries for use in celery include azoxystrobin,
triﬂoxystrobin, and propiconazole. Azoxystrobin and triﬂoxystrobin are members of the
strobilurin group of fungicides and are also classiﬁed as reduced risk products (6).
Strobilurins inhibit mitochondrial respiration by binding at the Q0 site of cytochrome b.
These reduced risk products exhibit very low toxicity to non-target organisms and have
low environmental risk. Both fungicides have systemic and curative properties (6).
Some growers choose to incorporate azoxystrobin into their Spray program Since it is
effective against late blight (15,19,36,64,79). Triﬂoxystrobin is also effective against S.
apiicola (36), but many Michigan celery growers use this ﬁmgicide less frequently than
azoxystrobin. Propiconazole is a member of the demethylation-inhibiting (DMI) group
of fungicides and has systemic and curative activity. DMIS inhibit strerol biosynthesis in
fungal cells thereby preventing cell membrane formation (44). The ﬁmgicide may be
applied up to eight days after inoculation without signiﬁcantly compromising control of
S. apiicola (95) but is generally more effective when applied earlier (96). Numerous
studies document the effectiveness of propiconazole for late blight management
(18,70,71 ,78,79,95,96). Some Michigan celery growers choose to use propiconazole in
rotation with chlorothalonil for control of late blight (3). Both strobilurin and triazole

fungicides act at highly speciﬁc yet distinct sites in fungal cells and must be rotated with

13

other fungicides that have different modes of action to prevent or delay the development
of fungicide-resistant fungal populations. Resistance of S. apiicola to currently registered

fungicides has not been observed in Michigan or documented elsewhere.

Early Blight Epidemiology and Management

Early blight, incited by the imperfect fungus Cercospora apii Fresen., is the most
destructive fungal disease of celery in Florida (10) and often warrants 25 to 40 fungicide
applications each season to produce a marketable crop (12). The disease was ﬁrst
reported on celery in Europe in 1863 (80,85). Early blight occurs wherever celery iS
grown (80,85) but occurs less frequently in California (42). The disease has become an
annual problem for Michigan celery growers (3) Since the early 199OS, although it
apparently occurred less frequently in the past (51).

Symptoms of early blight initially appear on older, outer leaves and are
characterized by light tan to gray colored circular lesions surround by a diffuse chlorotic
halo (51,80). The lesions expand to _>_1 cm in diameter and turn dry and papery in texture
(80). Lesions on petioles are generally elongate and lack a chlorotic halo. During
periods of high relative humidity, the lesions may appear gray due to sporulation of the
pathogen (51,80). Under favorable conditions, numerous lesions coalesce and infected
tissues turn necrotic and die. Symptoms of early blight can be distinguished from those
incited by S. apiicola due to the absence of pycnidia and the larger Size of early blight
lesions (80,85).

Cercospora apii hyphae are septate, brown, and 2 to 4 pm in diameter (80).

Conidiophores are 4 to 4.5 pm wide and 55 to 100 um long and arise from hyphal knots,

14

or pseudostromata, in substomatal spaces. Conidiophores are septate and emerge in
clusters through stomata. Several hyline conidia are produced on each conidiophore, and
each conidium is septate with 4 to 17 cells. Each conidium is straight or slightly curved,
3.5 to 5.5 pm wide and 40 to 200 um long, and tapered at the apex (80). Cercospora apii
requires temperatures of 15 to 30°C and RH near 100% for sporulation (7). Extensive
sporulation occurs when leaf wetness durations exceed 10 hours (80). Conidia germinate
and penetrate host tissues via stomata after _>_5 hours of leaf wetness (80). Lesions
usually appear 10 to 12 days after infection (7). C. apii conidia can be carried on the
clothing of farm laborers (56) although they are primarily disseminated by wind (7).

Berger (12) collected high numbers of spores on spore trapping slides following
periods when temperatures were 15 to 30°C and RH was nearly 100% for 310 h per day.
Early morning or late afternoon rains extended favorable RH periods and higher spore
counts were recorded during subsequent spore liberation events. Cercospora apii conidia
were found in rain droplets on leaf surfaces. Wind velocities did not affect the number of
trapped spores, and nighttime winds dried the foliage and resulted in minimal Spore
release during the following day. The highest numbers of Spores were trapped when the
RH was <90%, and peaks in spore liberation were associated with bright, sunny mornings
(0800 to 1000) with rapid decreases in RH. Morning cloud cover and fog impeded Spore
release until the weather cleared and RH decreased (12).

The number of trapped Spores per day provided an estimate of future disease
outbreaks: few lesions were found following spore counts of 0 to 25 spores/day; light
infection (1 to 10 lesions/leaf) was observed after 25 to 100 Spores/day were trapped;

moderate infection (5 to 20 lesions/leaf) occurred following spore releases of 100 to 300

15

spores/day; and severe blight developed when Spore counts were >300 spores/day (12).
Symptoms were observed on routinely sprayed celery plants following very high spore
liberation events (800 to 10,000 Spores/day). The onset and progression of disease in
ﬁelds where blight-free transplants were used was delayed compared with ﬁelds which
were planted with 5 or 40%-blighted transplants (12).

The effect of plant spacing was investigated as a possible cultural tactic to reduce
early blight on Florida celery. Berger (13) calculated higher C. apii infection rates during
the middle to latter half of the growing season in celery planted at high densities
compared with the infection rates calculated from low density plantings (13). Similarly,
disease severity in high-density plantings was approximately double the severity
observed in low-density plantings at eight weeks after transplanting, but these values did
not differ at harvest (13). In contrast, Strandberg and White (90) reported no signiﬁcant
differences among infection rates, ﬁnal disease levels, or leaf wetness durations in celery
planted at different densities. Their results indicated that plant spacing or planting on
raised beds did not improve air circulation and had no effect on early blight control (90).
Increased plant spacing may not be a useful disease management tactic for growers since
the small beneﬁt observed by Berger (13) would likely be offset by yield losses resulting
from reduced plant populations (13,90).

Cultural disease management tactics for early blight include methods to reduce
initial inoculum and the use of tolerant cultivars. Cercospora apii reportedly survives as
mycelium on infected seeds for >2 years and may provide a source of initial inoculum
(80,85), although these assumptions has not been adequately documented (R. N. Raid,

personal communication). Growers are instructed to plant pathogen-free seeds and limit

16

disease outbreaks on transplants in the greenhouse (80). Since infected celery debris left
in the ﬁeld after harvest serves as a source of inoculum for subsequent plantings (12),
adequate crop rotation may reduce the risk of early blight epidemics (80). Early blight-
tolerant cultivars include ‘Flora-belle’, ‘June-bell’, and ‘Earlibelle’ (80). Berger (10)
reported a three week lag in disease incidence on a tolerant cultivar when compared with
a susceptible cultivar, although season-long infection rates did not differ signiﬁcantly
between the cultivars. Despite the availability of semi—tolerant cultivars, fungicide sprays
in the greenhouse and in the ﬁeld are often required to protect the crop (80).

Florida celery growers are advised to apply the ﬁrst fungicide spray immediately
after transplanting (7,47). In 1969, subsequent applications were recommended at the
following frequencies, depending on the number of blight favorable hours per day and the
number of trapped spores per day: (i) Spray once per week if there are 0 to 8 blight
favorable hours and the number of trapped spores range from 1 to 100; (ii) Spray twice
per week if there are 8 to 12 blight favorable hours and the number of trapped spores
range from 100 to 300; (iii) Spray three times per week if there are 12 to 15 blight
favorable hours and the number of trapped spores range from 300 to 500; (iv) Spray three
to seven times per week if there is 15 to 24 blight favorable hours and the number of
trapped spores is >500 (7). Currently, the number of hours of blight-favorable weather
are used to determine the frequency of applications but the use of spore trapping has
apparently been discontinued (47,80). Fungicides may be applied as frequently as two to
four times per week during blight-favorable periods (47). In some cases, cultural
activities (i.e., ﬁeld work and harvesting) are more important for determining Spore

potential than environmental monitoring (12). Ten-fold increases in Spore counts were

17

recorded over 3-minute periods when ﬁeld workers walked 5 to 7 m upwind though a
blighted celery ﬁeld. In addition, Spore numbers peaked during harvest periods despite
the lack of favorable spore liberation conditions (12). A fungicide application is
currently recommended to celery ﬁelds located up to 0.5 miles downwind from
harvesting operations (47). Berger (12) noted that since Spore liberation is not always
linked to blight favorable hours, Spore trapping provides a more accurate measure of
early blight risk than environmental data.

Most fungicides currently registered to control late blight also control early blight.
In a fungicide efﬁcacy study where both S. apiicola and C. apii were present, Hausbeck
et al. (3 6) reported virtually equivalent control of both pathogens using alternating spray
programs with registered fungicides. Miller and Hernandez (68) reported a higher level
of early blight control when azoxystrobin was applied alone or in alternation with
chlorothalonil compared with a standard chlorothalonil program. Raid and Dyce (81)
found better early blight control was achieved in plots Sprayed with azoxystrobin
alternated with chlorothalonil compared with the disease control in plots treated with
propiconazole alternated with chlorothalonil or chlorothalonil applied alone. However,
early blight symptoms were detected on plants in the trial prior to initial fungicide
treatments (81). Mixing copper hydroxide with chlorothalonil had a synergistic effect on
early blight control Since the mixture provided better control compared with either of the
two fungicides applied independently (11). High and low rates of azoxystrobin were
applied in two or three-way alternating spray schedules with chlorothalonil or
propiconazole in a commercial celery ﬁeld in Michigan (16). Yield loss in the untreated

control was nearly 19% while fungicide treatments limited losses to <2% and provided

18

equivalent control of early blight (16). Signiﬁcant differences in early blight control
were not observed when azoxystrobin was applied at high vs. low rates which indicates

no rate response was observed with this ﬁmgicide in these studies (16,35,68,81).

Disease Predictive Systems for Septoria apiicola and Cercospora apii

Disease predictive systems use easily obtained measurements of environmental
conditions in the crop canopy and sometimes also rely on a spore trapping device to alert
growers when disease-favorable conditions exist. In turn, growers are able to apply
fungicides only when they are needed to manage diseases instead of applying fungicides
on a calendar-based schedule. Disease predictors frequently require fewer fungicide
applications compared with commonly used calendar-based schedules to achieve the
same level of disease control. Applying fewer fungicide applications reduces disease
management costs, the potential for fungicide residues on the crop, and the risk of
pathogens developing resistance to fungicides. Disease predictors have been developed
to manage foliar diseases incited by S. apiicola in Michigan (50) and C. apii in Florida
(7,8). A third disease predictor controls S. chopersici and other pathogens on tomato
(76,77) and may be useful for managing S. apiicola on celery (33).

The disease predictor developed in Michigan was used for timing fungicide
sprays to control late blight (50). Fungicides were applied when 312 h of continuous leaf
wetness were recorded if no fungicides were applied in the previous seven days. The
model was based on evidence that very few lesions developed on celery leaves exposed
to 12 h of leaf wetness. The role of temperature was excluded from the predictive system

since temperatures high enough to inhibit infection rarely occur during infection (leaf

l9

wetness) periods in Michigan. In the 3-year study, the predictive model required two
fewer applications relative to the weekly fungicide treatment without compromising
disease control (50). Field studies in 2000 indicated that the S. apiicola predictor
provides adequate disease control with two fewer applications when both S. apiicola and
C. apii were present, but the yield from this treatment was signiﬁcantly lower compared
to some treatments where plants were treated every seven days (3 6). Hausbeck (33)
reported that the S. apiicola predictor limited late blight and yield loss to levels Similar to
a 7-day fungicide schedule with three fewer applications. Despite substantial evidence
for reducing the number of fungicide sprays without compromising disease control, the
predictive system has not gained acceptance among Michigan celery growers.

A disease predictive system was developed to time fungicide sprays to manage C.
apii on celery (7,8). The system used hourly measurements of RH and temperature in
conjunction with a spore trap to identify sporulation periods. Fungicides were applied at
different frequencies based on the number of C. apii conidia collected on Spore trap slides
and based on the number of hours of early blight-favorable weather. The use of this
system in the 1968 winter growing season resulted in a savings of 5 to 15 fungicide
applications without compromising disease control (8). Florida celery growers typically
contract environmental monitoring and crop scouting activities to crop consultants
because growers seldom have the time to operate disease predictive systems on their own
(54). Also, the use of a spore trap requires expensive equipment and trained personnel to
count conidia.

The C. apii predictor was tested in Michigan and Florida without the spore-

trapping component. In Michigan, the predictor required four fewer applications and

20

provided equivalent control of both C. apii and S. apiicola compared with the standard
weekly fungicide schedule in 2001 (M. K. Hausbeck, unpublished data). When only late
blight was present in a 2002 study, the C. apii predictor limited disease and prompted six
fewer applications compared to the 7-day fungicide treatment (33). In Florida, the C. apii
predictor provided inconsistent control of early blight. Studies in 2002 and 2004 were
conducted when environmental conditions were considered “conducive” and “moderately
conductive”, respectively, for early blight development and the C. apii predictor required
ﬁve applications each season (82,84). The early blight control provided by the predictor
was signiﬁcantly lower compared to the weekly fungicide schedule which required 13
applications (82,84). In 2003, environmental conditions were considered “very
conducive” for early blight development and the C. apii predictor required nine
applications (83). Disease control and marketable yields were similar between plants
treated according to the C. apii predictor and weekly application schedule (83).

The TOM-CAST disease predictor was originally developed to manage foliar
diseases (incited by Alternaria solani and Septoria lycopersici) and anthracnose fruit rot
(incited by Colletotrichum coccodes) on tomato (Lycopersicon esculentum) (76). For
each 24-hour period (1100 to 1100), TOM-CAST uses the hours of leaf wetness and the
average temperature during the wetness periods to calculate a disease severity value
(DSV) ranging from 0 to 4, corresponding to environmental conditions unfavorable to
highly favorable for disease development, respectively (76,77). A fungicide application
is made when the cumulative DSV reaches a predetermined threshold. The predictor was
recently validated for managing foliar pathogens on crops for which it was not originally

developed to manage, including Cercospora carotae and Alternaria dauci on carrot

21

(Daucus carota) (23) and Stemphylium vesicarium on asparagus (Asparagus oﬂicinalis)
(66). A preliminary study indicated that the TOM-CAST disease predictor may be
appropriate for managing S. apiicola on celery, but only a Single DSV threshold was
tested (33).

Raid and Pernezny (82,83) and Raid et al. (84) tested the TOM-CAST 15-DSV
program for control of C. apii in Florida. This program only required two to four
fungicide sprays each season compared with the weekly application program which
required 11 to 13 Sprays. Fungicides applied according to the TOM-CAST lS-DSV
program resulted in signiﬁcantly less disease control each season and oﬁen resulted in
lower marketable yields than the calendar-based fungicide program (82,83,84).

Since celery is successively planted over several months, the crop canopy and
microclimate in the crop canopy varies from one ﬁeld to the next depending on the age of
the plants (1 2). Therefore, it is important to monitor environmental conditions separately
in plantings of different maturities. Lacy et al. (54) lists environmental monitoring
instruments that may be use to collect data for disease predictors, although newer

instruments are often less expensive and are relatively less complicated.

Crater Rot Epidemiology and Management
Crater rot of celery is incited by the soil borne fungus Rhizoctonia solani Kuhn
(teleomorph: Thanatephorous cucumeris (A.B. Frank) Donk.). Florida celery production
has been affected by the disease since the early 19003 (85). The disease probably occurs
wherever celery is grown (40). Initial symptoms are small, tan lesions located near the

base of older petioles in contact with the soil (38). Lesions may develop as early as one

22

week after transplanting (74). Older lesions are sunken, red to dark brown, circular or
oval, and up to one inch in diameter (38,93). Crater rot is difﬁcult to detect in ﬁelds (93),
as most of the damage is observed during harvesting and packing operations. Yield
losses are incurred when diseased petioles are manually removed; a labor-intensive
process resulting in reduced plant Size (38). In Florida, yields are up to 28% lower when
the disease is left uncontrolled (75). In Michigan, yield losses of 5 to 7% are reported
(17,22).

The severity of the disease varies from year to year (3 8), presumably due to
environmental changes in microclimate that have not been thoroughly identiﬁed.
Pieczarka (74) observed signiﬁcant increases in petiole infection following rain events,
especially during the mid to latter half of the growing season. The disease is usually
exacerbated by continuous celery culture (38), although R. solani incites disease on a
wide variety of plants (40). Rhizoctonia solani isolates are classiﬁed by anastomosis
groups (AGS), but isolates from celery have not been classiﬁed (40). R. solani isolates
obtained from sugar beet and potato are pathogenic on celery (38), and isolates from
celery incite disease on bean, cotton, and tomato (40).

Rhizoctonia solani produces relatively large (5 to 10 pm in diameter), tan to
brown colored septate hyphae but does not produce Spores (40). The fungus may
produce sclerotia which are brown and variable in appearance. Both mycelium in
infested crop debris and sclerotia can persist in the soil and serve as sources of initial
inoculum for subsequent celery plantings or other crops (40).

The basidiomycete, T. cucumeris, may occur during periods of high humidity and

cool temperatures, especially when the crop is nearing maturity (40). T. cucumeris

23

produces a white mat of superﬁcial hyphae, basidia, and basidiospores (93) which
resemble a light coating of talcum powder on the petiole. This growth does not cause
disease symptoms, nor is there any evidence to support its role in crater rot epidemiology
(40).

A disorder termed “celery petiole lesion” resembled crater rot damage and
resulted in substantial losses to the California celery industry in the early 19905 (43).
Rhizoctonia solani was not recovered from affected petioles, and other tests to detect
pathogenic organisms were negative. Pesticides commonly used in celery production
were tested for possible phytotoxic effects, and the insecticide naled (e.g., Dibrom,
various manufacturers) was implicated with “celery petiole lesion” dar'nage (43). Due to
the similarity between symptoms of crater rot and symptoms of naled phytotoxicity, a
laboratory analysis is recommended to distinguish between the problems (40). However,
naled is no longer registered for use on celery.

Crater rot is minimized with a multitude of cultural control tactics and with
effective fungicides. Celery growers are advised to avoid double cropping celery during
the same season and to avoid planting celery to ﬁelds that have a large amount of
undecomposed crop debris (40). Shallow planting or setting transplants on ridged rows
reduces contact between petioles and infested soil or crop debris. If cultivation is used,
soil should not be thrown at the base of plants to further reduce the chance of inoculum
coming into contact with petioles. Rotation with non-host crops such as small grains may
inhibit R. solani from reaching high populations (40). In California, the use of soil
fumigants in rotational crops (e. g., strawberry) limits the R. solani population in the soil

(4). Directing fungicide sprays toward the base of celery plants is recommended (40), but

24

may not always improve control compared with sprays applied over the top of the foliage
(75).

Guzman et al. (32) recommend the practice of crop scouting at Six weeks after
transplanting to determine the percentage large (marketable) petioles with crater rot
damage. A fungicide spray Should be applied when 20% of plants Show symptoms of
crater rot, and subsequent sprays Should be applied if the average number of infected
petioles on each plant increases or if the percentage of infected plants increases. Lastly,
they recommend one fungicide application just prior to the pre-harvest interval in the
event that disease favorable weather persists until harvest (3 2).

In Florida, crater rot was reduced by modifying the planting depth and by
applying the fungicide chlorothalonil (7 5). Untreated transplants with crowns set 1.3 cm
below the soil surface on ridged rows had fewer infected petioles when compared to
transplants set deeper (4.0 cm) on unridged rows. Chlorothalonil applications applied
either one or Six weeks after transplanting and reapplied weekly signiﬁcantly reduced
crater rot, regardless of planting depth or row conﬁguration. Some Spray programs were
initiated at week Six Since most of the infected petioles during the ﬁrst ﬁve weeks are not
marketable. F ungicide sprays directed at the base of the plants did not improve disease
control compared to sprays applied over the top of the foliage (75).

Fungicide efficacy studies were conducted in commercial celery ﬁelds with a
history of crater rot in Michigan. Over 46% of untreated control plants had petiole
lesions after plants were trimmed to fresh market speciﬁcations (17). All ﬁmgicide
treatments limited disease incidence to <23%. Azoxystrobin alternated with

chlorothalonil was the most effect treatment in reducing the incidence of crater rot and

25

minimizing yield loss (1 7). Both fungicides are currently registered for control of crater
rot (14). In a separate commercial ﬁeld, fungicides initially applied one or four weeks
after transplanting provided complete control of crater rot and signiﬁcantly reduced yield
loss compared with the untreated control (22). Applying the ﬁrst application on a
preventive basis four weeks after transplanting reduced the number of applications and
reduced ﬁrngicide costs by approximately $40/ha compared to the fungicide program

initiated one week after transplanting (22).

Bacterial Leaf Spot Epidemiology and Management

Bacterial leaf spot is incited by Pseudomonas syringae pv. apii (Jagger) Young et
al. The bacterium was ﬁrst reported to infect celery in 1921 (3 9) and has Since been
reported to infect fennel (F oeniculum vulgare) and parsley (Petroselinum crispum) (58).
The disease was previously known as northern bacterial blight to distinguish it from
southern bacterial blight, incited by P. cichorii, which occurs on Florida-grown celery
(85). Unlike the bacterial leaf spot pathogen, P. cichorii causes a petiole necrosis known
as brown stem (73). Symptoms of bacterial leaf Spot on celery leaves are initially small,
water-soaked lesions which are circular to angular in shape (42,85). The lesions turn
necrotic, coalesce, and produce a blighted appearance under favorable conditions.
Lesions may appear as light tan necrotic spots and have a papery texture under dry
conditions. Bacterial leaf spot lesions can be distinguished from those of late blight due
to the absence of pycnidia (42,58). Similarly, lesions incited by P. syringae pv. apii are
smaller, less circular in shape, and lack the gray color during periods of high humidity

when compared with appearance of early blight lesions (42). Petioles are not typically

26

affected by the disease (42,5 8). Direct losses from the disease are reported to reach 5%
due to the extra cost of removing infected foliage (58,85). Indirectly, the disease reduces
the vigor and quality of transplants (42,57) and increases disease management costs in the
greenhouse (42) and in the ﬁeld.

Pseudomonas syringae pv. apii is a ﬂuorescent bacterium which has the following
biochemical and physiological characteristics when subjected to LOPAT tests: positive
for levan production; negative for cytochrome oxidase; negative for potato rot; negative
for arginine dihydrolase; and positive for tobacco hypersensitivity (42,57). An individual
bacterium is 0.7 to 1.2 pm wide and 1.5 mm long and is motile by more than one polar
ﬂagella (58). The bacterium survives on celery seed for a period of two to three years,
and infected seeds are considered the primary source of inoculum (58). The bacterium
lives epiphytically on seedlings until favorable environmental conditions allow for
populations to increase. Pseudomonas syringae pv. apii enters host tissue through
wounds or stomata (57,58).

Environmental conditions and cultural practices associated with celery transplant
production in greenhouses typically favor the development of bacterial leaf spot. Warm
temperatures and frequent watering which are necessary for celery seedling establishment
are also favorable conditions for P. syringae pv. apii (42,57). Close plant Spacing allows
the bacterium to spread between plants via splash dispersal during irrigation and via
mechanical dispersal during mowing. High-pressure overhead irrigation and mowing
also create wounds that allow the pathogen to easily infect seedlings (42,57). Although
the disease may become established in the greenhouse, symptoms may not become

evident until 7 to 10 days later (58) or until the transplants are set in the ﬁeld. The use of

27

sprinkler irrigation in the ﬁeld promotes disease development and Spread of the bacterium
(42).

Several cultural control tactics are recommended for managing bacterial leaf spot
of celery, with an emphasis on reducing inoculum prior to transplanting. Growers should
plant certiﬁed disease-free seed or treat the seed in a hot water at 50°C for 25 min (58).
The hot water treatment drastically reduces P. syringae pv. apii from celery seed but does
not provide completely eradicate the pathogen (57). This seed treatment may reduce seed
germination especially if the seed is 22 years old. Celery ﬂats and greenhouse benches
Should be disinfected between growing seasons. Growers Should irrigate at times that
allow leaves to dry quickly and avoid the use of high pressure irrigation (57). Proper
greenhouse ventilation and air circulation can be used to reduce humidity and the
duration of leaf wetness (54). Mowing should be conducted when the leaves are dry, and
mowers should be disinfected at regular intervals (57). Vigilant sanitation of equipment
and workers should be implemented to avoid contamination between greenhouses (57).

Celery growers in Michigan and California use copper hydroxide-based products
in greenhouse transplant production and in the ﬁeld to limit the disease, but these
products only provide fair to good control (3,4). Bounds and Hausbeck (21) evaluated
various products for bacterial leaf Spot management at the request of Michigan celery
growers (3). Weekly copper hydroxide (Kocide 2000 53.8DF at 1 and 1.5 lb/A)
treatments that growers often use reduced bacterial leaf spot severity by >5 0% compared
to the untreated control (21). Weekly applications copper hydroxide mixed with of an
unregistered product classiﬁed as a plant defense activator, acibenzolar-S-methyl

(Actigard 50WG, Syngenta Crop Protection, Inc.), resulted in the highest level of disease

28

control but did not differ signiﬁcantly from the standard treatment of copper hydroxide
applied alone. Other products were mixed with copper hydroxide and included the harpin
protein biopesticide (Messenger STS 3WDG, Eden Bioscience Corporation) and the
Bacillus subtilis biocontrol agent (Serenade Max 14.6WP, AgraQuest, Inc.). Although
both of these products are registered for use on celery in the greenhouse and in the ﬁeld,
neither improved disease control relative to the standard copper hydroxide treatment.

The severity of bacterial leaf spot in this study did not warrant additional trimming of

diseased leaves or reduce yields even when plants were left untreated (21).

29

10.

11.

12.

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37

CHAPTER 1.

Comparing Disease Predictors and Fungicide Programs for Late Blight
Management in Celery

38

ABSTRACT

Late blight, incited by the fungus Septoria apiicola, is the most important foliar
disease of celery in Michigan and results in necrotic lesions on leaves and petioles that
can reduce yields by up to 80%. Most celery growers apply the fungicide chlorothalonil
as frequently as every seven days to protect the crop, but some of these applications may
not be needed when environmental conditions do not favor disease development. The
purpose of this study was to evaluate disease predictors using a standard (chlorothalonil)
and a reduced-risk (azoxystrobin alternate chlorothalonil) fungicide program for
managing late blight on ‘Dutchess’ celery in 2003 to 2005. Fungicides were initiated one
week after transplanting and reapplied every seven days or according to the Septoria
predictor, the Cercospora predictor, or TOM-CAST using intervals of 10, 15, and 20
disease severity values (DSV). In each year of this study, timing sprays according to the
TOM-CAST lO-DSV predictor resulted in disease control comparable to the 7-day
interval, but required up to ﬁve fewer sprays, and reduced ﬁmgicide costs (53215/ha)
compared to the 7-day interval. The Septoria, Cercospora, and TOM-CAST 15-DSV
predictors oﬁen provided control similar to the 7-day interval, but these predictors were
somewhat inconsistent compared to TOM-CAST lO-DSV. The TOM-CAST 20-DSV
predictor required the fewest number of sprays but unacceptable levels of disease
resulted. The standard and reduced-risk fungicide programs frequently provided similar
control, but azoxystrobin alternated with chlorothalonil was more effective than

chlorothalonil alone when disease pressure was high.

39

INTRODUCTION

Late blight, incited by Septoria apiicola Speg., is an important foliar disease of
celery (Apium graveolens L. var. dulce (Mill.) Pers.) in Michigan (1 ,16,27) and
California (2,23,36). The disease also occurs in Florida (6) but has not been an annual
problem in recent years (24). Late blight symptoms typically appear ﬁrst on the older,
outer leaves and are characterized by irregularly Shaped, necrotic lesions (16) which are
sometimes surrounded by a diffuse, chlorotic margin (17). The disease is easily
distinguished from other common foliar diseases of celery in Michigan, such as early
blight (incited by Cercospora apii) (29) and bacterial leaf spot (incited by Pseudomonas
syringae pv. apii) (32), because black pycnidia are embedded in S. apiicola lesions (15).
Each pycnidium is capable of releasing an average of 3,675 conidia (30). Conidial
germination is highest in the presence of free moisture (53) and infection occurs when
celery leaves remain wet for 212 h (26,33). Conidia are splash-dispersed by rainfall or
overhead irrigation (6) and are moved by mechanical means in conidia-laden water
droplets by farm laborers and machinery (31). Severely blighted tissues wither and
senesce (16). Diseased petioles must be manually removed, a time consuming process
which increases harvesting costs and reduces yields. Untreated plants can weigh 56 to
82% less aﬁer diseased petioles are removed (12,46); these plants are not marketable
since only a few small, healthy petioles remain.

F oliar fungicides are often the primary tool for control of late blight (28) and can
be applied as frequently as every seven days (8). The calendar-based management
approach may result in excessive applications when environmental conditions do not

favor disease development. Most growers use the protectant fungicide chlorothalonil,

4O

which is classiﬁed as a BZ carcinogen, to manage late blight because it is effective
(11,25,45,46) and relatively inexpensive. Azoxystrobin is a relatively new, reduced-risk
fungicide labeled for use on celery that protects against S. apiicola (12,21,49).
Azoxystrobin has systemic and curative properties and must be alternated with fungicides
that have a different mode of action to prevent or delay the occurrence of fungicide-
resistant strains of plant pathogens (3). The use of chlorothalonil in alternation with
azoxystrobin is typically recommended since both products effectively limit late blight
and they have different modes of action. Including azoxystrobin in a spray program to
manage late blight may displace some of the applications of the B2 carcinogen,
chlorothalonil.

Applying fungicides only when environmental conditions are favorable for S.
apiicola may reduce the overall number of applications needed for effective disease
control, reducing pesticide inputs to the environment. Two disease predictors have been
developed to manage foliar diseases of celery. Lacy (26) developed a S. apiicola model
(hereafter referred to as the Septoria predictor) which requires measurements of leaf
wetness duration. Berger (4,5) developed a C. apii model (hereafter referred to as the
Cercospora predictor) which requires measurements of relative humidity (RH) and
temperature use in conjunction with a spore trap. Both disease predictors reduced the
number of fungicide applications needed to obtain disease control comparable to
calendar-based application schedules (5,26). A disease predictor that includes both leaf
wetness duration and temperature may be appropriate for managing S. apiicola Since both

parameters are important for disease development (3 3).

41

The TOM-CAST disease predictor was originally developed to manage foliar
diseases (incited by Alternaria solani and Septoria lycopersici) and anthracnose fruit rot
(incited by Colletotrichum coccodes) on tomato (43,44). The system utilizes the duration
of leaf wetness and average temperature during the wetness period to calculate a daily
disease severity value (DSV). A fungicide is applied when the cumulative DSV reaches
a predetermined threshold (43,44). Since its development for use on tomatoes, TOM-
CAST has been validated for use in managing Cercospora carotae and Alternaria dauci
on carrot (14,51) and Stemphylium vesicarium on asparagus (35).

Michigan celery growers are interested in reducing their reliance on the calendar-
based fungicide regime and desire an alternative late blight management strategy in order
to lower production costs while maximizing quality and yield (1). A preliminary study
indicated that the TOM-CAST disease predictor may be appropriate for managing late
blight of celery, but only a single DSV threshold was tested (20). The objective of this
study was to compare disease predictors using a grower standard and a reduced-risk

fungicide program to manage late blight on celery.

MATERIALS AND METHODS
Plot establishment and inoculation. The study was conducted at the Michigan
State University Muck Soils Research Farm in Laingsburg, Michigan from 2003 to 2005.
Seven— to eight-week old ‘DutcheSS’ celery transplants were set 17.8 cm apart in rows
spaced 0.8 m apart on 28 May 2003, 21 June 2004, and 25 May 2005 in Houghton muck
soil, previously planted with potato (2003 and 2004) or carrot (2005). Each treatment

plot consisted of one row 6.1 m long which included approximately 34 plants. Sections

42

of inoculated buffer row (0.8 to 1.5 m) bordered the ends of each plot, and two inoculated
buffer rows separated adjacent treatment rows. Thirteen treatments which included one
untreated control and 12 ftmgicide programs were arranged in a randomized complete
block design in four blocks. Weed, insects, and fertilization requirements were managed
according to standard production practices (8,59,60). Sprinkler irrigation was used to
maintain the water requirements of the crop and to promote S. apiicola infection.

Inoculum was prepared and applied similar to the procedures described by Lacy et
al. (28). Dried celery leaves infected with S. apiicola were soaked in tap water for 10
min, and the debris was removed by straining the spore suspension through two layers of
cheesecloth. Spore concentration was determined using a hemacytometer, and the ﬁnal
spore suspension was adjusted to 1 x 106 conidia/ml with water just prior to inoculation.
Plots were sprinkler-irrigated to wet the foliage for approximately 60 min prior to
inoculation. Inoculum was applied at dusk with a hand-pump sprayer equipped with one
hollow-cone or ﬂat-fan nozzle delivering approximately 168 liters/ha. All buffer rows
were inoculated (i.e., every plant except those in treatment plots); care was taken to avoid
inoculating plants in treatment rows. This indirect inoculation approach was used to
allow S. apiicola to Spread under natural splash-dispersal conditions. Twelve or more
hours of continuous leaf wetness were maintained by Short intervals (10 to 15 min) of
sprinkler irrigation in the morning after each inoculation. All buffer rows were
inoculated twice each season at 42 and 49 days after planting (DAP) (2003 and 2005) and
39 and 51 DAP (2004).

Disease predictors and fungicide programs. The Septoria predictor requires a

fungicide application when 312 h of consecutive leaf wetness occurs if no fungicides

43

were applied during the previous seven days (26). The Cercospora predictor used in this
study was Similar to the program developed in Florida (4), but without the Spore-trapping
component. The spore-trapping component of the predictor was omitted in an effort to
test the system in a manner growers or crop consultants could operate without requiring
excessive time or expensive equipment. A fungicide Spray was applied if all of the
following criteria were met: (i) no fungicides were applied during the previous seven
days; (ii) 212 h of RH 290% were recorded the previous day (0700 yesterday to 0600
today); (iii) mean temperature was 15 to 30°C during previous day; (iv) temperatures
three days ago were __>_12°C, or, if the temperature fell below 12°C, the mean night
temperature (2200 to 0700) on each of the two succeeding nights was 315°C and had a
mean RH 295%.

For each 24-h period (1100 to 1100), TOM-CAST uses the hours of leaf wetness
and the average temperature during the wetness periods to calculate a DSV, ranging from
O to 4, corresponding to environmental conditions unfavorable to highly favorable for
disease development, respectively (44). Daily DSVS are summed and accumulated until
a threshold value is reached, a fungicide Spray is applied, and the DSV total is reset to
zero. Unlike the Septoria or Cercospora predictors, the TOM-CAST program was not
tested with a minimum ﬁmgicide reapplication interval since none was Speciﬁed when
the predictor was developed for tomato (43,44) or when the predictor was validated for
use in asparagus (35) or carrot (14).

Hourly measurements of temperature and leaf wetness were obtained using a
digital data recorder (WatchDog Leaf Wetness and Temperature Logger 3610TWD;

Spectrum Technologies, Inc., Plainﬁeld, IL) located in the upper 75% of the crop canopy

44

in an unsprayed row at a 45° angle facing north. A second digital data recorder
(WatchDog Data Logger Model 450; Spectrum Technologies, Inc.) made hourly
measurements of RH, temperature, and rainfall and was located in the upper 75% of the
crop canopy. A tipping-bucket rain gauge (Model 3665R; Spectrum Technologies, Inc.)
collected rainfall and sprinkler irrigation and was located 1.2 m above the soil surface.
Data recorders were placed in the plots after transplanting and were set to record
temperatures from 0 to 100°C and leaf wetness (i.e., wetness threshold set to 0) where
applicable. Data were downloaded at least every other day to a laptop computer using a
computer program (Specware 6.02; Spectrum Technologies, Inc.) equipped to calculate
DSVS for the TOM-CAST predictor. Raw hourly data were examined to determine the
number of hours of consecutive leaf wetness for the Septoria predictor. Similarly, raw
hourly data were examined to determine the number of hours of RH 290 or _>_95% and to
calculate average temperatures for the Cercospora predictor.

The reduced-risk fungicide azoxystrobin (Quadris 2.08F at 0.17 kg a.i./ha,
Syngenta Crop Protection, Inc., Greensboro, NC) and the fungicide chlorothalonil (Bravo
Ultrex 82.5WDG at 1.7 kg a.i./ha, Syngenta Crop Protection, Inc.) were either applied in
an alternating program (hereafter referred to as azoxystrobin/chlorothalonil), where
azoxystrobin was applied at the initial application, or in a grower standard program where
chlorothalonil was applied exclusively. The thirteen treatments included in this study
were composed of an untreated control plus 12 fungicide treatments where both fungicide
programs (azoxystrobin/chlorothalonil and chlorothalonil) were tested with each of the
six application programs: (i) 7-day interval; (ii) Septoria predictor; (iii) Cercospora

predictor; (iv) TOM-CAST 10-DSV; (v) TOM-CAST lS-DSV; (vi) TOM-CAST 20-

45

DSV. To allow for equal comparison among application programs, all fungicide
treatments received an initial Spray one week after transplanting. Applications were
made with a C02 backpack boom Sprayer (R & D Sprayers, Opelousas, LA) equipped
with three Teejet XR8003VS ﬂat-fan nozzles (Spraying Systems Co., Wheaton, IL)
spaced 45.7 cm apart, operating at a boom pressure of 359 kPa, and delivering 467.6
liters/ha. The cost of fungicides applied for each treatment was calculated by multiplying
the number of applications by estimated costs of azoxystrobin ($53/ha) or chlorothalonil
($28/ha) per application.

Disease assessment. Weekly visual evaluations of leaf blight severity were
conducted on 20 plants from the middle of each treatment plot using a celery leaf blight
assessment key developed by Strandberg (unpublished). According to the key, plots were
assigned values of 0, l, 5, 10, 15, 20, 25, 30, 35, 40, 45, or 50% leaf blight. The celery
leaf blight assessment key was similar to the leaf blight assessment key developed for
carrot (55). Plots were evaluated once prior to disease developing in treatment rows and
three (2005) or four (2003 and 2004) times after disease symptoms were detected.

Ten plants were hand-harvested from each treatment plot and trimmed to fresh
market speciﬁcations at 90 (2003), 95 (2004), and 85 DAP (2005). Disease severity was
evaluated by visually estimating the percentage of symptomatic petiole tissue on all
trimmed plants. Total yield was recorded, diseased petioles were removed, and plants
were weighed again to obtain disease-free yield. The yield loss percentage was expressed
as the weight of diseased petioles removed (total yield minus disease-free yield) divided
by the total yield. The methods of disease assessment used in this study conform with the

methods suggested by Kavanagh and Ryan (22).

46

Statistical analysis. Disease and yield data were analyzed using an analysis of
variance (AN OVA) for a randomized complete block experiment with the Proc GLM
procedure of the Statistical Analysis System (SAS Institute, Cary, NC). The equal
variance assumption was examined using Levene’s Robust test (3 9). All data were
analyzed separately by year Since the equal variance assumption between years was
usually not met. Normality was examined using the residuals from each ANOVA and
Proc Univariate procedure of SAS, and all data were square-root transformed to meet the
normality assumption. The treatments in this experiment represent a two (fungicide
program) by six (application program) factorial experiment with an additional untreated
control treatment. Signiﬁcant treatment differences were further examined using the
following four linear contrasts to compare the treatments: (i).the difference between the
untreated control and the average of fungicide treatments; (ii) differences among
application programs across fungicide programs (main effect of application program);
(iii) the difference between ﬁmgicide programs across application programs (main effect
of fungicide program); (iv) the interaction between application program and fungicide
program. Differences among means for any signiﬁcant effect, giving priority to

interaction effects, were examined using Tukey’s Studentized Range test (54).

RESULTS
Environmental conditions were favorable for S. apiicola establishment and
dissemination during each year of this study but were particularly favorable in 2003 when
the rainfall and irrigation total was highest. Rainfall and irrigation totaled 21.9, 16.3, and

16.5 cm from the day after the ﬁrst inoculation until harvest in 2003, 2004, and 2005,

47

respectively. During the same time period, the average leaf wetness was 12.7, 13.9, and
14.2 h/day and extended periods of leaf wetness (212 h) occurred on 62%, 78%, and 85%
of the days in 2003, 2004, and 2005. Mean temperatures recorded in the crop canopy
were similar in 2003 (193°C) and 2004 (19.1°C) but were Slightly warmer in 2005
(222°C).

In each year of the study, late blight symptoms were observed on the leaves of
inoculated plants 12 to 15 days after the buffer rows were ﬁrst inoculated (DAFI). S.
apiicola Spread to treatment rows, and symptoms were detected 26 to 31 DAP I on
untreated plants. Leaf blight severity on the untreated plants was 55% at 33 to 40 DAF I
and was 35% (2004 and 2005) to 41% (2003) at the ﬁnal evaluation (Fig. 1). Untreated
plants trimmed to fresh market Speciﬁcations had severe late blight symptoms on petioles
(>3 8%) (Table 1), which required additional trimming and resulted in a yield loss of 54 to
69% (Table 2). Untreated plants were not marketable for fresh market use since fewer
than four small, healthy petioles remained aﬁer blighted petioles were removed.

Fungicide-treated plants had Signiﬁcantly lower levels of disease and yield loss
compared to untreated plants (Tables 1 and 2). Fungicide applications limited leaf blight
severity to <22% (2003) or 55% (2004 and 2005) and petiole blight severity to <23%
(2003) or 510% (2004 and 2005) (data not shown). When averaged across fungicide
programs, similar control of petiole blight was provided by the following application
programs during 2003: 7-day interval; Septoria predictor; Cercospora predictor; TOM-
CAST lO-DSV predictor (Table 1). Only the 7-day interval and the Septoria predictor
completely prevented yield loss (Table 2). The TOM-CAST 10-DSV and Cercospora

predictors were comparable in limiting yield loss to <3%, but only the TOM-CAST 10-

48

 

 

 

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60

row plants were inoculated with Septoria apiicola at approximately six and seven weeks

after transplanting in 2003-2005.

49

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51

Table 2. Effects of application programs and fungicides" on yield loss resulting from
Septoria apiicola on ‘Dutchess’ celery during 2003 to 2005

 

 

 

Yield loss (%)
Treatment main effect 2003 2004 2005
Untreated controlW 54.9 69.1 63.7
Application program"
7-day 0.0 ay 0.0 a 1.0
Septoria predictor 0.0 a 0.0 a 2.6
Cercospora predictor 2.9 b 0.0 a 0.9
TOM-CAST lO-DSV 1.5 ab 0.6 a 0.8
TOM-CAST 15-DSV 8.4 c 0.3 a 7.0
TOM-CAST 20-DSV 32.5 d 2.5 b 15.6
F ungicide programz
chlorothalonil 6.8 b 0.7 a 3.9
azoxystrobin/chlorothalonil 3.1 a 0.3 a 3.4
Source P value P value P value
Treatment <0.0001 <0.0001 <0.0001
Untreated vs. avg. treated <0.0001 <0.0001 <0.0001
Application program <0.0001 <0.0001 <0.0001
Fungicide program 0.0004 0.1215 0.5918
Application-fungicide interaction 0.0747 0.65 16 0.0003

 

V Chlorothalonil (1.7 kg a.i.lha) was applied exclusively or in alternation with
azoxystrobin (0.17 kg a.i./ha) where azoxystrobin was applied ﬁrst. Both fungicide
programs were tested with each application program.

Untreated control data are shown for comparison purposes only and are not included
in the statistical analyses for main effects or the interaction.

Application program data (n = 8/mean) averaged across ﬁmgicide programs.

y Means within a column for each treatment main effect followed by the same letter are
not significantly different according to Tukey’s Studentized Range test (P 5 0.05).
Mean separation results are not presented for main effects when the application-
fungicide interaction was signiﬁcant.

F ungicide program data (n = 24/mean) averaged across application programs.

52

DSV predictor was comparable to the 7-day interval. Applying fungicides according to
the TOM-CAST 15- or 20-DSV predictors resulted in signiﬁcantly higher yield loss
when compared to all other application programs (Table 2).

Across application programs, azoxystrobin/chlorothalonil resulted in similar
reductions in disease severity and yield loss compared with chlorothalonil (Tables 1 and
2). An exception was noted in 2003 when signiﬁcant differences were detected under
high disease pressure: azoxystrobin/chlorothalonil signiﬁcantly reduced petiole blight
severity (Table 1) and yield loss (Table 2) compared to chlorothalonil.

A signiﬁcant interaction between application program and fungicide occurred for
leaf blight severity in 2003, indicating the most effective program was dependent on the
fungicide (Table 1). The 7-day interval, Septoria predictor, Cercospora predictor, and
TOM-CAST 10-DSV predictor were equally effective in limiting leaf blight to <2%
when chlorothalonil was applied (Table 3). While the TOM-CAST 15-DSV predictor
was less effective than these programs, it resulted in signiﬁcantly lower leaf blight than
the TOM-CAST 20-DSV predictor. When azoxystrobin/chlorothalonil was used, the 7-
day interval, Septoria predictor, and TOM-CAST 10-DSV predictor signiﬁcantly reduced
leaf blight (_<_1%) compared to the Cercospora predictor. The TOM-CAST 15-DSV
predictor signiﬁcantly reduced leaf blight compared to the TOM-CAST 20-DSV
predictor and provided control similar to the Septoria, Cercospora, and TOM-CAST 10-
DSV predictors when azoxystrobin/chlorothalonil was used (Table 3).

Under moderate disease pressure in 2004 and 2005, all disease predictors except
TOM-CAST 20-DSV limited petiole blight to levels similar to the 7-day interval when

averaged across fungicide programs (Table 1). In 2005, the TOM-CAST 20-DSV

53

Table 3. Interaction of application programs and fungicides on control of Septoria
apiicola on ‘Dutchess’ celery in 2003 and 2005y

 

 

 

Disease assessment (year) Fungicide programz
Application program Chlorothalonil Azoxystrobin/chlorothalonil

Leaf blight severity (%) (2003)
7-day 0.7 a B 0.2 a A
Septoria predictor 1.0 a B 0.5 ab A
Cercospora predictor 1.8 a A 2.7 c B
TOM-CAST lO-DSV 1.0 a A 1.0 ab A
TOM-CAST 15-DSV 8.3 b B 1.8 be A
TOM-CAST 20-DSV 18.3 c A 21.2 d B

Yield loss (%) (2005)
7-day 1.3 a A 0.8 a A
Septoria predictor 6.1 b B 0.3 a A
Cercospora predictor 0.2 a A 1.8 a B
TOM-CAST lO-DSV 0.9 a A 0.7 a A
TOM-CAST 15-DSV 12.6 b B 2.9 a A
TOM-CAST 20-DSV 8.1 b A 25.5 b B

 

y Means under each disease assessment-year combination in the same column
(fungicide program) followed by the same lowercase letter and means in the same
row (application program) followed by the same uppercase letter are not signiﬁcantly
different according to Tukey’s Studentized Range test (P 5 0.05).

Chlorothalonil (1.7 kg a.i./ha) was applied exclusively or in alternation with
azoxystrobin (0.17 kg a.i./ha) where azoxystrobin was applied ﬁrst.

54

predictor had a signiﬁcantly higher level of leaf blight than the 7-day interval (Table 1).
Leaf blight severity did not differ signiﬁcantly among application programs in 2004
(Table 1), but differences in yield loss occurred (Table 2). Yield loss was signiﬁcantly
higher when fungicides were applied according to the TOM-CAST 20-DSV predictor
compared to all other application programs (Table 2).

In 2005, a signiﬁcant interaction between the application program and fungicide
was detected in the analysis of yield loss (Table 2). The Septoria, TOM-CAST 15-DSV,
and TOM-CAST 20-DSV predictors resulted in yield losses that were statistically similar
and signiﬁcantly higher compared to the other application programs when chlorothalonil
was applied exclusively (Table 3). All application programs were equally effective in
limiting yield loss when azoxystrobin/chlorothalonil was used, with the exception of the
TOM-CAST 20-DSV predictor (Table 3).

Signiﬁcant interactions between application program and fungicide indicated that
fungicides performed differently depending on the application program. Frequently,
azoxystrobin/chlorothalonil outperformed chlorothalonil under high disease pressure in
2003, with two exceptions (Table 3). Chlorothalonil applied according to the Cercospora
or TOM-CAST 20-DSV predictors was signiﬁcantly more effective in limiting leaf blight
compared to azoxystrobin/chlorothalonil (Table 3). Similar results were noted in the
analysis of yield loss in 2005. When chlorothalonil or azoxystrobin/chlorothalonil was
applied at 7-day intervals in 2005, there was no signiﬁcant difference in yield losses.
Fungicides did not differ signiﬁcantly in leaf blight severity (2003) or yield loss (2005)

when applied according to the TOM-CAST lO-DSV predictor (Table 3).

55.

Overall, disease predictors increased the average number of days between sprays
compared to the 7-day interval (Table 4). Fungicides were applied as frequently as every
seven days for the Septoria and Cercospora predictors, which is the minimum interval
speciﬁed by these programs. When environmental conditions did not favor disease, a
maximum application interval of 17 days occurred for the Septoria and Cercospora
predictors. The TOM-CAST 10-DSV predictor required a fungicide application as
frequently as every four days or as infrequently as every 21 days. The TOM-CAST
predictor prompted applications every 7 to 20 days (1 S-DSV) or every 11 to 28 days (20-
DSV) (Table 4).

The Septoria and Cercospora predictors frequently provided equivalent disease
control compared to the 7-day interval and required up to two and ﬁve fewer sprays,
respectively (Table 5). The TOM-CAST 10-DSV predictor required either the same
number or up to ﬁve fewer sprays compared to the 7-day program and always resulted in
equivalent disease control regardless of fungicide. The TOM-CAST 15-DSV predictor
required three to seven fewer sprays than the 7-day interval and often provided similar
control under moderate disease levels in 2004 and 2005 but not when disease was severe
in 2003. The TOM-CAST 20-DSV predictor required as many as nine fewer sprays
compared to the 7-day interval, but equivalent disease control was not achieved
regardless of fungicide. The 7-day interval was often the most costly since it required the
most fungicide applications. Disease predictors which required two to seven fewer
sprays than the 7-day interval reduced fungicides costs by $56 to $196/ha with

chlorothalonil and $81 to $296 with azoxystrobin/chlorothalonil, respectively (Table 5).

56

Table 4. Frequency of applications as required by the 7-day interval and disease
predictors for control of Septoria apiicola during 2003 to 2005

 

Average no. days between applications (minimum, Three-year

 

 

maximum no. days between applications) ave. no. days
Application program 2003 2004 2005 between app.
7-dayz 7.0 (6,9) 7.0 (6,8) 7.1 (7,8) 7.0
Septoria predictor 8.6 (7,12) 8.] (7,15) 8.3 (7,11) 8.3
Cercospora predictor 11.7 (7,15) 8.8 (7,14) 9.0 (7,17) 9.6
TOM-CAST 10-DSV 10.9 (5,16) 10.6 (5,21) 7.2 (4,13) 9.3
TOM-CAST 15-DSV 16.3 (12,20) 14.8 (13,19) 10.6 (7,15) 13.3

TOM-CAST 20-DSV 21.7 (15,28) 21.0 (17,26) 14.2 (11,23) 18.1

 

F orecasted or actual rain events either shortened or extended the days between
applications for the 7-day interval.

57

Table 5. Number of fungicide applications and cost of fungicides applied according to
different application programs for control of Septoria @iicola during 2003 to 2005
No. fungicide sprays
Cost of chlorothalonil--azoxystrobin alternate chlorothalonil

 

 

($lha)z
Application program 2003 2004 2005
7-da 12 13 11
y 336--486 364--539 308--458
Se toria redictor 10 11 10
p p 280--405 308--458 280--405
Cercospora predictor 7 10 9
196--296 280--405 252--377
TOM-CAST 10st 3 3 11
224--324 224--324 308--458
TOM-CAST 15-st 5 6 8
140-215 168-243 224424
4 4 6
TOM-CAST 20st 1 12"] 62 112--162 168-‘243

 

Z

($28/ha) and azoxystrobin ($53/ha) per application.

58

The cost of fungicide applied was calculated using estimated costs of chlorothalonil

DISCUSSION

Failure of a disease predictor to limit disease in celery has signiﬁcant economic
consequences since the crop value is relatively high and the plant parts affected by S.
apiicola are the marketable portions of the crop. Consumers demand blemish-free celery,
so an alternative disease management program that requires fewer fungicide applications
than the standard program must not compromise disease control. In Michigan, the TOM-
CAST system is predominantly used by producers of carrots and asparagus (38) that are
valued at $3,424 and $2,420/ha, respectively. In contrast, the value of Michigan celery
grown for processing and fresh market is six- to eight-fold higher at $20,43 um (58). '
TOM-CAST is used to manage diseases of carrot and asparagus that occur on plant parts
that are not usually marketed or consumed. In carrot, TOM-CAST is used to manage
foliar blight on leaves and petioles (14,51), plant parts which supply photosynthetic
energy to the edible root and must be healthy to withstand the pull of mechanical
harvesters. In asparagus, TOM-CAST is used to manage purple spot on the fern after the
edible spears are harvested (3 5). In our experiments with celery, management programs
which allowed S. apiicola to affect the marketable portion of the crop were unacceptable.

The highest disease severity was observed in 2003 when the relatively high
amount of rainfall and irrigation favored dissemination and infection by S. apiicola
(6,52). Parker et al. (41) reported a linear relationship between rainfall, expressed as
cumulative rainfall and cumulative hours with rain, and Septoria leaf spot severity
(incited by S. lycopersici) on tomato foliage. In our trials, differences in disease severity
among years likely impacted the efﬁcacy of some of the fungicide treatments. For

example, our 2003 results indicate that the TOM-CAST 15-DSV predictor was less

59

effective when compared with the 7-day interval when disease pressure was high.
However, differences in disease control were not detected when TOM-CAST lS-DSV
was tested under lower disease pressure in 2004 and 2005, especially when azoxystrobin
was included in the spray program. Incorporating rainfall and irrigation parameters
might be beneﬁcial to the TOM-CAST predictor for managing diseases incited by
Septoria spp. as others have recommended (19,40,41). Reﬁning the TOM-CAST
predictor to include these parameters could bolster the predictor for use in high risk
crops, such as celery.

Our inoculated ﬁeld plots may have had higher levels of S. apiicola inocula than
the levels that typically occur in commercial celery ﬁelds. Even the 7-day interval did
not entirely prevent disease development. Under conditions of naturally-occurring S.
lycopersici inoculum, the TOM-CAST 20-DSV predictor provided equivalent disease
control on tomato compared to the 7-day interval with ﬁve to six fewer applications (37).
Perhaps the TOM-CAST lS-DSV predictor would be acceptable under natural conditions
in commercial celery ﬁelds where the inoculum load is limited by cultural control
measures. Most Michigan celery growers plant seed that is hot-water treated, which may
not completely eradicate S. apiicola (34), and some growers use a one-year crop rotation
(25) to reduce inoculum associated with infested celery debris in the soil. However, due
to the uneven distribution of late blight under natural conditions, artiﬁcial inoculation is
recommended when conducting ﬁmgicide efﬁcacy studies with S. apiicola (28). When
the Septoria predictor was ﬁrst tested in the early 1990s, S. apiicola inoculum was
applied once to the experimental plots (26). In our study, we followed the

recommendation of Lacy et al. (28) and inoculated the buffer rows twice to ensure

60

adequate infection. The higher inoculum load in our experimental plots may explain why
the Septoria predictor was less effective in limiting yield loss than the 7-day interval
when chlorothalonil alone was applied in 2005.

In Florida, Raid and Pernezny (47,48) and Raid et al. (50) found that the TOM-
CAST 15-DSV predictor resulted in signiﬁcantly less C. apii control and oﬁen resulted in
lower marketable yields than the weekly interval. However, the TOM-CAST 15-DSV
predictor only required two to four fungicide sprays compared to the weekly interval
which required 11 to 13 sprays (47,48,50). Under the environmental conditions in our
study, the TOM-CAST 15-DSV predictor required a minimum of six sprays and provided
control similar to the 7-day interval in 2004 and 2005. In Canada, Trueman et a1. (57)
recently evaluated the TOM-CAST 20-DSV predictor for controlling late blight of celery
and determined that it was not as effective as the 7-day interval. Although our results
with the TOM-CAST 20—DSV predictor agree with those of Trueman et a1. (57), the
fungicide programs used in our study differ greatly from those tested in Canada.
Trueman et al. (57) applied copper hydroxide in alternation with either chlorothalonil or a
new fungicide that is not currently registered for use on celery. Michigan celery growers
often tank-mix copper hydroxide with chlorothalonil in their disease control programs
because copper hydroxide limits bacterial leaf spot (1 3) and improves early blight control
compared to chlorothalonil alone (7). Reports from Michigan (1,12,20) and California
(2,42) indicate that copper-based fungicides provide little, if any, protection from S.
apiicola when used alone. Although copper-based fungicides were not tested with

disease predictors under the experimental conditions in our study, we are reluctant to

61

recommend copper-based fungicides for use with disease predictors in Michigan due to
their limited efficacy against S. apiicola.

Overall, azoxystrobin/chlorothalonil was more effective than chlorothalonil in
limiting petiole blight and yield loss under high disease pressure in 2003. Also, the
alternating program was more eﬁ‘ective than chlorothalonil alone when applied every
seven days in 2003 or applied according to the Septoria or TOM-CAST 15 DSV
predictors in 2003 and 2005. Raid (46) also obtained signiﬁcantly better control of S.
apiicola and higher yields when azoxystrobin was applied in alternation with
chlorothalonil compared with chlorothalonil only. The beneﬁt of including azoxystrobin
in the spray program can likely be attributed to the systemic property, translarninar
movement (3), and, perhaps the long residual activity, of this fungicide.

An unexpected outcome in our study was that chlorothalonil resulted in
signiﬁcantly lower levels of leaf blight (2003) and yield loss (2005) than
azoxystrobin/chlorothalonil when applied according to the Cercospora and TOM-CAST
20-DSV predictors. Potential factors that may have contributed to this outcome were our
use of robust inoculum loads and lowest labeled fungicide rates. These factors may have
selected for S. apiicola strains with reduced-sensitivity to azoxystrobin. Resistance in S.
tritici to azoxystrobin is conferred by an amino acid substitution of glycine with alanine
at position 143 on the cytochrome b gene (18) or by alternative respiration; however, the
latter mechanism has no effect on disease control in vivo (61). Similar evidence has not
been reported for S. apiicola, nor have baseline sensitivities of S. apiicola been
established for strobilurin fungicides (including azoxystrobin and triﬂoxystrobin, which

are both registered for use on celery in the United States). Whether a target site mutation

62

or alternative respiration occurred in S. apiicola in our study is unknown but appears
unlikely since the inferior control provided by azoxystrobin/chlorothalonil was only
evident in plots treated according to the Cercospora and TOM-CAST 20-DSV predictors.
Further, spray programs that included azoxystrobin were among the most efﬁcacious in
fungicide efﬁcacy trials simultaneously conducted and located adjacent to the
experimental plots of the present study (9,10,11,12). Nevertheless, the Cercospora and
TOM-CAST 20-DSV predictors provided inconsistent and unacceptable control,
respectively, compared with the 7-day interval.

The TOM-CAST lO-DSV predictor was the only disease predictor that
consistently provided disease control comparable to the 7-day interval when using either
chlorothalonil or azoxystrobin/chlorothalonil. This program required up to ﬁve fewer
applications than the 7-day program, while reducing fungicide costs up to $215/ha. The
use of a 7 -day minimum reapplication interval, that is required with other disease
predictors (26,56), is a further adjustment to the TOM-CAST predictor that may improve
the usefulness of this program for celery growers. For instance, the TOM-CAST lO-DSV
predictor occasionally required applications at 4- to 5-day intervals, although this did not
provide additional disease control beneﬁts compared to the 7-day interval. Further,
chlorothalonil cannot be used more frequently than every seven days. As new fungicides
are developed for celery, these products should be tested in conjunction with disease
predictors to verify their efﬁcacy in management program that require fewer overall
sprays. The environmental conditions in Michigan differ from those in other important
celery-producing areas in the United States, such as California and Florida; ﬁirther

validation in other areas is needed. Implementation of disease predictors that reduce

63

fungicide inputs for late blight management in celery will likely beneﬁt growers,

consumers and the environment.

ACKNOWLEDGEMENTS

This work was funded by the United States Department of Agriculture
Cooperative State Research, Education, and Extension Service, Risk Avoidance and
Mitigation Program, Project Award No. 2002-51101-01908, A strategy to advance IPM
for celery growers in Michigan, California, and Florida; American Farmland Trust and
the Environmental Protection Agency, project Ecoindicators 2003-2004, Advancing IPM
for celery growers in Michigan, California, and Florida; and Project GREEEN (a
cooperative effort by Michigan plant-based commodities and businesses with Michigan
State University Extension, the Michigan Agricultural Experiment Station, and the
Michigan Department of Agriculture) project GR00-076, Using resistance and reduced
risk fungicides to manage F usarium yellows and foliar diseases in celery. Thanks to K.
L. Pernezny (University of Florida, Everglades Research and Education Center, Belle
Glade) for reviewing an earlier version of this manuscript, R. N. Raid (University of
Florida, Everglades Research and Education Center, Belle Glade) and S. T. Koike
(University of California Cooperative Extension, Salinas) for helpful discussion on this

project, and R. H. Podolsky (Medical College of Georgia, Augusta) for statistical advice.

64

10.

11.

12.

13.

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69

CHAPTER II.

Evaluation of a Reduced-Risk Fungicide Program Applied According to Disease
Thresholds for Late Blight Management in Celery

70

ABSTRACT

Late blight of celery, incited by Septoria apiicola, results in necrotic lesions on
leaves and petioles and reduces yield when it occurs on marketable petioles. Michigan
celery growers typically begin applying fungicides one to three weeks after transplanting
and reapply fungicides at 7- to 10-day intervals to manage late blight. Delaying the
initial fungicide application or using disease thresholds to prompt the ﬁrst fungicide spray
may reduce the number of applications needed for effective disease control. Weekly
treatments of azoxystrobin alternated with chlorothalonil were initiated early (one week
after transplanting), preventively (four weeks after transplanting), or when disease
symptoms were detected at a trace, 5%, or 10% level on ‘Dutchess’ celery plants in 2003
and 2004. The early and preventive initiation programs were equally effective in
preventing petiole blight and yield loss. The preventive disease management approach
required three fewer applications, saving up to $134/ha, compared to the early fungicide
program initiated one week after transplanting. Delaying the initial fungicide application
until any level of disease developed subsequently resulted in high disease levels at
harvest that were often similar to untreated control plants. Delaying the initial fungicide
application until initial late blight symptoms are observed in the ﬁeld is not

recommended in Michigan due to the risk of extensive yield loss.

71

INTRODUCTION

Late blight, incited by Septoria apiicola Speg., is an important disease of celery
(Apium graveolens L. var. dulce (Mill.) Pers.) worldwide (28) and is the most serious
foliar disease of celery in Michigan (18). Symptoms of the disease are ﬁrst observed on
older, outer leaves (10) and consist of irregularly shaped, necrotic lesions which
sometimes exhibit a faint chlorotic margin (12). An identifying sign of late blight is the
presence of pycnidia, which are embedded in infected tissues (12) and resemble ﬂecks of
ground black pepper when observed without magniﬁcation. Severely blighted tissues
wither and senesce (10). Diseased tissues must be manually removed which increases
harvesting costs and reduces yield.

Celery growers are advised to use preventive disease control tactics such as
planting disease-free seed (20) and implementing a two-year crop rotation (21) to avoid
late blight epidemics. Most celery seed planted in Michigan is hot-water treated, which
may not completely eradicate S. apiicola (20), and later pelleted for precise seeding into
transplant trays. Pelleted seed may have a shorter shelf life compared to raw seed, so
storing pelleted seed for the recommended two to three years to avoid seed-borne S.
apiicola (10,16) may reduce germination. The majority of Michigan celery growers do
not practice crop rotation, although some use a one-year rotation (17) with onion. Since
genetic resistance to S. apiicola is not currently available in commercial celery cultivars
(l l), foliar fungicides are often relied on as the primary tool for disease control in the
ﬁeld (19).

Most growers use the protectant fungicide chlorothalonil to manage late blight

because it is effective (7,17,25) and relatively inexpensive; however, chlorothalonil is

72

classiﬁed as a BZ carcinogen. Some Michigan growers incorporate azoxystrobin into
their spray program since it is effective against late blight (6,7,8,14,27) and is more
effective than chlorothalonil in some cases (23,26). Azoxystrobin is a newer fungicide
registered for use on celery that has systemic and curative properties (4). It is considered
to pose a low risk to non-target organisms and the environment, and has been registered
as a reduced-risk fungicide by the United States Environmental Protection Agency (4).
Including azoxystrobin in a spray program to manage late blight often displaces some
applications of the B2 carcinogen, chlorothalonil.

Michigan celery growers typically begin applying fungicides one to three weeks
after transplanting and reapply fungicides at 7- to lO-day intervals to manage late blight
(5). Delaying the initial fungicide application until the level of disease reaches a critical
threshold often reduces the total number of fungicide applications needed for effective
disease control and has been successfully used in disease management programs for
carrot (9,13) and onion (29). Similar programs have not been developed for celery.
Mudita and Kushalappa (24) applied chlorothalonil three weeks after celery transplanting
and every seven to ten days thereafter to plots containing 0 to 16% diseased transplants
per plot. Due to the relatively low cost of fungicides in relation to the crop value and the
occurrence of yield loss at low disease incidence thresholds, they were not able to
establish an economical threshold for initiating ﬁmgicide treatment (24). Lacy (17)
observed “fair to good” control of S. apiicola when benomyl, a systemic fungicide no
longer registered for use on celery, was applied at 14-day intervals when the initial
application was applied three days after lesions were detected on the leaves of inoculated

plants. Nearly 10% of the foliage of benomyl-treated plants was blighted and was mostly

73

conﬁned to the leaves, but this treatment was not directly compared with one that was
initiated on preventive basis prior to disease symptom development (17). Since
azoxystrobin is a relatively new fungicide and has a mode of action distinct from that of
chlorothalonil or benomyl, an opportunity exists to test this fungicide in disease
management programs where applications are withheld until low disease thresholds are
reached. Delaying the initial fungicide application may reduce the number of
applications without compromising control.

Consumers are concerned about pesticide residues on their food, and celery has
been targeted as one of the most “contaminated” vegetables (2). Michigan celery
growers are interested in alternative disease management practices for S. apiicola that
reduce fungicide use without compromising disease control and yield (1). The purpose of
this research was to incorporate a reduced-risk fungicide into late blight management
programs and examine methods to reduce ﬁmgicide use in celery for the beneﬁt of
consumers, celery growers, and the environment. The objective of this study was to
evaluate weekly, reduced-risk fungicide schedules initiated prior to disease development

or according to disease thresholds for control of S. apiicola on celery.

MATERIALS AND METHODS
Plot establishment and inoculation. The study was conducted at the Michigan
State University Muck Soils Research Farm in Laingsburg, Michigan in 2003 and 2004.
Seven- to eight-week old ‘Dutchess’ celery transplants were set 17.8 cm apart in rows
spaced 0.8 m apart on 28 May 2003 and 21 June 2004 in Houghton muck soil, previously

planted with potato. Each treatment plot consisted of one row 6.1 m long which included

74

approximately 34 plants. Sections of inoculated buffer row (0.8 to 1.5 m) bordered the
ends of each plot, and two inoculated buffer rows separated adjacent treatment rows. Six
treatments which included one untreated control and ﬁve fungicide programs were
arranged in a randomized complete block design in four blocks. Weed, insects, and
fertilization requirements were managed according to standard production practices
(5,33,34). Sprinkler irrigation was used to maintain the water requirements of the crop
and to promote S. apiicola infection.

Inoculum was prepared and applied similar to the procedures described by Lacy et
al. (19). Dried celery leaves infected with S. apiicola were soaked in tap water for 10
min, and the debris was removed by straining the spore suspension through two layers of
cheesecloth. Spore concentration was determined using a hemacytometer, and the ﬁnal
spore suspension was adjusted to 1 x 106 conidia/ml with water just prior to inoculation.
Plots were sprinkler irrigated to wet the foliage for approximately 60 min prior to
inoculation. Inoculum was applied at dusk with a hand-pump sprayer equipped with one
hollow-cone or ﬂat-fan nozzle delivering approximately 168 liters/ha. All buffer rows
were inoculated (i.e., every plant except those in treatment plots); care was taken to avoid
inoculating plants in treatment rows. This indirect inoculation approach was used to
allow S. apiicola to spread under natural splash-dispersal conditions. Twelve or more
hours of continuous leaf wetness were maintained by short intervals (10 to 15 min) of
sprinkler irrigation in the morning after each inoculation. All buffer rows were
inoculated twice each season at 42 and 49 days after planting (DAP) in 2003 and 39 and

51 DAP in 2004.

75

Timings of initial fungicide applications. The six treatments tested in this study
were (i) an untreated control, (ii) an early fungicide treatment initiated one week after
transplanting (grower standard), (iii) a preventive fungicide treatment initiated four
weeks after transplanting, (iv) a ﬁmgicide treatment initiated when a trace level of late
blight was ﬁrst detected in uninoculated treatment rows, (v) a fungicide treatment
initiated when 5% leaf blight was observed in uninoculated treatment rows, and (vi) a
fungicide treatment initiated when 10% leaf blight was observed in uninoculated
treatment rows. Early treatments were applied on 5 June 2003 (8 DAP) and 28 June 2004
(7 DAP). Preventive treatments were applied on 27 June 2003 (30 DAP) and 19 July
2004 (28 DAP). Treatments made at a trace level of disease were applied on 7 August
2003 (71 DAP) and 30 August 2004 (70 DAP). Although symptoms of late blight were
detected on 5 August 2003, rain occurred on 5 and 6 August and delayed the initial
application by two days. A celery leaf blight assessment key developed by Strandberg
(unpublished) was used to determine disease severity for the 5 and 10% thresholds. The
5% blight initiation treatments were applied on 14 August 2003 (78 DAP) and 13
September 2004 (84 DAP). The 10% blight initiation treatments were applied on 21
August 2003 (85 DAP) and 17 September 2004 (87 DAP). Subsequent fungicide
applications were applied at 7-day intervals for all fungicide treatments until harvest.

The fungicides azoxystrobin (Quadris 2.08F at 0.17 kg a.i./ha, Syngenta Crop
Protection, Inc., Greensboro, NC) and chlorothalonil (Bravo Ultrex 82.5WDG at 1.7 kg
a.i.lha, Syngenta Crop Protection, Inc.) were applied in an alternating program where
azoxystrobin was applied ﬁrst. Applications were made with a C02 backpack boom

sprayer (R & D Sprayers, Opelousas, LA) equipped with three Teejet XR8003VS ﬂat-fan

76

nozzles (Spraying Systems Co., Wheaton, IL) spaced 45.7 cm apart, operating at a boom
pressure of 359 kPa, and delivering 467.6 liters/ha. The cost of fungicides applied for
each treatment was calculated by multiplying the number of applications by estimated
costs of azoxystrobin ($5 3/ha) or chlorothalonil ($28/ha) per application.

Disease assessment and statistical analysis. Weekly visual evaluations of leaf
blight severity were conducted on 20 plants from the middle of each treatment plot using
a celery leaf blight assessment key developed by Strandberg (unpublished). According to
the key, plots were assigned values of 0, 1, 5, 10, 15, 20, 25, 30, 35, 40, 45, or 50% leaf
blight. The celery leaf blight assessment key was similar to the leaf blight assessment
key developed for carrot (31). Plots were evaluated once prior to disease developing in
treatment rows and four times aﬁer disease symptoms were detected.

Ten plants were hand-harvested from each treatment plot and trimmed to fresh
market speciﬁcations at 90 DAP in 2003 and 95 DAP in 2004. Disease incidence was
determined on trimmed plants by counting the number of plants with one or more lesions
on the petioles. Disease severity was evaluated by visually estimating the percentage of
symptomatic petiole tissue on all trimmed plants. Total yield was recorded, diseased
petioles were removed, and plants were weighed again to obtain disease-free yield. The
yield loss percentage was expressed as the weight of diseased petioles removed (total
yield minus disease-free yield) divided by the total yield. The methods of disease
assessment used in this study conform with the methods suggested by Kavanagh and
Ryan (15).

Disease and yield data were analyzed using an analysis of variance (ANOVA) for

a randomized complete block experiment with the Proc GLM procedure of the Statistical

77

Analysis System (SAS Institute, Cary, NC). Normality was examined using the residuals
from each AN OVA and Proc Univariate procedure of SAS. Differences among means

for any signiﬁcant effect were examined using Tukey’s Studentized Range test (30).

RESULTS

Late blight symptoms were observed on the leaves of inoculated plants in the
buffer rows 15 to 16 days after the ﬁrst inoculation (DAFI) each year. S. apiicola spread
to uninoculated plants in treatment rows, and symptoms were detected 27 to 31 DAFI
each year. At harvest, all untreated control plants had lesions on marketable petioles and
when diseased petioles were removed, the yield loss was 50 (2003) and 74% (2004).
None of the untreated control plants were marketable for fresh market use since only two
to four small, healthy petioles remained after diseased petioles were removed.

In 2003, only the early and preventive treatments signiﬁcantly reduced leaf and
petiole blight (_<_1%) compared to the untreated control (Table 1). Total yields recorded
prior to the removal of diseased petioles were signiﬁcantly different among treatments in
2003, where the early and preventive treatments resulted in the highest yields and the
trace disease treatment resulted in the lowest yield (data not shown). The early and
preventive treatments resulted in signiﬁcantly higher disease-free yields compared to the
untreated control and the trace, 5%, or 10% disease treatments (Table 1).

In 2004, both the early and preventive treatments produced healthy leaves and
marketable petioles (Table 1). Applying the initial ﬁmgicide at 5 or 10% disease resulted
in severe leaf and petiole blight similar to the untreated. When fungicides were initiated

at trace disease, leaf and petiole blight were signiﬁcantly reduced compared to the

78

Table 1. Late blight severity and yield of ‘Dutchess’ celery plants not treated or treated

with fungicidesx initiated at different timings for control of Septoria apiicola in 2004 and
2005

 

 

 

Leaf blight severity Petiole blight Disease-free yield
(%) severity (%) (kg)

Treatmenty 2003 2004 2003 2004 2003 2004
Untreated control 50.0 b2 38.8 c 42.5 be 52.5 c 5.5 b 3.2 c
Early 0.8 a 0.0 a 0.0 a 0.0 a 11.9 a 13.1 a
Preventive 0.3 a 0.0 a 0.0 a 0.0 a 12.0 a 13.7 a
Trace threshold 50.0 b 27.5 b 47.5 c 27.5 b 4.8 b 5.8 b
5% blight threshold 50.0 b 38.8 c 36.3 bc 48.8 c 5.4 b 3.8 c
10% blight threshold 50.0 b 35.0 c 32.5 b 46.3 c 5.1 b 4.1 bc
X

 

Azoxystrobin (0.17 kg a.i./ha) alternated with chlorothalonil (1.7 kg a.i./ha).
Treatments were applied early (one week after transplanting), preventively (four
weeks after transplanting), or when disease was detected in treatment rows at a trace,
5%, or 10% level. Subsequent sprays were applied every seven days.

Means within a column followed by the same letter are not signiﬁcantly different
according to Tukey’s Studentized Range test (P 5 0.05).

y

79

untreated but this treatment was not as effective as the early and preventive treatments.
Total yields did not differ signiﬁcantly among treatments (data not shown), but
differences among disease-free yields occurred (Table 1). The early and preventive
treatments resulted in the highest disease-free yields. The yield of plants treated at trace
disease was signiﬁcantly higher compared to the untreated and those treated at 5%
disease but was signiﬁcantly lower compared to the early and preventive treatments
(Table 1).

Three fewer fungicide sprays were made by initiating the ﬁrst application four
weeks after transplanting (preventive) compared with the early treatment initiated one
week after transplanting. The preventive treatment reduced fungicide costs by $109/ha in
2003 and $134/ha in 2004 compared to the early treatment without compromising disease

control (Table 2).

80

Table 2. Number of fungicide applications and cost of fungicides applied when initiated

at different timings for control of Septoria apiicola in 2003 and 2004

No. fungicide applications (fungicide cost [$/ha]y)

 

 

Treatment" 2003 2004

Untreated control 0 (0) 0 (0)
Early 12 (486) 13 (539)
Preventive 9 (377) 10 (405)
Trace 3 (134) 4 (162)
5% blight threshold 2 (81) 2 (81)
10% blight threshold 1 (53) l (53)

 

y

Azoxystrobin (0.17 kg a.i./ha) was alternated with chlorothalonil (1.7 kg a.i./ha). The

cost of fungicide applied was calculated using estimated costs of azoxystrobin

($5 3/ha) and chlorothalonil ($28/ha) per application.
Treatments were applied early (one week after transplanting), preventively (four

weeks after transplanting), or when disease was detected in treatment rows at a trace,
5%, or 10% level. Subsequent sprays were applied every seven days.

81

DISCUSSION

Failure of a disease management program to limit late blight on celery has severe
economic consequences. Since S. apiicola affects the petioles, which are the marketable
portions of the crop, there is a zero tolerance for disease. Consumers demand blemish-
free celery, yet desire reduced pesticide use. An integrated pest management (IPM)
program which involves the use of disease thresholds to initiate fungicide treatment
cannot compromise the value of the marketable product. Disease thresholds are currently
used to initiate fungicide treatment for control of foliar blight of carrot (incited by
Cercospora carotae and Altemaria dauci) (9) and are recommended for Botrytis leaf
blight of onion (incited by Botrytis squamosa) (29). However, the value of celery grown
for processing and fresh market is $20,431/ha in Michigan, which is substantially higher
than the values of other important vegetable crops in Michigan such as processing carrot
($3,424/ha) and onion ($6,445/ha) (3 2). In carrot and onion, fewer sprays are typically
applied since applications are withheld until speciﬁc disease thresholds are reached
(9,29). These programs do not present a direct risk to the marketable portion of the crop
since yields are not compromised (9,29) and disease symptoms do not occur on the
marketable product. In our experiments with celery, delaying the initial spray until just a
trace level of disease was observed allowed S. apiicola to become well established and
subsequently destroyed the marketable portion of the crop.

Several studies have been conducted to examine the efﬁcacy of fungicides applied
after the appearance of late blight symptoms in inoculated ﬁeld trials (3,17,22,25). In
California, Paulus et al. (25) made weekly applications of chlorothalonil or benomyl and

reported equivalent and “satisfactory” control of S. apiicola when the initial spray was

82

applied aﬁer disease symptoms were severe on leaves and petioles of inoculated plants.
Both ﬁmgicide programs resulted in complete crop loss when the application interval was
increased to 14 days (25). In a separate experiment, chlorothalonil or benomyl were
initially applied when disease symptoms were ﬁrst detected on the leaves of inoculated
plants and fungicides provided adequate control when applied at 14-day intervals (25). In
Michigan, Lacy (17) obtained “fair to good” control with benomyl when applied at a 14-
day interval when the ﬁrst application was applied three days after late blight lesions
were detected on the leaves of inoculated plants. In England, Bambridge et al. (3)
reported better control with benomyl than chlorothalonil when biweekly sprays were
initiated after all plants were symptomatic, although the chlorothalonil rate was less than
one-third of what is currently recommended. Under low disease pressure in Canada,
McDonald et al. (22) controlled late blight with applications of chlorothalonil that were
initiated after disease symptoms were present. These studies illustrate the variability in
disease control based on geographic location, fungicides, and probably inocula levels
since they were seldom speciﬁed. The fungicides used in the aforementioned studies act
either on the surface of plant tissues to protect against infection (e. g., chlorothalonil) or
act systemically to cure existing infections (e. g., benomyl). Although benomyl has both
protectant and curative properties, it is no longer registered for use on celery in the
United States. Our study was the ﬁrst to examine the use of the reduced-risk fungicide
azoxystrobin, which was labeled for use on celery in the United States in 2000, in
situations where late blight symptoms were present on celery foliage prior to the ﬁrst
application. Although azoxystrobin has a unique biochemical mode of action (4) that

differs from that of benomyl and chlorothalonil, it did not demonstrate eradicant activity

83

(i.e., disease control aﬁer disease symptoms are present) on S. apiicola under the
conditions in our study. Our study illustrates the potential risk associated with delaying
the initial fungicide application until disease symptoms were observed because S.
apiicola was not controlled with weekly fungicide sprays once it was established in the
celery crop.

Mudita and Kushalappa (24) suggest postponing the initial fungicide treatment
until late blight symptoms are observed in commercial ﬁelds. In our study, waiting to
apply the ﬁrst fungicide treatment until any level of disease symptoms was observed
resulted in complete crop loss during each year. Our results suggest a preventive disease
management approach where fungicides are initially applied prior to disease symptom
appearance. Perhaps our conclusion and that of Mudita and Kushalappa differ because
the overall amount of S. apiicola inoculum in our experiment may have been higher than
the inoculum level in their commercial ﬁelds. However, any damage to the marketable
portion of the crop, resulting from any amount of S. apiicola inoculum, must be manually
removed and thus increases harvesting costs and reduces yields. Since late blight damage
is often unevenly distributed under natural (commercial ﬁeld) conditions, artiﬁcial
inoculation ensures uniform distribution of the pathogen (19) and allows for equal
comparison among treatments. We used an indirect inoculation approach by applying
inoculum to buffer rows only (rather than treatment rows) to allow S. apiicola to spread
under natural, splash-dispersal conditions. The treatments that failed to provide adequate
control were those that did not have fungicide active ingredient in place to provide
protectant activity prior to the splash-dispersal of S. apiicola conidia to unprotected

plants.

84

Disease risk factors such as environmental conditions and the levels of S. apiicola
inoculum on transplants and on celery debris in the soil are important in the
understanding of late blight epidemiology and management. In our study, the transplants
were considered disease-free since no symptoms were detected prior to those resulting
from inoculations. Also, the soil in which our plots were located had not been planted to
celery for at least three years. Delaying the initial fungicide application until four weeks
after transplanting may not result in acceptable disease control when sources of initial
inoculum are not considered. Celery growers should continue their efforts to reduce or
eliminate initial inoculum sources and apply fungicides preventively, as dictated by
fungicide labels, to avoid late blight epidemics. Delaying the initial application until four
weeks after transplanting did not reduce disease control or marketable yield. This
preventive treatment required three fewer sprays, saving $109 to $134/ha, compared to
the early treatment initiated one week after transplanting. Although fungicide costs
account for only a small fraction of celery production inputs, the reduction in fungicide
use is likely to have societal or environmental impacts that were not addressed in this
study. The use of delayed initial fungicide applications could be used in conjunction with
an effective disease predictive system to not only reduce the number of unnecessary
early-season applications but also to reduce the number of fungicide applications needed

for effective control of late blight for the duration of the growing season.

ACKNOWLEDGEMENTS
This work was funded by the United States Department of Agriculture

Cooperative State Research, Education, and Extension Service, Risk Avoidance and

85

Mitigation Program, Project Award No. 2002-51101-01908, A strategy to advance IPM
for celery growers in Michigan, California, and Florida; American Farmland Trust and
the Environmental Protection Agency, project Ecoindicators 2003-2004, Advancing IPM
for celery growers in Michigan, California, and Florida; and Project GREEEN (a
cooperative effort by Michigan plant-based commodities and businesses with Michigan
State University Extension, the Michigan Agricultural Experiment Station, and the
Michigan Department of Agriculture) project GR00-076, Using resistance and reduced
risk fungicides to manage Fusarium yellows and foliar diseases in celery. Thanks to R.
N. Raid and K. L. Pernezny (University of Florida, Everglades Research and Education
Center, Belle Glade) and S. T. Koike (University of California Cooperative Extension,

8 alinas) for helpful discussion on this project.

86

10.

11.

12.

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Anonymous. 2003. Group names most-contaminated produce. The New York
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Bambridge, J. M., Maude, R. B., and Spencer, A. 1985. Tests of fungicides as
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Bartlett, D. W., Clough, J. M., Godwin, J. R., Hall, A. A., Hamer, M., and Parr-
Dobrzanski, B. 2002. The strobilurin fungicides. Pest Manag. Sci. 58:649-662.

Bird, G., Bishop, 3., Graﬁus, E, Hausbeck, M., Jess, L. J ., Kirk, W., and Pett, W.
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Bounds, R. S., and Hausbeck, M. K. 2004. Evaluation of a biopesticide and
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Bounds, R. S., and Hausbeck, M. K. 2005. Evaluation of fungicides for
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Bounds, R. S., and Hausbeck, M. K. 2006. Evaluation of fungicides for
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Bounds, R. S., Podolsky, R. H., and Hausbeck, M. K. 2006. Integrating disease
thresholds with TOM-CAST for carrot foliar blight management. Plant Dis. 90
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Coons, G. H., and Levin, E. 1916. The Septoria leaf spot disease of celery or
celery blight. Mich. Agric. Exp. Stn. Bul. 77.

Edwards, S. J ., Collin, H. A., and Isaac, S. 1997. The response of different celery
genotypes to infection by Septoria apiicola. Plant Pathol. 46:264-270.

Gabrielson, R. L., and Grogan, R. G. 1964. The celery late blight organism
Septoria apiicola. Phytopathology 54: 1251-1257.

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Hausbeck, M. K., Cortright, B. D., and Linderman, S. D. 2002. Control of foliar
blights of celery using standard fungicide programs and a disease forecaster,
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Kavanagh, T., and Ryan, E. W. 1971. Methods of assessment of celery leaf spot
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Krout, W. S. 1921. Treatment of celery seed for the control of Septoria blight. J.
Agric. Res. 21 :396-372.

Lacy, M. L. 1973. Control of Septoria leafspot of celery with systemic and
nonsystemic fungicides. Plant Dis. Rep. 57:425-428.

Lacy, M. L., and Graﬁus, E. J. 1980. Disease and insect pests of celery. Mich.
State Univ. Ext. Bull. E-1427.

Lacy, M. L., Strandberg, J. O., and Paulus, A. O. 1986. Field evaluation of
fungicides for control of celery diseases. Pages 182-184 in: Methods for
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Maude, R. B. 1970. The control of Septoria on celery seed. Ann. Appl. Biol.
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Maude, R. B., and Shuring, C. G. 1970. The persistence of Septoria apiicola on
diseased celery debris in soil. Plant Pathol. 19:177-179.

McDonald, M. R., Janse, S., and Vander Kooi, K. 1999. Evaluation of fungicides
for the control of Septoria late blight on celery, 1998. Fungic. Nematicide Tests
54:134.

McDonald, M. R., Vander Kooi, K., and Hovius, M. 2004. Evaluation of new
chemistry fungicides for the control of late blight on celery, 2003. Fungic.
Nematicide Tests 59:V094.

Mudita, I. W., and Kushalappa, A. C. 1993. Ineffectiveness of the ﬁrst ﬁmgicide
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control of late blight on celery, 2001. Fungic. Nematicide Tests 57:V025.

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of Statistics. A Biometrical Approach. McGraw-Hill, New York.

Strandberg, J. O. 1988. Establishment of Altemaria leaf blight on carrots in
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Univ. Ext. Bull. 433.

89

CHAPTER III.

Coupling Delayed Initial Fungicide Applications and Disease Predictors for Late
Blight, Early Blight, and Crater Rot Management in Michigan Celery Production

90

 

 

ABSTRACT

Michigan celery growers rely on foliar fungicides applied one to three weeks after
transplanting and reapplied at weekly intervals to control Septoria apiicola, Cercospora
apii, and Rhizoctonia solani, the fungi that incite late blight, early blight, and crater rot,
respectively. Delaying the initial fungicide application and using disease predictors to
time subsequent sprays may reduce the number of applications without compromising
control. Field trials were established in 2004 and 2005 at a research farm where S.
apiicola inoculum was applied and at a commercial ﬁeld where early blight and crater rot
developed from naturally-occurring inoculum. A reduced-risk fungicide program
(chlorothalonil alternated with azoxystrobin) was initiated preventively (four to ﬁve
weeks after transplanting) and reapplied weekly or according to the Septoria predictor or
TOM-CAST 10-DSV and was compared with the standard, weekly application program
initiated early (one to two weeks after transplanting). Combining the use of preventive
initial applications with the Septoria predictor or TOM-CAST lO-DSV reduced the
number of sprays by two to six while providing disease control that was comparable to
the standard weekly fungicide program initiated early. These programs reduced
fungicide expenditures by $71 to $213/ha compared to the weekly fungicide program

initiated early.

91

INTRODUCTION

Michigan celery (Apium graveolens L. var. dulce (Mill.) Pers.) production is
affected by three fungal pathogens that damage the marketable portion of plants every
year. Diseased tissues must be manually removed which increases harvesting costs and
reduces yield. Late blight, incited by Septoria apiicola Speg., is the most important foliar
disease of celery in the state (26) and can result in yield losses near 80% (10,18). Early
blight, incited by Cercospora apii Fresen., has become an annual problem for Michigan
celery growers (1) since the early 1990s. Early blight can result in yield losses ranging
from 19 to 58% in Michigan (5,20). Crater rot is incited by Rhizoctonia solani Kuhn and
is reported to lower celery yields by 5 to 7% in Michigan (6,13).

The symptoms of each disease are distinct from one another, and an accurate
diagnosis is usually conducted in the ﬁeld by growers or experienced crop consultants.
Late blight is identiﬁed by the appearance of small, tan to brown colored, irregularly
shaped lesions on leaves and petioles (19,26). Lesions are initially detected on the older,
outer leaves (16) which are sometimes surrounded by a diffuse chlorotic margin (17).
The identifying feature of S. apiicola is the presence of pycnidia which are embedded in
nearly all infected tissues and resemble ﬂecks of ground black pepper without the aid of
magniﬁcation (17). Symptoms of early blight also initially appear on older, outer leaves
and are characterized by light tan to gray colored circular lesions surround by a diffuse
chlorotic halo (26,37). The lesions expand to 21 cm in diameter and turn dry and papery
in texture (37). Lesions on petioles are generally elongate and lack a chlorotic halo.
During periods of high relative humidity, the lesions may appear gray due to sporulation

of the pathogen (26,37). Symptoms of early blight can be distinguished from those

92

incited by S. apiicola due to the absence of pycnidia and the larger size of early blight
lesions (37,42). Initial symptoms of crater rot are small, tan lesions located near the base
of older petioles in contact with the soil (22). Mature lesions are sunken, red to dark
brown, circular or oval, and up to 2.5 cm in diameter (22,46). Crater rot is difﬁcult to
detect in ﬁelds (46), as most of the damage is observed during harvesting and packing
operations.

Michigan celery growers typically begin applying the fungicide chlorothalonil
one to three weeks after transplanting and reapply at weekly intervals to control all three
diseases. Chlorothalonil is a broad-spectrum, protectant fungicide with activity against S.
apiicola (7,9,10,18,36), C. apii (3,5), and R. solani (6,32), and is classiﬁed as a B2
carcinogen. Strategies to reduce the number of chlorothalonil applications have been
implemented by producers of carrots for processing in Michigan (30). These strategies
include the use of (i) a disease threshold to prompt the ﬁrst fungicide spray, (ii) the
TOM-CAST 15-DSV disease predictor to schedule subsequent sprays according to
disease-favorable environmental conditions, and (iii) the reduced-risk ﬁmgicide
azoxystrobin to displace some of the chlorothalonil applications (14). Azoxystrobin is a
relatively new fungicide labeled for use on celery and is effective against S. apiicola
(7,21,35), C. apii (20,29,38), and R. solani (6,13). Azoxystrobin has systemic and
curative properties and must be alternated with fungicides that have a different mode of
action to prevent or delay the occurrence of fungicide-resistant strains of plant pathogens
(2). The use of chlorothalonil in alternation with azoxystrobin is typically recommended

since both products effectively limit late blight and they have different modes of action.

93

Including azoxystrobin in a spray program to manage celery diseases may displace some
of the applications of the B2 carcinogen, chlorothalonil.

By implementing some of the same disease control strategies used in carrot,
Michigan celery growers may be able to reduce their dependence on chlorothalonil and
apply fewer sprays to achieve the same level of disease control. Attempts to delay the
initial fungicide application until late blight symptoms are detected on celery results in
substantial yield loss (8). However, the initial fungicide spray can be applied on a
preventive basis by delaying the ﬁrst application until four and ﬁve weeks after
transplanting without compromising control of crater rot (l 3) and late blight (12),
respectively. The preventive fungicide program reduces the number of sprays by three
while decreasing fungicide costs by approximately $100/ha compared to an early-
initiation program started one to two weeks after transplanting (12,13). Disease
predictors, such as the TOM-CAST lO-DSV program and the Septoria predictor, require
up to four fewer sprays than the 7-day application program while providing comparable
control of late blight (11). Disease predictors have not been tested in commercial celery
ﬁelds in Michigan, nor have studies documented the efficacy of predictors in controlling
diseases other than late blight, such as early blight and crater rot, that often occur in
commercial celery ﬁelds.

Michigan celery growers are interested in reducing their reliance on early and
repeated fungicide applications and desire an alternative disease management strategy in
order to lower production costs while maximizing quality and yield (1). When
implemented separately, delayed initial applications and disease predictors reduce the

total number of fungicide applications without compromising disease control. Coupling

94

the use of delayed initial applications with a disease predictor may further reduce the
number of fungicide applications needed to manage celery diseases. The objective of this
study was to compare a standard fungicide application program with alternative disease

management programs which utilized delayed initial applications and disease predictors.

MATERIALS AND METHODS

Plot establishment. Plots were established at the Michigan State University
Muck Soils Research Farm in Laingsburg, Michigan and at a commercial celery ﬁeld in
Hudsonville, Michigan in 2004 and 2005. Seven-week old ‘Dutchess’ celery transplants
were set 17.8 cm apart in rows spaced 0.8 m apart on 21 June 2004 and 25 May 2005 at
the Muck Soils Research Farm (hereafter referred to as the research farm) in Houghton
muck soil, previously planted with potato (2004) or carrot (2005). The study located in
Hudsonville (hereafter referred to as the commercial ﬁeld) was established with eight-
week old ‘Dutchess’ celery transplants set 19.1 cm apart in rows spaced 0.5 m apart on
24 June 2004 and 16 June 2005 in Houghton muck soil, previously planted with onion.
Each treatment plot consisted of a 6.1 m-long row which included approximately 34 and
32 plants at the research farm and commercial ﬁeld, respectively. Sections of unsprayed
buffer row (0.8 to 1.5 m) bordered the ends of each plot, and two unsprayed buffer rows
separated adjacent treatment rows. Five treatments which included one untreated control
and four fungicide programs were arranged in a randomized complete block design in
four blocks. Weed, insects, and fertilization requirements were managed according to
standard production practices (4,45,47). Sprinkler irrigation was used to maintain the

water requirements of the crop.

95

 

At the research farm, S. apiicola inoculum was prepared and applied similar to the
procedures described by Lacy et al. (27). Dried celery leaves infected with S. apiicola
were soaked in tap water for 10 min, and the debris was removed by straining the spore
suspension through two layers of cheesecloth. Spore concentration was determined using
a hemacytometer, and the ﬁnal spore suspension was adjusted to l x 106 conidia/ml with
water just prior to inoculation. Plots were sprinkler irrigated to wet the foliage for
approximately 60 min prior to inoculation. Inoculum was applied at dusk with a hand-
pump sprayer equipped with one hollow-cone or ﬂat-fan nozzle delivering approximately
168 liters/ha. All buffer rows were inoculated (i.e., every plant except those in treatment
plots); care was taken to avoid inoculating plants in treatment rows. This indirect
inoculation approach was used to allow S. apiicola to spread under natural splash-
dispersal conditions. Twelve or more hours of continuous leaf wetness were maintained
by short intervals (10 to 15 nrin) of sprinkler irrigation in the morning after each
inoculation. All buffer rows were inoculated twice each season at 39 and 51 days after
planting (DAP) in 2004 and at 42 and 49 DAP in 2005. Disease(s) developed from
naturally-occurring inoculum at the commercial ﬁeld.

Fungicide initiation and reapplication programs. The fungicide chlorothalonil
(Bravo Ultrex 82.5WDG at 1.7 kg a.i.lha, Syngenta Crop Protection, Inc., Greensboro,
NC) was applied in an alternating program with the reduced-risk fungicide azoxystrobin
(Amistar 80WG at 0.17 kg a.i.lha, Syngenta Crop Protection, Inc.) where chlorothalonil
was applied ﬁrst. Applications were made with a C02 backpack boom sprayer (R & D
Sprayers, Opelousas, LA). The spray boom used at the research farm was equipped with

three Teejet XR8003VS ﬂat-fan nozzles (Spraying Systems Co., Wheaton, IL) spaced

96

45.7 cm apart, operating at a boom pressure of 359 kPa, and delivering 467.6 liters/ha.
The spray boom used at the commercial ﬁeld was smaller in relation to the one used at
the research farm since the row spacing was less at the commercial ﬁeld. It was equipped
with two Teejet XR8003VS ﬂat-fan nozzles spaced 49.5 cm apart, operating at a boom
pressure of 366 kPa, and delivering 467.6 liters/ha. The cost of fungicides applied for
each treatment was calculated by multiplying the number of applications by estimated
costs of chlorothalonil ($28/ha) and azoxystrobin ($43/ha) per application.

Initial fungicide treatments were either applied early according to the typical
management practice at one (commercial ﬁeld) to two (research farm) weeks after
transplanting or applied preventively at four (commercial ﬁeld) to ﬁve (research farm)
weeks after transplanting. Early initiation sprays were reapplied at 7-day intervals.
Preventive initiation sprays were reapplied every seven days or as required by the
Septoria predictor or TOM-CAST using an interval of 10 DSVs.

The Septoria predictor requires a fungicide application when 212 h of consecutive
leaf wetness occurs if no fungicides were applied during the previous seven days (25).
For each 24-h period (1100 to 1100), TOM-CAST uses the hours of leaf wetness and the
average temperature during the wetness periods to calculate a DSV, ranging from 0 to 4,
corresponding to environmental conditions unfavorable to highly favorable for disease
development, respectively (34). Daily DSVs are summed and accumulated until a
threshold value is reached, a fungicide spray is applied, and the DSV total is reset to zero.
Unlike the Septoria predictor, the TOM-CAST program was not tested with a minimum

fungicide reapplication interval since none was speciﬁed when the forecaster was

97

developed for tomato (33,34) or when the forecaster was validated for use in asparagus
(28) or carrot (15).

Hourly measurements of temperature and leaf wetness were obtained using digital
data recorders (WatchDog Leaf Wetness and Temperature Logger 3610TWD; Spectrum
Technologies, Inc., Plainﬁeld, IL) located in the upper 75% of the crop canopy in an
unsprayed row at a 45° angle facing north. Data recorders were placed in the plots after
transplanting and were set to record temperatures from 0 to 100°C and leaf wetness (i.e.,
wetness threshold set to 0). Data were downloaded at least every three days to a lapt0p
computer using a computer program (Specware 6.02; Spectrum Technologies, Inc.)
equipped to calculate DSVS for the TOM-CAST system. Raw hourly data were
examined to determine the number of hours of consecutive leaf wetness for the Septoria
predictor. A tipping-bucket rain gauge (Model 3665R; Spectrum Technologies, Inc.)
collected rainfall and sprinkler irrigation and was located 1.2 m above the soil surface at
the research farm only.

Disease assessment and statistical analysis. Visual evaluations of leaf blight
severity were conducted on 20 plants from the middle of each treatment plot using a
celery leaf blight assessment key developed by Strandberg (unpublished). According to
the key, plots were assigned values of 0, 1, 5, 10, 15, 20, 25, 30, 35, 40, 45, or 50% leaf
blight. The celery leaf blight assessment key was similar to the leaf blight assessment
key developed for carrot (44). Plots at the research farm were evaluated once prior to
disease developing in treatment rows and three (2005) or four (2004) times at weekly
intervals after disease symptoms were detected. Plots at the commercial ﬁeld were

evaluated once on the day of harvest.

98

Ten plants were hand-harvested from each treatment plot and trimmed to fresh
market speciﬁcations at 92 and 85 DAP at the research farm and at 85 and 76 DAP at the
commercial ﬁeld in 2004 and 2005, respectively. At the research farm, late blight
severity on petioles was evaluated by visually estimating the percentage of symptomatic
petiole tissue on all trimmed plants. At the commercial ﬁeld, crater rot incidence was
determined by counting the number of trimmed plants with one or more lesions. Total
yield was recorded, diseased petioles were removed, and plants were weighed again to
obtain disease-free yield. The yield loss percentage was expressed as the weight of
diseased petioles removed (total yield minus disease-free yield) divided by the total yield.
The methods of disease assessment used in this study conform with the methods
suggested by Kavanagh and Ryan (23).

Disease and yield data were analyzed using an analysis of variance (ANOVA) for
a randomized complete block experiment with the Proc GLM procedure of the Statistical
Analysis System (SAS Institute, Cary, NC). Normality was examined using the residuals
from each ANOVA and Proc Univariate procedure of SAS. Differences among means

for any signiﬁcant effect were examined using Tukey’s Studentized Range test (43).

RESULTS
Environmental conditions were considered favorable for disease development
during each year of the study. The mean temperature recorded in the crop canopy at the
research farm was 19.1 and 222°C in 2004 and 2005, respectively; similar mean

temperatures were recorded at the commercial ﬁeld. Combined rainfall and irrigation at

the research farm totaled 16.3 (2004) and 16.5 cm (2005) from the day after the ﬁrst

99

inoculation until harvest. At the commercial ﬁeld, rainfall was recorded approximately
0.5 km from the experimental plots at a MAWN (Michigan Automated Weather
Network) weather station and totaled 16.4 (2004) and 10.8 cm (2005).

Late blight symptoms were observed on the leaves on inoculated plants at the
research farm 16 and 12 days after the buffer rows were ﬁrst inoculated (DAFI) in 2004
and 2005, respectively. S. apiicola spread to uninoculated treatment rows, and symptoms
were detected 31 (2004) and 26 (2005) DAF I on untreated plants. At harvest, all
untreated plants trimmed to fresh market speciﬁcations had lesions on marketable
petioles, and yield loss was >63% when diseased petioles were removed (Table 1). None
of the untreated plants were marketable for fresh market use since only two to four small,
healthy petioles remained after diseased petioles were removed. All fungicide treatments
provided similar control and limited leaf blight severity to < 1% each year (Table 1). All
ﬁmgicide treatments prevented petiole blight, with one exception. In 2005, the
preventive TOM-CAST 10-DSV management program resulted in a nominal amount of
petiole blight and yield loss that did not differ signiﬁcantly from the management
programs that prevented petiole disease (Table 1).

At the commercial ﬁeld, early blight symptoms were detected on the leaves of
untreated plants 55 and 69 DAP in 2004 and 2005, respectively. At harvest, leaf blight
severity was 51% on untreated plants, and the disease did not occur on petioles during
either year (Table 2). All fungicide treatments signiﬁcantly reduced leaf blight severity
compared to the untreated control. Symptoms of crater rot were observed on 20% of
untreated plants trimmed to fresh market speciﬁcations during the harvest in 2004, and

symptoms were not found on fungicide-treated plants. Yield loss due to crater rot was

100

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101

>5% when diseased petioles were removed (Table 2).

Fungicides were applied as frequently as every seven days for the preventive
Septoria predictor, which was the minimum interval dictated by this program, with one
exception (Table 3). A single application was made six days after the previous
application at the commercial ﬁeld in 2005. A maximum of 10 days occurred between
applications during periods when leaf wetness durations were <12 hours. The preventive
TOM-CAST lO-DSV management program required fungicides as frequently as every
four days or as infrequently as every 17 days depending on the weather. When averaged
across both years and locations, the preventive Septoria predictor and TOM-CAST 10-
DSV programs called for a fungicide every 7.7 to 8.2 days (Table 3).

Applying fungicides according to the standard management practice at one to two weeks
after transplanting and at weekly intervals thereafter was the most costly management
program because it required the most fungicide applications (Table 4). The preventive 7-
day program did not compromise disease control and required three fewer applications,
reducing fungicide costs by $99 to $1 l4/ha, since it was initiated later in the season. The
preventive Septoria predictor program often required the same number of applications
compared to the preventive 7-day program, with one exception. In 2004, this
management program required two fewer sprays than the preventive 7-day program at the
commercial ﬁeld. The preventive TOM-CAST lO-DSV program required two to six
fewer applications, reducing fungicide costs by $71 to $213/ha, compared with the
standard weekly fungicide program. When compared to the preventive 7-day program,
the TOM-CAST lO-DSV program required three fewer sprays and one more spray in

2004 and 2005, respectively (Table 4).

102

Table 3. Frequency of fungicide applications required by different management

programs for control of Septoria apiicola at the research farm and commercial ﬁeld in
2004 and 20052

 

 

 

Ave. no. days between app. Two-year

Location (min.,max. no. dajs between app.) ave. no. days

Management program 2004 2005 between app.
Research farm

Early 7-day 7.0 (6,8) 7.1 (7,8) 7.]

Preventive 7-day 7.0 (6,8) 7.2 (7,8) 7.1

Preventive Septoria predictor 7.6 (7,9) 7.8 (7,10) 7.7

Preventive TOM-CAST lO-DSV 11.5 (7,17) 6.3 (4,10) 8.2
Commercial ﬁeld

Early 7-day 6.9 (6,7) 7.1 (6,8) 7.0

Preventive 7-day 6.9 (6,7) 7.2 (6,8) 7.0

Preventive Septoria predictor“ 8.2 (7,10) 7.2 (6,8) 7.7

Preventive TOM-CAST lO-DSV 9.8 (7,12) 6.1 (4,8) 7.7

 

Forecasted or actual rain events either shortened or extended the days between
applications for the 7-day interval at both locations each year. The interval was also

shortened for the preventive Septoria predictor on one occasion at the commercial
ﬁeld in 2005.

Table 4. Number of fungicide applications and cost of fungicides applied according to
different management programs for control of Septoria apiicola at the research farm and
at the commercial ﬁeld in 2004 and 2005

No. fungicide applications (fungicide cost [$/ha]z)

 

 

 

Research farm Commercial ﬁeld
Management program 2004 2005 2004 2005
Early 7-day 11 (383) 10 (355) 12 (426) 10 (355)
Preventive 7-day 8 (284) 7 (241) 9 (312) 7 (241)
Preventive Septoria predictor 8 (284) 7 (241) 7 (241) 7 (241)
Preventive TOM-CAST lO-DSV 5 (170) 8 (284) 6 (213) 8 (284L

 

Z

Chlorothalonil (1.7 kg a.i./ha) was alternated with azoxystrobin (0.17 kg a.i.lha). The
cost of fungicide applied was calculated using estimated costs of chlorothalonil
($28/ha) and azoxystrobin ($43/ha) per application.

103

DISCUSSION

Combining the use of delayed initial applications with the Septoria predictor or
TOM-CAST lO-DSV reduced the number of sprays by two to six while providing disease
control that was comparable to the standard weekly fungicide program initiated early.
These programs reduced fungicide expenditures by $71 to $213/ha compared to the
weekly fungicide program initiated one to two weeks aﬁer transplanting. Our results
were consistent under high disease pressure where S. apiicola inoculum was applied at
the research farm and under conditions of naturally-occurring inoculum at the
commercial ﬁeld.

High levels of disease likely failed to develop at the commercial ﬁeld due to low
levels of inoculum. Some Michigan celery growers practice a one-year crop rotation (24)
and plant celery seed which is hot-water treated to reduce initial inoculum, but foliar
fungicides are often the primary means of disease control (27). The celery planted in the
same ﬁeld and surrounding ﬁelds were routinely sprayed with effective fungicides. We
relied on natural inoculum to infect plants since it was desirable to evaluate the efﬁcacy
of the management programs in a commercial ﬁeld situation. Differences among
management programs may have become evident if the plots were artiﬁcially inoculated
or if higher levels of inoculum were present in the growing area. Our results from the
commercial ﬁeld indicate that there is probably a lower risk for disease epidemics than
growers realize. The use of cultural control measures, including the use of hot-water
treated seed and limited crop rotation could account for the low level of inoculum and
subsequent low disease risk. It is possible that Michigan celery growers underestimate

the effectiveness of their efforts to reduce initial inoculum.

104

The preventive Septoria predictor required the same number of applications as the
preventive 7-day interval during each year at the research farm and in 2005 at the
commercial ﬁeld. Leaf wetness durations of Z 12 hours frequently occur in celery crop
canopies during the mid to latter portions of the celery growing season in Michigan.
Thus, it was not surprising that the Septoria predictor prompted applications at regular 7-
to 10-day intervals and often required the same number of sprays at the preventive 7-day
program. Lacy (25) apparently applied the initial spray according to the Septoria
predictor the ﬁrst time that 212 hours of leaf wetness occurred after transplanting. This
program resulted in a reduction of two sprays per season compared to a weekly spray
schedule over a three year period (25), however, it was not clearly stated when initial
sprays were applied for either management program. Our results, with regard to the
frequent similarity in number of sprays applied according to the preventive 7-day spray
schedule and the Septoria predictor, probably differ from those of Lacy (25) since we
initiated both spray programs at the same time and Lacy (25) apparently did not.

Warmer temperatures in 2005 increased DSV accumulation, thereby prompting
sprays as frequently as every four days according to TOM-CAST lO-DSV. This
management program required one more spray at both locations than the weekly
management program initiated at the same time, without any signiﬁcant disease control
beneﬁt. Celery growers that exclusively apply chlorothalonil would not be permitted to
follow this application program since the fungicide label dictates a minimum
reapplication interval of seven days. A further adjustment to the TOM-CAST system
should include a 7-day minimum reapplication interval, similar to the requirement for the

Septoria predictor. This would allow growers to spray as frequently as every seven days

105

when the DSV total exceeds 10 and spray according to the TOM-CAST lO-DSV
threshold when environmental conditions are less favorable for disease development.
The use of a higher DSV threshold, such as TOM-CAST lS-DSV, may be appropriate
but was not tested in conjunction with preventive initial applications or evaluated in
commercial ﬁelds.

Raid and Pernezny (39,40) and Raid et al. (41) tested the TOM-CAST 15-DSV
threshold for control of C. apii in Florida. This program only required two to four
fungicide sprays each season compared with the weekly application schedule which
required 11 to 13 applications. Sprays applied according to the TOM-CAST 15-DSV
program resulted in signiﬁcantly less disease control each season and often resulted in
lower marketable yields compared to the calendar-based fungicide schedule (3 9,40,41).
In Michigan, the TOM-CAST 15-DSV program required eight sprays and provided
comparable control to the weekly application schedule (12 sprays) without compromising
control of S. apiicola (11). Although the target pathogens differed between locations, the
difference in efﬁcacy of TOM-CAST 15-DSV between Florida and Michigan is likely
due to the number of applications the predictor required.

With regard to crater rot, our results from the commercial ﬁeld in 2004 agree with
those ofPieczarka (31,32) who delayed initial fungicide applications until six weeks aﬁer
transplanting and obtained equivalent control compared with applications initiated one
week after transplanting. Some Michigan celery growers are especially concerned about
crater rot when early-season sprays are not applied. However, most growers are reluctant
to use azoxystrobin, a fungicide that is more expensive than chlorothalonil, early in the

season. We applied chlorothalonil ﬁrst in our study since celery growers prefer to apply

106

em .. -.

this fungicide as the ﬁrst spray. Our results and those ofPieczarka (31,32) identify
effective crater rot management strategies that lack early-season applications and reduce
fungicide expenditures.

Our results indicate that Michigan celery growers who applied 10 to 12
applications in 2004 and 2005 probably applied more fungicide than was needed to
manage disease. We managed S. apiicola under high disease pressure at the research
farm with up to 55% fewer applications and managed C. apii and R. solani with up to
50% fewer applications under natural conditions at the commercial ﬁeld. Future research
could evaluate the use of a less conservative disease predictor, such as TOM-CAST 15-

DSV, in an effort to further reduce unnecessary fungicide Sprays.

ACKNOWLEDGEMENTS

This work was funded by the United States Department of Agriculture
Cooperative State Research, Education, and Extension Service, Risk Avoidance and
Mitigation Program, Project Award No. 2002-51101-01908, A strategy to advance IPM
for celery growers in Michigan, California, and Florida; American Farmland Trust and
the Environmental Protection Agency, project Ecoindicators 2003-2004, Advancing IPM
for celery growers in Michigan, California, and Florida; and Project GREEEN (a
cooperative effort by Michigan plant-based commodities and businesses with Michigan
State University Extension, the Michigan Agricultural Experiment Station, and the
Michigan Department of Agriculture) project GR00-076, Using resistance and reduced
risk ﬁmgicides to manage F usarium yellows and foliar diseases in celery. Thanks to R.

N. Raid and K. L. Pernezny (University of Florida, Everglades Research and Education

107

Center, Belle Glade) and S. T. Koike (University of California Cooperative Extension,

Salinas) for helpful discussion on this project.

108

10.

11.

12.

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112

 

APPENDIX A.

Summary of Disease Predictor Data and Fungicide Application Dates in 2003-2005

113

 

 

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APPENDIX B.

Summary of Field Experiments Conducted to Evaluate Fungicide Efﬁcacy, Disease
Predictors, and/or Delayed Initial Applications for Managing Late Blight, Early
Blight, Crater Rot, and/or Bacterial Leaf Spot in Michigan Celery Production in

2003-2005

122

CELERY (Apium graveolens ‘Dutchess’) R.S. Bounds and MK. Hausbeck
Late blight; Septoria apiicola Michigan State University
Department of Plant Pathology
East Lansing, MI 48824

Evaluation of fungicides and biopesticides for managing late blight of celery, 2003.

This study was conducted at the Michigan State University Muck Soils Research
Farm in Laingsburg, M1 on a Houghton muck ﬁeld previously planted to potato. Seven-
week old celery ‘Dutchess’ transplants were planted 7 in. apart in rows spaced 32 in.
apart on 29 May. Treatment plots consisted of one row 22.5 ft long with 5 ft of
unsprayed buffer between plots in the same row. Two buffer rows were left unsprayed
between each treatment row. Ten treatments were replicated four times in a randomized
complete block design. The ﬁeld was fertilized with applications of 8-21-29 (400 lb/A
on 22 May) plus micronutrients (0.5% Cu, 1% Mn, and 0.5% Zn), 0-0-62 (400 lb/A on 22
May), 46-0-0 (100 lb/A on 23 Jun), Techmangan (4 lb/A on 2 Jul), and calcium nitrate
(10 lb/A on 14 Jul). Insects were controlled with applications of Sevin XLR Plus (0.5
qt/A on 2 Jul), Sevin 80S (1.5 lb/A on 14 Jul), Vydate C-LV (4 pt/A on 19 Jul), and
Lannate LV (3 pt/A on 9 Aug). Weeds were controlled with applications of Lorox SODF
(2 lb/A on 16 Jun) and Caparol 4L (1 qt/A on 8 Jul); supplemental hand weeding was
performed as needed. Septoria inoculum (2.9 x 10 spores/ﬂ oz) was prepared by soaking
dried infected celery leaves for 10 min in water and straining through two layers of
cheesecloth. Unsprayed buffer rows were inoculated 30 Jul, and all rows were inoculated
on 8 Aug. Inoculum was applied with a hand-pump backpack sprayer using one hollow
cone nozzle that delivered 12 gal/A. Fungicides were applied with a C02 backpack
sprayer equipped with three XRSOO3 ﬂat fan nozzles spaced 18 in. apart and calibrated to
deliver 50 gal/A at a nozzle pressure of 52 psi. Ten applications were made at weekly
intervals on 3, 12, 18, 25 and 30 Jul; 8, 15, 22, and 29 Aug; and 5 Sep. Leaf blight
severity was evaluated and ten plants from the middle of each treatment row were hand-
harvested and trimmed to fresh market speciﬁcations (14 in. length) on 11 Sep. Petiole
disease incidence and severity were assessed, diseased petioles were removed from the
plants, and yields were recorded. Average minimum and maximum air temperatures (F)
were 41.2 and 64.7, 48.2 and 76.4, 54.6 and 81.4, 53.6 and 82.0, and 44.1 and 72.3 for
May, Jun, Jul, Aug, and Sep, respectively. Rainfall totals (in.) were 3.8, 1.6, 1.0, 1.4, and
2.1 for the same respective months.

Disease symptoms appeared in inoculated buffer rows on 15 Aug. At harvest,
petioles of untreated plants were severely diseased which resulted in extensive trimming
and low yield. Applications of Quadris 2.08F alternated with Tilt 3.6EC, Bravo Ultrex
82.5WDG applied alone or applied in alternation with Pristine 38WG or Quadris 2.08F
were highly effective in controlling disease and resulted in no disease symptoms on
leaves or petioles. Plots treated with Endura 7OWG or Bravo Ultrex 82.5WDG alternated
with Cabrio 20WG, Serenade lOWP, or Messenger 3WDG resulted in limited amounts of
leaf blight (0.3 — 13.0%) and were not signiﬁcantly different from treatments where
disease was absent. Plants treated with 710-145f 0.14% w/v were severely diseased and

123

 

similar to the untreated plants for all parameters measured. No phytotoxicity occurred on
any of the plants.

 

 

 

PetiO‘C blight Leaf blight Trimmed yield
Treatment and rate/A (application sequence’) Incidence (%)y Severity" (%) (lb)
Untreated ......................................................... 100.0 b“ 5.8 b 47.5 b 13.2 b
Cabrio 20WG 1 lb (1,3,5,7,9)
Bravo Ultrex 82.5WDG 1.8 lb (2,4,6,8,10).... 0.0 a 1.0 a 0.3 a 30.8 a
Pristine 38WG 0.66 lb (1,3,5,7,9)
Bravo Ultrex 82.5WDG 1.8 lb (2,4,6,8,10).... 0.0 a 1.0 a 0.0 a 38.4 a
Quadris 2.08F 14.9 ﬂ oz (1,3,5,7,9)
Bravo Ultrex 82.5WDG 1.8 lb (2,4,6,8,10).... 0.0 a 1.0 a 0.0 a 32.3 a
Quadris 2.08F 14.9 ﬂ oz (1,3,5,7,9)
Tilt 3.6EC 4 ﬂ oz (2,4,6,8,10) ......................... 0.0 a 1.0 a 0.0 a 35.6 a
Endura 70WG 0.7 lb (1-10) ............................ 0.0 a 1.0 a 2.8 a 33.8 a
Bravo Ultrex 82.5WDG 1.8 lb (1-10) ............. 0.0 a 1.0 a 0.0 a 33.1 a
Serenade lOWP 6 lb (1,3,5,7,9)
Bravo Ultrex 82.5WDG 1.8 lb (2,4,6,8,10).... 0.0 a 1.0 a 1.8 a 33.5 a
Messenger 3WDG 0.6 lb (1,3,5,7,9)
Bravo Ultrex 82.5WDG 1.8 lb (2,4,6,8,10) ..... 15.0 a 2.0 a 13.0 a 31.4 a
710-145f0.14% w/v 5.7 pt (1-10) ................... 100.0 b 6.3 b 48.8 b 9.3 b

 

2 Application sequence: 1 = 3 Jul; 2 = 12 Jul; 3 = 18 Jul; 4 = 25 Jul; 5 = 30 Jul; 6 = 8
Aug; 7 =15 Aug; 8 = 22 Aug; 9 = 29 Aug; 10 = 5 Sep.

y Percentage of trimmed plants with one or more petiole lesions. Variable could not be

normalized.

Severity of petiole blight evaluated using a 1-10 scale where 1 = no disease to 10 = all

petioles severely blighted. Variable could not be normalized.

Evaluated using a leaf blight assessment key representing the percentage of diseased

leaf area. Variable could not be normalized.

Ten plants from the center of each plot were hand-harvested, trimmed to 14 in.

length, and stripped of diseased petioles prior to weighing.

Means within a column followed by the same letter are not signiﬁcantly different

according to Tukey’s Studentized Range Test (P=0.05).

124

 

I
a-..

 

CELERY (Apium graveolens ‘Dutchess’) R.S. Bounds and MK. Hausbeck
Late blight; Septoria apiicola Michigan State University
Department of Plant Pathology
East Lansing, MI 48824

Evaluation of a biopesticide and fungicides for managing late blight of celery, 2003.

This study was conducted at the Michigan State University Muck Soils Research
Farm in Laingsburg, M1 on a Houghton muck ﬁeld previously planted to potato. Seven-
week old celery ‘Dutchess’ transplants were planted 7 in. apart in rows spaced 32 in.
apart on 29 May. Treatment plots consisted of one row 22.5 ft long with 5 ft of
unsprayed buffer between plots in the same row. Two buffer rows were left unsprayed
between each treatment row. Nine treatments were replicated four times in a randomized
complete block design. The ﬁeld was fertilized with applications of 8-21-29 (400 lb/A
on 22 May) plus micronutrients (0.5% Cu, 1% Mn, and 0.5% Zn), 0-0-62 (400 lb/A on 22
May), 46-0-0 (100 lb/A on 23 Jun), Techmangan (4 lb/A on 2 Jul), and calcium nitrate
(10 lb/A on 14 Jul). Insects were controlled with applications of Sevin XLR Plus (0.5
qt/A on 2 Jul), Sevin 80S (1.5 lb/A on 14 Jul), Vydate C-LV (4 pt/A on 19 Jul), and
Lannate LV (3 pt/A on 9 Aug). Weeds were controlled with applications of Lorox 50DF
(2 lb/A on 16 Jun) and Caparol 4L (1 qt/A on 8 Jul); supplemental hand weeding was
performed as needed. Septoria inoculum (2.9 x 10 spores/ﬂ 02) was prepared by soaking
dried infected celery leaves for 10 min in water and straining through two layers of
cheesecloth. Unsprayed buffer rows were inoculated 30 Jul, and all rows except the
untreated uninoculated plots were inoculated on 8 Aug. Inoculum was applied with a
hand-pump backpack sprayer using one hollow cone nozzle that delivered 12 gal/A.
Fungicides were applied with a C02 backpack sprayer equipped with three XR8003 ﬂat
fan nozzles spaced 18 in. apart and calibrated to deliver 50 gal/A at a nozzle pressure of
52 psi. Seven applications were made on 18 and 30 Jul; 8, 15, 22, and 29 Aug; and 5
Sep. Leaf blight severity was evaluated and ten plants from the middle of each treatment
row were hand-harvested and trimmed to fresh market speciﬁcations (14 in. length) on 11
Sep. Petiole disease incidence and severity were assessed, diseased petioles were
removed from the plants, and yields were recorded. Average minimum and maximum air
temperatures (F) were 41.2 and 64.7, 48.2 and 76.4, 54.6 and 81.4, 53.6 and 82.0, and
44.1 and 72.3 for May, Jun, Jul, Aug, and Sep, respectively. Rainfall totals (in.) were
3.8, 1.6, 1.0, 1.4, and 2.1 for the same respective months.

Disease symptoms appeared in inoculated buffer rows on 15 Aug. Leaf blight
severity of the uninoculated untreated plants at harvest was 35%, indicating substantial
disease spread. The incidence of petiole blight in both the untreated and 710-145f 0.14%
w/v only treatments was 100%. All treatments with Bravo Ultrex 82.5WDG or Quadris
2.08F provided excellent disease control and signiﬁcantly reduced disease on petioles and
leaves and increased marketable yield when compared with the untreated or 710-145f
0.14% w/v only treatments. No phytotoxicity occurred on any of the plants.

125

 

 

 

 

Petiole blight Leaf blight Trimmed yield

 

 

Treatment and rate/A (application sequence’) Incidence (%)Y Severity" (%)w (lb)"
Untreated uninoculated .................................... 100.0 b"l 4.8 b 35.0 b 20.7 b
Untreated inoculated ........................................ 100.0 b 7.3 c 41.3 c 16.3 b
Bravo Ultrex 82.5WDG 1.8 lb (1-7) ................ 0.0a 1.0a 0.0a 41.6a
Quadris 2.08F 14.9 ﬂ oz (1-7) ......................... 0.0a 1.0a 0.0a 37.3 a
710-145f0.14% w/v 2.8 pt (1-7) ..................... 100.0 b 7.5 c 43.8 c 12.3 b
7]0-145f0.14% w/v 5.7 pt (1-7) ..................... 100.0 b 7.5 c 43.8 c 14.3 b
Bravo Ultrex 82.5WDG 1.8 lb (l,3,5,7)

710-145f0. 14% w/v 5.7 pt (2,4,6) ................... 0.0a 1.0 a 3.0 a 36.8 a
Quadris 2.08F 14.9 ﬂ oz (1 ,3,5,7)

710-145f0. 14% w/v 5.7 pt (2,4,6) ................... 0.0 a 1.0 a 3.0 a 35.4 a

Bravo Ultrex 82.5WDG 1.8 lb (1,4,7)
710-145f 0. 14% w/v 5.7 pt (2,5)
Quadris 2.08F 14.9 ﬂ oz (3,6) .......................... 0.5 a 1.3 a 0.5 a 34.4a

 

2

Application sequence: 1 = 18 Jul; 2 = 30 Jul; 3 = 8 Aug; 4 = 15 Aug; 5 = 22 Aug; 6 =
29 Aug; 7 = 5 Sep.

Percentage of trimmed plants with one or more petiole lesions. Variable could not be
normalized.

Severity of petiole blight evaluated using a 1-10 scale where 1 = no disease to 10 = all
petioles severely blighted.

Evaluated using a leaf blight assessment key representing the percentage of diseased
leaf area.

Ten plants from the center of each plot were hand-harvested, trimmed to 14 in.
length, and stripped of diseased petioles prior to weighing.

Means within a column followed by the same letter are not signiﬁcantly different
according to Tukey’s Studentized Range Test (P=0.05).

126

 

CELERY (Apium graveolens ‘Sabroso’) R.S. Bounds and MK. Hausbeck
Early blight; Cercospora apii Michigan State University
Department of Plant Pathology
East Lansing, MI 48824

Evaluation of fungicides for managing early blight of celery and impact on yield,
2003.

This study was conducted at cooperator’s farm in Kent County, M1 on a
Houghton muck ﬁeld previously planted to onion. Seven-week old celery ‘Sabroso’
transplants were planted 6.75 in. apart in rows staggered 32-28-32 in. apart on 17 Jun.
Treatment plots were 20 ﬁ long with 5 ft of unsprayed buffer between plots in the same
row and consisted of two rows spaced 28 in. apart. Two buffer rows were left unsprayed
between each treatment plot. Eight treatments were replicated four times in a randomized
complete block design. Weeds, insects, and fertilization were managed according to
standard production practices. The ﬁeld was irrigated with drip irrigation tape placed 3
in. beneath the soil surface between rows spaced 28 in. apart. Fungicides were applied
with a C02 backpack sprayer equipped with three XR8003 ﬂat fan nozzles spaced 18 in.
apart and calibrated to deliver 50 gal/A at a nozzle pressure of 52 psi. Eight applications
were made at weekly intervals on 23 and 29 Jul; 6, 12, 20, and 27 Aug; and 4 and 10 Sep.
Leaf blight severity was evaluated weekly from 12 Aug to 15 Sep. Petiole blight was
assessed and yields were recorded on 15 Sep. Average minimum and maximum air
temperatures (F) were 52.2 and 77.3, 58.2 and 81.3, 59.9 and 83.6, and 49.9 and 72.6 for
Jun, Jul, Aug, and Sep, respectively. Rainfall totals (in.) were 0.9, 2.7, 1.9, and 2.5 for
the same respective months.

 

Disease symptoms appeared in several localized areas throughout the ﬁeld on 12
Aug. Leaf blight severity on untreated plants was moderate (26.3%) at the time of
harvest, while all fungicide treatments limited leaf blight to 5 2.0%. All fungicide
treatments signiﬁcantly reduced disease and yield loss when compared to the untreated.
Treatments of Bravo Ultrex 82.5WDG applied alone or in alternation with Quadris 2.08F
(15.4 ﬂ oz/A) were highly effective in controlling early blight and resulted in no petiole
blight or yield reduction. The three-way alternation program of Quadris 2.08F (9.2 ﬂ
oz/A), Bravo Ultrex 82.5WDG, and Tilt 3.6EC was the least effective of the fungicide
treatments in limiting petiole blight and reducing yield loss; however, it did not differ
when compared to treatments that prevented petiole disease. The treatments did not have
a signiﬁcant effect on total yield, indicating the fungicides did not adversely affect plant
growth. No phytotoxicity occurred on any of the plants.

127

 

Leaf blighty Petiole blight (%) Yield

 

 

Treatment and rate/A (application At harvest Total Loss
sequence)z AUDPC" (%) Incidencew SeverityV (lb)“ (%)t
Untreated ................................................. 391.0 b‘ 26.3 b 77.5 b 11.8 b 21.7 18.7 b
Bravo Ultrex 82.5WDG 1.8 lb (1-8) ........ 18.8 a 1.0 a 0.0 a 0.0 a 21.1 0.0 a
Quadris 2.08F 9.2 ﬂ oz (1,3,5,7)

Bravo Ultrex 82.5WDG 1.8 lb (2,4,6,8).. 22.5 a 1.0 a 5.0 a 0.3 a 21.5 0.2 a
Quadris 2.08F 15.4 ﬂ oz (1,3,5,7)

Bravo Ultrex 82.5WDG 1.8 lb (2,4,6,8).. 30.5 a 2.0 a 0.0 a 0.0 a 19.9 0.0 a

 

Quadris 2.08F 9.2 ﬂ oz (1,4,7)
Bravo Ultrex 82.5WDG 1.8 lb (2,5,8)
Tilt 3.6EC 4 ﬂ oz (3,6) ............................ 47.9 a 2.0 a 25.0 a 1.0 a 21.8 1.9 a

Quadris 2.08F 15.4 ﬂ oz (1,4,7)
Bravo Ultrex 82.5WDG 1.8 lb (2,5,8)

Tilt 3.6EC 4 ﬂ 02 (3,6) ............................ 30.5 a 2.0 a 7.5 a 0.6 a 22.0 0.7 a .
1L-

Bravo Ultrex 82.5WDG 1.8 lb (1-3,7)

Quadris 2.08F 9.2 ﬂ oz (4-6,8) ................ 20.6 a 1.0 a 15.0 a 0.3 a 20.7 0.7 a

Bravo Ultrex 82.5WDG 1.8 lb (1-3,7)

Quadris 2.08F 15.4 ﬂ oz (4-6,8) .............. 33.9 a 2.0 a 7.5 a 0.6 a 22.0 0.5 a

 

Z

Kocide 2000 53.8DF 1.5 lb/A was added to each fungicide treatment to limit bacterial
blight. Application sequence: 1 = 23 Jul; 2 = 29 Jul; 3 = 6 Aug; 4 = 12 Aug; 5 = 20
Aug; 6 = 27 Aug; 7 = 4 Sep; 8 = 10 Sep.

Evaluated using a leaf blight assessment key representing the percentage of diseased
foliage. Variables could not be normalized.

Area under the disease progress curve.

Percentage of trimmed plants with one or more petiole lesions.

Severity of petiole blight assessed as the percentage of petiole area with disease
symptoms. Data were transformed using square root (Y) to stabilize the variance.
The table shows back-transformed data.

Ten plants from the center of one row per plot were hand-harvested, trimmed to 14 in.
length, and weighed on 15 Sep.

Harvested plants were stripped of disease petioles and re-weighed. Yield loss was
calculated as a percentage of stripped weight divided by the total yield. Data were
transformed using square root (Y) to stabilize the variance. The table shows back-
transforrned data.

Means within a column followed by the same letter or no letter are not signiﬁcantly
different according to Tukey’s Studentized Range Test (P=0.05).

128

CELERY (Apium graveolens ‘Dutchess’) R.S. Bounds and MK. Hausbeck
Crater rot; Rhizoctonia solani Michigan State University
Department of Plant Pathology
East Lansing, MI 48824

Evaluation of fungicides for managing crater rot of celery, 2003.

This study was conducted at cooperator’s farm in Allegan County, M1 on a
Houghton muck ﬁeld previously planted to celery. Seven-week old celery ‘Dutchess’
transplants were planted 8 in. apart in rows staggered 26-14-26 in. apart on 15 May.
Treatment plots were 20 ft long with 5 ft of unsprayed buffer between plots in the same
row and consisted of two rows spaced 14 in apart. Two buffer rows were left unsprayed
between each treatment plot. Six treatments were replicated four times in a randomized
complete block design. Weeds, insects, and fertilization were managed according to
standard production practices. The ﬁeld was irrigated with a high-volume hard hose
traveling irrigation unit that generated extensive soil splashing onto petioles. Fungicides
were applied with a C02 backpack sprayer equipped with three XR8003 ﬂat fan nozzles
spaced 19 in. apart and calibrated to deliver 50 gal/A at a nozzle pressure of 52 psi. Nine
applications were made at weekly intervals on 3, 11, 18, and 25 Jun; 2, 9, 16, 23, and 29
Jul. A directed spray pattern with three XR8002 nozzles was used for the last four
applications by replacing the outer nozzles with 6 in. drop nozzles directed at 45-degree
angles inward. The incidence of crater rot was assessed and yields were recorded on 6
Aug. Average minimmn and maximum air temperatures (F) were 45.0 and 66.3, 52.2
and 77.3, 58.2 and 81.3, and 59.9 and 83.6 for May, Jun, Jul, and Aug, respectively.
Rainfall totals (in.) were 4.5, 0.9, 2.7, and 1.9 for the same respective months.

Disease pressure was moderately low when compared to previous seasons with
warmer and more humid weather during Jul. Crater rot symptoms were observed only
during harvest and not during random destructive sampling of plants in untreated buffer
rows in the latter part of the season. Over 45% of untreated plants had petiole lesions
while all fungicide treatments limited disease to 5 22.1%. Only the alternation of
Quadris 2.08F and Bravo Ultrex 82.5WDG signiﬁcantly limited the incidence of crater
rot when compared to the untreated. Total yield and marketable yield were not affected
by treatments. Yield loss from plants treated with Bravo Ultrex 82.5WDG alone or Tilt
3.6EC alternated with Bravo Ultrex 82.5WDG did not differ from the untreated.
Applications of Quadris 2.08F, Topsin M 70WP, or Scholar 50WP alternated with Bravo
Ultrex 82.5WDG were effective in limiting yield loss when compared to the untreated.
No phytotoxicity occurred on any of the plants.

129

 

 

 

Yield

 

 

Crater rot
Treatment and rate/A (application sequence)z rncrdence’ Total (lb)" Marketable (lb)w Loss (%)”
Untreated .......................................................... 46.3 b“ 44.6 41.6 6.7 c
Bravo Ultrex 82.5WDG 1.8 lb (1-9) ................. 17.9 ab 48.7 47.0 2.7 bc
Quadris 2.08F 9.2 ﬂ oz (1,3,5,7,9)
Bravo Ultrex 82.5WDG 1.8 lb (2,4,6,8) ........... 1.0 a 50.4 50.3 0.1 a
Tilt 3.6EC 4 ﬂ oz (1,3,5,7,9)
Bravo Ultrex 82.5WDG 1.8 lb (2,4,6,8) ........... 22.1 ab 49.2 47.9 2.3 bc
Topsin M 70WP 0.5 lb (1,3,5,7,9)
Bravo Ultrex 82.5WDG 1.8 lb (2,4,6,8) ........... 19.6 ab 46.8 45.9 1.7 ab
Scholar 50WP 0.44 lb (1 ,3,5,7,9)
Bravo Ultrex 82.5WDG 1.8 lb (2,4,6,8) ........... 16.1 ab 46.2 45.3 1.8 ab

 

z Kocide 2000 53.8DF 1.5 lb/A was added to each fungicide treatment to limit bacterial
blight. Application sequence: 1 = 3 Jun; 2 = 11 Jun; 3 = 18 Jun; 4 = 25 Jun; 5 = 2 Jul;
6=9Jul;7= 16Jul; 8=23 Jul;9=29Jul.

y Crater rot incidence was assessed by counting the number of plants with one or more
petiole lesions out of the total number of consecutive plants from 15 ft. of one row.

weighed.

Plants from 15 ft. of one row were hand-harvested, trimmed to 14 in. length, and

Harvested plants were stripped of disease petioles and weighed again.
Yield loss was calculated as a percentage of stripped weight divided by the total

yield. Data were transformed using log (Y+l) to stabilize the variance. The table

shows back-transformed data.

130

Means within a column followed by the same letter or no letter are not signiﬁcantly
different according to Tukey’s Studentized Range Test (P=0.05).

 

CELERY (Apium graveolens var. dulce ‘Dutchess’) R.S. Bounds and MK. Hausbeck
Late blight; Septoria apiicola Michigan State University
Department of Plant Pathology
East Lansing, MI 48824

Evaluation of two spray initiation timings, two application intervals, and seven
fungicide programs for managing late blight of celery, 2003.

This study was conducted at the Michigan State University Muck Soils Research
Farm in Laingsburg, M1 on a Houghton muck ﬁeld previously planted to potato. Seven-
week old celery ‘Dutchess’ transplants were planted 7 in. apart in rows spaced 32 in.
apart on 28 May. Treatment plots consisted of one row 20 ft long with 5 ft of unsprayed
buffer between plots in the same row. Two buffer rows were left unsprayed between
each treatment row. Twenty-nine treatments were replicated four times in a randomized
complete block design. Septoria inoculum (2.9 x 10 spores/ﬂ oz) was prepared by
soaking dried infected celery leaves for 10 min in water and straining through two layers
of cheesecloth. Unsprayed buffer rows were inoculated on 16 Jul. Inoculum was applied
with a hand-pump backpack sprayer using one hollow cone nozzle that delivered
approximately 12 gal/A. The initial fungicide application was applied either (i) one week
after transplanting on 5 Jun (preventive)or (ii) when disease symptoms were observed in
uninoculated treatment rows on 22 Jul (disease detection). Fungicides were reapplied
either (i) weekly or (ii) according to the TOM-CAST disease forecaster using a threshold
of 15 disease severity values (DSV). Seven fungicide programs tested: (i) Kocide 2000
53.8DF; (ii) Bravo Ultrex 82.5WDG; (iii) Quadris 2.08F; (iv) Bravo Ultrex 82.5WDG
alternated with Kocide 2000 53.8DF; (v) Quadris 2.08F alternated with Kocide 2000
53.8DF; (vi) Quadris 2.08F alternated with Bravo Ultrex 82.5WDG; and (vii) Quadris
2.08F alternated with Kocide 2000 53.8DF alternated with Bravo Ultrex 82.5WDG.
Fungicides were applied with a C02 backpack sprayer equipped with three XR8003 ﬂat
fan nozzles spaced 18 in. apart and calibrated to deliver 50 gal/A at a boom pressure of
52 psi. Ten plants from each treatment row were hand-harvested, trimmed to 14 in.
length, and weighed on 26 Aug. Trimmed plants were evaluated for disease, stripped of
diseased petioles, and weighed again.

Disease symptoms appeared on inoculated plants in untreated buffer rows and on
a few uninoculated plants in treatment rows which were not previously treated on 22 Jul.
The infection of uninoculated plants was not the result of the manual inoculation, but
rather from natural infection of transplants. Since disease symptoms did not develop
uniformly in uninoculated treatment rows, this factor likely confounded the experiment.
Due to signiﬁcant three-way interactions among the factors (initiation timing, application
interval, and fungicide program), the data were analyzed for differences between the
combined effects of application interval and initiation timing within each fungicide
program. All fungicide programs, except Kocide 2000 53.8DF applied alone, were
effective in limiting disease and yield loss to low levels when applied one week after
transplanting and reapplied every seven days. The preventive 7-day interval was often
signiﬁcantly more effective in limiting disease and yield loss than the TOM-CAST
intervals initiated either preventively or at disease detection. The Quadris 2.08F

131

alternated with Bravo Ultrex and Bravo Ultrex applied alone fungicide programs often
provided the highest level of control. No phytotoxicity occurred on any of the plants.

 

 

 

Treatment and rate/A No. of Disease severity (%) Yield loss
Application interval / initiation timing‘ sprays Leavesy Petioles" (%)W
Untreated control .................................................. 0 27.5 23.8 39.5
Kocide 2000 53.8DF 1.5 lb
7-day / preventive ......................................... 12 19.3 a" 14.8 a 31.6 a
7-day / disease detection ............................... 5 23.4 a 21.5 a 33.0 a
TOM-CAST 15 DSV / preventive ................ 5 33.4 a 24.2 a 36.9 a
TOM-CAST 15 DSV / disease detection ...... 2 25.1 a 20.0 a 35.0 a
Bravo Ultrex 82.5WDG 1.8 lb
7—day / preventive ......................................... 12 0.0 a 0.0 a 0.0 a
7-day / disease detection ............................... 5 1.0 ab 0.0 a 0.0 a
TOM-CAST 15 DSV / preventive ................ 5 2.6 b 3.0 b 8.9 a
TOM-CAST 15 DSV / disease detection ...... 2 5.1 b 3.4 b 11.9 a
Quadris 2.08F 9.2 ﬂ oz
7-day / preventive ......................................... 12 0.1 a 0.0 a 0.0 a
7-day / disease detection ............................... 5 1.7 ab 0.6 a 2.6 a
TOM-CAST 15 DSV / preventive ................ 5 5.0 bc 0.7 a 1.8 a
TOM-CAST 15 DSV / disease detection ...... 2 13.4 c 9.3 b 18.0 b
Bravo Ultrex 82.5WDG 1.8 lb alt.
Kocide 2000 53.8DF 1.5 lb
7-day / preventive ......................................... 12 0.3 a 0.1 a 0.3 a
7-day / disease detection ............................... 5 5.5 b 3.7 b 13.1 b
TOM-CAST 15 DSV / preventive ................ 5 13.0 b 15.8 c 25.7 c
TOM-CAST 15 DSV / disease detection ...... 2 4.7 b 1.7 ab 12.3 ab
Quadris 2.08F 9.2 ﬂ oz alt.
Kocide 2000 53.8DF 1.5 lb
7-day / preventive ......................................... 12 0.6 a 0.1 a 0.5 a
7-day / disease detection ............................... 2.6 ab 3.2 ab 13.7 b
TOM-CAST 15 DSV / preventive ................ 5 8.3 be 5.7 b 16.2 b
TOM-CAST 15 DSV / disease detection ...... 2 14.6 c 8.3 b 21.7 b
Quadris 2.08F 9.2 ﬂ oz alt.
Bravo Ultrex 82.5WDG 1.8 lb
7-day/preventive ......................................... 12 0.1 a 0.1 a 0.1 a
7-day / disease detection ............................... 0.3 a 0.0 a 0.0 a
TOM-CAST 15 DSV / preventive ................ 5 1.0 a 0.6 a 4.8 a
TOM-CAST 15 DSV / disease detection ...... 2 10.8 b 7.3 b 10.2 a
Quadris 2.08F 9.2 ﬂ oz alt.
Kocide 2000 53.8DF 1.5 lb alt.
Bravo Ultrex 82.5WDG 1.8 lb
7-day / preventive ......................................... 12 0.0 a 0.0 a 0.0 a
7-day / disease detection ............................... 5 2.4 b 1.1 ab 7.5 a
TOM-CAST 15 DSV / preventive ................ 5 8.3 be 7.7 c 9.4 a
TOM-CAST 15 DSV / disease detection ...... 2 11.8 c 2.6 be 10.7 a

 

132

Preventive sprays were initially applied one week after transplanting; disease
detection sprays were applied when disease symptoms were observed in treatment
rows.

Percentage of petiole area with disease symptoms. Data were transformed using
square root (Y) to stabilize the variance. The table shows back-transformed data.
Evaluated using a leaf blight assessment key representing the percentage of diseased
leaf area. Data were transformed using square root (Y) to stabilize the variance. The
table shows back-transformed data.

Harvested plants were stripped of disease petioles and weighed again. Yield loss was
calculated as a percentage of stripped weight divided by the total yield.

Means within each fungicide program in a column followed by the same letter are not
signiﬁcantly different according to Tukey’s Studentized Range Test (P=0.05).

133

CELERY (Apium graveolens L. var. dulce ‘Dutchess’) R.S. Bounds and MK. Hausbeck
Late blight; Septoria apiicola Michigan State University
Department of Plant Pathology
East Lansing, MI 48824

Evaluation of fungicides for managing late blight of celery, 2004.

This study was conducted at the Michigan State University Muck Soils Research
Farm in Laingsburg, M1 on a Houghton muck ﬁeld previously planted to potato. Eight-
week old celery ‘Dutchess’ transplants were planted 7 in. apart in rows spaced 32 in.
apart on 21 Jun. Treatment plots consisted of one row 20 ft long with 5 ft of unsprayed
buffer between plots in the same row. Two buffer rows were left unsprayed between
each treatment row. Eleven treatments were replicated four times in a randomized
complete block design. Septoria inoculum (2.9 x 107 spores/ﬂ oz) was prepared by
soaking dried infected celery leaves for 10 min in water and straining through two layers
of cheesecloth. Inoculum was applied with a hand-pump backpack sprayer to all plants
in the trial on 30 Jul using one hollow cone nozzle that delivered 12 gal/A. Fungicides
were applied with a C02 backpack sprayer equipped with three XR8003 ﬂat fan nozzles
spaced 18 in. apart and calibrated to deliver 50 gal/A at a nozzle pressure of 52 psi. Nine
applications were made at weekly intervals on 19 and 26 Jul; 2, 9, 17, 23, and 30 Aug;
and 6 and 14 Sep. Leaf blight severity was evaluated on 8 and 20 Sep, and ten plants
from the middle of each treatment row were hand-harvested and trimmed to fresh market
speciﬁcations (14 in. length) on 21 Sep. Petiole disease incidence and severity were
assessed, diseased petioles were removed from the plants, and yields were recorded.
Data were analyzed using the general linear models procedure in SAS, and differences
among treatments were examined using the Waller-Duncan Bayesian k-ratio t-test.
Average monthly minimum and maximum air temperatures (°F) were: Jun (52.0 and
74.8); Jul (55.9 and 79.1); Aug (52.3 and 75.7); and Sep (46.9 and 77.4). Rainfall totals
(in.) were 4.2, 3.7, 1.8, and 0.9 for the same respective months.

Disease symptoms appeared throughout the plot on 11 Aug. Untreated plants
were severely diseased on leaves (50%) and petioles (71%) at harvest which resulted in
extensive trimming and low yield. Three fungicide programs provided season-long
disease prevention on leaves and petioles and included Endorse 2.5WP alternated with
Bravo Ultrex 82.5WDG, Amistar 80WG alternated with Tilt 3.6EC, and Amistar 80WG
alternated with Bravo Ultrex 82.5WDG. Applications of Pristine 38WG alternated with
Bravo Ultrex 82.5WDG or Bravo Ultrex 82.5WDG (1.8 lb/A) applied alone were also
effective programs. Applications of Endura 70WG resulted in a lower trimmed yield
than programs that prevented disease. Mixing EXP 1 with a reduced rate of Bravo Ultrex
82.5 (0.6 lb/A) provided no disease control beneﬁt when compared to the reduced rate
Bravo Ultrex 82.5WDG (0.6 lb/A) treatment. Disease control and yield of the Serenade
Max 20WP alternated with Bravo Ultrex 82.5WDG treatment and the reduced rate Bravo
Ultrex 82.5WDG (0.6 lb/A) treatment did not differ. Applications of Switch 62.5WG
were ineffective and resulted in a low yield similar to the untreated control. No
phytotoxicity was observed.

134

 

 

 

Leaf blight (%)” Petiole blight
Treatment and rate/A (application Incidence Severityw ngdv
sequence’) 8 Sep 20 Sep (%)x yre ( )
Untreated control .................................... 39.9 d“ 50.0 e 100.0 e 71.3 d 2.7 d
Endura 70WG 4.6 oz (1-10) ................... 6.3 72.5 d 7.8 b 23.1 c
Pristine 38WG 10.5 oz (1,3,5,7,9)
Bravo Ultrex 82.5WDG 1.8 lb
(2,4,6,8,10) ............................................. 0.0 a 0.0 a 2.5 ab 0.3 a 29.8 a
Endorse 2.5WP 2.2 lb (1,3,5,7,9)
Bravo Ultrex 82.5WDG 1.8 lb
(2,4,6,8,10) ............................................. 0.0 a 0.0 a 0.0 a 0.0 a 28.5 ab
Amistar 80WG 5 oz (1,3,5,7,9)
Tilt 3.6EC 4 ﬂ oz (2,4,6,8,10) ................ 0.0 a 0.0 a 0.0 a 0.0 a 28.0 ab
Amistar 80WG 5 oz (1,3,5,7,9)
Bravo Ultrex 82.5WDG 1.8 lb
(2,4,6,8,10) ............................................. 0.0 a 0.0 a 0.0 a 0.0 a 28.4 ab
Switch 62.5WG 11 02 (1-10) .................. 32.5 c 46.3 d 100.0 c 55.0 c 6.6 (1
EXP 1 12.8 ﬂ 02
+ Bravo Ultrex 82.5WDG 0.6 lb (1-10). 3.0 b 42.5 c 3.3 a 25.0 bc
Bravo Ultrex 82.5WDG 0.6 lb (1-10) ..... 0.6 b 3.0 b 25.0 be 1.8 a 27.5 ab
Bravo Ultrex 82.5WDG 1.8 lb (1-10) ..... 0.1 a 0.3 a 0.0 a 0.0 a 28.8 ab
Serenade Max 20WP 2 lb (1,3,5,7,9)
Bravo Ultrex 82.5WDG 1.8 lb
(2,4,6,8,10) ............................................. 0.6 b 3.0 b 10.0 ab 2.5 a 27.8 ab

 

2 Application sequence: 1 = 19 Jul; 2 = 26 Jul; 3 = 2 Aug; 4 = 9 Aug; 5 = 17 Aug; 6 =
23 Aug; 7 = 30 Aug; 8 = 6 Sep; 9 = 14 Sep.

y Evaluated using a leaf blight assessment key representing the percentage of diseased
leaf area. Data from 8 Sep were transformed using square root (Y) to stabilize the
variance; the table shows back-transformed data. Data from 20 Sep could not be

normalized.

normalized.

Percentage of petiole area with disease symptoms.

Percentage of trimmed plants with one or more petiole lesions. Variable could not be

Ten plants from the center of each plot were hand-harvested, trimmed to 14 in.
length, and stripped of diseased petioles prior to weighing.

according to Waller-Duncan Bayesian k-ratio t-test (k-ratio = 100).

135

Means within a column followed by the same letter are not signiﬁcantly different

CELERY (Apium graveolens var. dulce ‘Dutchess’) R.S. Bounds and MK. Hausbeck
Late blight; Septoria apiicola Michigan State University
Dept. of Plant Pathology
East Lansing, MI 48824

Evaluation of fungicide programs and disease predictors for managing late blight of
celery, 2004.

This study was conducted at the Michigan State University Muck Soils Research
Farm in Laingsburg, M1 on a Houghton muck ﬁeld previously planted to potato. Seven-
week old celery transplants were planted 7 in. apart in rows spaced 32 in. apart on 21 Jun.
Each plot consisted of a 20 ft—long row with 5 ft of unsprayed row between adjacent plots
in the same row. Two unsprayed buffer rows separated adjacent treatment rows.
Treatments were replicated four times in a randomized complete block design. Weeds,
insects, and fertilization were managed according to standard production practices.
Septoria apiicola inoculum (2.9 x 107 spores/ﬂ oz) was prepared by soaking dried
infected celery leaves for 10 min in water and straining the spore suspension through two
layers of cheesecloth. Inoculum was applied to the buffer rows on 30 Jul and 11 Aug
using a hand-pump backpack sprayer with one hollow cone nozzle that delivered
approximately 12 gal/A. Fungicides were applied with a C02 backpack boom sprayer
equipped with three XR8003 ﬂat fan nozzles spaced 18 in. apart and calibrated to deliver
50 gal/A at a boom pressure of 52 psi. Fungicides were initially applied one week after
transplanting and were reapplied every seven days or according to the Septoria or TOM-
CAST 10 DSV disease predictors. The 7-day interval required 12 applications, the
Septoria predictor required 11 applications, and the TOM-CAST lO-DSV predictor
required 8 applications. Ten plants from the middle of each plot were hand-harvested,
trimmed to fresh market speciﬁcations (14 in. length), and weighed on 22 Sep. Trimmed
plants were evaluated for late blight, stripped of diseased petioles, and weighed again.
Data were analyzed using the general linear models procedure in SAS. Average monthly
minimum and maximum air temperatures (°F) were 52.0 and 74.8 in Jun, 55.9 and 79.1
in Jul, 52.3 and 75.7 in Aug, and 46.9 and 77.4 in Sep. Rainfall totaled 4.2, 3.7, 1.8, and
0.9 in. for the same respective months.

Disease symptoms were detected in inoculated buffer rows on 15 Aug and were
observed on all uninoculated untreated plants by 2 Sep. At harvest, all untreated plants
were severely diseased, with 70% yield loss recorded when diseased petioles were
removed. Nominal amounts of leaf blight (0.3 to 1.0%) were observed for 10 of the 15
fungicide spray programs. All fungicide programs, regardless of application interval,
were equally effective in preventing petiole blight and signiﬁcantly reduced leaf blight
severity compared to the untreated control plants. Likewise, all application intervals
were equally effective against late blight. However, the Septoria and TOM-CAST 10-
DSV predictors required 1 and 4 fewer applications, respectively, relative to the weekly
schedule. This study identiﬁed effective fungicide spray programs that can be used in
conjunction with the Septoria and TOM-CAST lO-DSV predictors while reducing the
number of applications needed to achieve equivalent disease control. Symptoms of
phytotoxicity were not observed.

136

 

Treatment and rate/A (application sequencez) No. of Petiole blight (%)y Leaf blight Yield loss

 

 

Application interval sprays Incidence Severity (%)x (%)W
Untreated control .................................................... 0 100.0 b" 42.5 b 38.8 b 70.0 b
Bravo Ultrex 82.5WDG 1.8 lb (1-12)

7-day ................................................................... 12 0.0 a 0.0 a 0.0 a 0.0 a

Septoria predictor ............................................... 1 l 0.0 a 0.0 a 0.0 a 0.0 a

TOM-CAST lO-DSV ......................................... 8 0.0 a 0.0 a 0.5 a 0.0 a

Bravo Ultrex 82.5WDG 1.8 1b(1,2,4,5,7,8,10,11)
Tilt 3.6EC 4 ﬂ oz (3,6,9,12)

7-day ................................................................... 12 0.0 a 0.0 a 0.0 a 0.0a
Septoria predictor ............................................... 1 1 0.0 a 0.0 a 0.3 a 0.0 a
TOM-CAST lO-DSV ......................................... 8 0.0 a 0.0 a 0.3 a 0.0 a

Bravo Ultrex 82.5WDG 1.8 lb (1,2,4,5,7,8,10,11)
Amistar 80WG 3 oz (3 ,6,9,12)

7—day ................................................................... 12 0.0 a 0.0 a 0.3 a 0.0 a
Septoria predictor ............................................... 1 1 0.0 a 0.0 a 0.0 a 0.0 a
TOM-CAST lO-DSV ......................................... 8 0.0 a 0.0 a 0.3 a 0.0 a

Bravo Ultrex 82.5WDG 1.8 lb (1,3,5,7,9,11)

Amistar 80WG 3 oz (2,6,10)

Tilt 3.6EC 4 ﬂ oz (4,8,12)
7-day ................................................................... 12 0.0a 0.03 0.3 a 0.0a
Septoria predictor ............................................... 1 l 0.0 a V 0.0 a 0.3 a 0.0 a
TOM-CAST lO-DSV ......................................... 8 0.0 a 0.0 a 1.0 a 0.0 a

Bravo Ultrex 82.5WDG 1.8 1b(1,4,7,10)
Amistar 80WG 3 oz (2,5,8,l 1)
Tilt 3.6EC 4 ﬂ oz (3,6,9,12)

 

7-day ................................................................... 12 0.0 a 0.0 a 0.0 a 0.0 a

Septoria predictor ............................................... l l 0.0 a 0.0 a 0.5 a 0.0 a

TOM-CAST 10-DSV ......................................... 8 0.0 a 0.0 a 0.3 a 0.0 a
2

Application sequence indicates the order in which fungicides were applied. For the 7-
day interval, fungicides were applied on: 1 = 28 Jun; 2 = 5 Jul; 3 = 12 Jul; 4 = 19 Jul;
5 =26Jul; 6=2Aug;7=8Aug; 8= 15 Aug;9=22Aug; 10=30 Aug; 11 =6Sep;
12 = 13 Sep. For the Septoria predictor, fungicides were applied on: 1 = 28 Jun; 2 =
13Jul;3=20Jul;4=28Jul;5=5Aug;6=13 Aug;7=20Aug;8=27Aug;9=3
Sep; 10 = 10 Sep; 11 = 17 Sep. For the TOM-CAST lO-DSV predictor, fungicides
were applied on: 1 = 28 Jun; 2 = 8 Jul; 3 = 18 Jul; 4 = 28 Jul; 5 = 4 Aug; 6 = 25 Aug;
7 = 30 Aug; 8 =10 Sep.

Incidence = % of trimmed plants with one or more petiole lesions; severity = % of
petiole surface area with symptoms.

Evaluated prior to harvest using a leaf blight assessment key of the % of diseased leaf
area on 22 Sep.

w Yield loss was calculated as the % of total yield lost due to late blight.

Means within a column followed by the same letter are not signiﬁcantly different
according to Tukey’s Studentized Range Test (P 5 0.05).

137

CELERY (Apium graveolens var. dulce ‘Dutchess’) R.S. Bounds and MK. Hausbeck
Late blight; Septoria apiicola Michigan State University
Dept. of Plant Pathology
East Lansing, MI 48824

Evaluation of fungicide initiation timing for managing late blight of celery, 2004.

This study was conducted at the Michigan State University Muck Soils Research
Farm in Laingsburg, M1 on a Houghton muck ﬁeld previously planted to potato. Seven-
week old celery transplants were planted 7 in. apart in rows spaced 32 in. apart on 21 Jun.
Each plot consisted of a 20 ft-long row with 5 ft of unsprayed row between adjacent plots
in the same row. Two unsprayed buffer rows separated adjacent treatment rows.
Treatments were replicated four times in a randomized complete block design. Weeds,
insects, and fertilization were managed according to standard production practices.
Septoria apiicola inoculum (2.9 x 107 spores/ﬂ 02) was prepared by soaking dried
infected celery leaves for 10 min in water and straining the spore suspension through two
layers of cheesecloth. Inoculum was applied to all plants in the trial on 30 Jul and 11
Aug with a hand-pump backpack sprayer using one hollow cone nozzle that delivered
approximately 12 gal/A. Fungicides were applied with a C02 backpack boom sprayer
equipped with three XR8003 ﬂat fan nozzles spaced 18 in. apart and calibrated to deliver
50 gal/A at a boom pressure of 52 psi. Fungicide spray programs were initiated two
(early) or ﬁve (preventive) weeks after transplanting. Ten applications were made at
weekly intervals for the early fungicide program which commenced on 5 Jul. Seven
applications were made at weekly intervals for the preventive fungicide program which
commenced on 26 Jul. Ten plants from the middle of each treatment row were hand-
harvested, trimmed to fresh market speciﬁcations (14 in. length), and weighed on 21 Sep.
Trimmed plants were evaluated for late blight, stripped of diseased petioles, and weighed
again. Data were analyzed using the general linear models procedure in SAS. Average
monthly minimum and maximum air temperatures (°F) were 52.0 and 74.8 in Jun, 55.9
and 79.1 in Jul, 52.3 and 75.7 in Aug, and 46.9 and 77.4 in Sep. Rainfall totaled 4.2, 3.7,
1.8, and 0.9 in. for the same respective months.

Late blight lesions were ﬁrst detected on 15 Aug, and disease progressed rapidly
until harvest. Leaving plants untreated resulted in a signiﬁcant yield loss of 80%. The
early and preventive fungicide programs were equally effective at preventing petiole
blight and yield loss. Three fewer fungicide applications were made by initiating the ﬁrst
application ﬁve weeks after transplanting compared with the early program initiated two
weeks after transplanting. The preventive fungicide program reduced fungicide costs by
approximately $40/A compared to the early fungicide program. Symptoms of
phytotoxicity were not observed.

138

 

Petiole blight (%)’

 

 

Treatment and rate/A (application Initial Leaf; blight Yield LOSS
sequence‘) appllcatlon Incidence Severity ( /°) ( /°)
Untreated control ......................................... -- 100.0 b" 70.0 b 48.4 b 80.8 b
Bravo Ultrex 82.5WDG 1.8 lb (1,3,5,7,9)

Amistar 80WG 3 oz (2,6,10)

Tilt 3.6EC 4 ﬂ oz (4,8) ................................ early 0.0 a 0.0 a 0.5 a 0.0 a
Bravo Ultrex 82.5WDG 1.8 lb (4,6,8,10)

Amistar 80WG 3 oz (5,9)

Tilt 3.6EC 4 ﬂ oz (7) ................................... preventive 0.0 a 0.0 a 0.3 a 0.0 a

 

2 Application sequence: 1 = 5 Jul; 2 = 12 Jul; 3 = 19 Jul; 4 = 26 Jul; 5 = 8 Aug; 6 = 15
Aug; 7 = 22 Aug; 8 = 30 Aug; 9 = 6 Sep;10 =13 Sep.

y Incidence = % of trimmed plants with one or more petiole lesions; severity = % of
petiole surface area with symptoms.

" Evaluated prior to harvest using a leaf blight assessment key representing the % of
diseased leaf area on 21 Sep.

w Yield loss was calculated as the % of total yield lost due to late blight.

V Means within a column followed by the same letter are not signiﬁcantly different
according to Tukey’s Studentized Range Test (P _<_ 0.05).

139

CELERY (Apium graveolens var. dulce ‘Dutchess’) R.S. Bounds and MK. Hausbeck
Crater rot; Rhizoctonia solani Michigan State University
Dept. of Plant Pathology
East Lansing, MI 48824

Evaluation of fungicide initiation timing for managing crater rot of celery, 2004.

This study was conducted at a cooperator’s farm in Ottawa County, M1 on a
Houghton muck ﬁeld previously planted to celery. Eight-week old celery transplants
were planted 6 in. apart in rows spaced 28 in. apart on 24 Jun. Each plot consisted of a
row 20 ft-long row with 5 ft of unsprayed row between adjacent plots in the same row.
Two unsprayed buffer rows separated adjacent treatment rows. Treatments were
replicated four times in a randomized complete block design. Weeds, insects, and
fertilization were managed according to standard production practices. Cultivation was
not implemented for weed control. Fungicides were applied with a C02 backpack boom
sprayer equipped with two XR8003 ﬂat fan nozzles spaced 19.5 in. apart and calibrated
to deliver 50 gal/A at a boom pressure of 53 psi. F ungicide spray programs were initiated
one (early) or four (preventive) weeks aﬁer transplanting. Ten applications were made at
weekly intervals for the early fungicide program which commenced on 1 Jul. Seven
applications were made at weekly intervals for the preventive fungicide program which
commenced on 22 Jul. Ten plants from the middle of each treatment row were hand-
harvested, trimmed to fresh market speciﬁcations (14 in. length), and weighed on 9 Sep.
The incidence of crater rot symptoms on the petioles was assessed on trimmed plants,
diseased petioles were counted and removed from the plants, and marketable yields were
weighed. Data were analyzed using the general linear models procedure in SAS.
Average monthly minimum and maximum air temperatures (°F) were 55.1 and 74.9 in
Jun, 59.0 and 79.7 in Jul, 54.3 and 76.4 in Aug, and 50.5 and 79.4 in Sep. Rainfall
totaled 2.4, 2.6, 3.0, and 0.3 in. for the same respective months.

This trial was originally designed to evaluate early and preventive fungicide
applications for managing foliar blights of celery. A few early blight lesions (caused by
Cercospora apii) were detected on 18 Aug in an unsprayed buffer row, but the disease
did not spread to treatment plots. Symptoms of crater rot were observed on petioles
during harvest. Over 22% of untreated plants had crater rot lesions after trimming, and a
signiﬁcant yield loss of 5.2% resulted when diseased petioles were removed. The early
and preventive ﬁlngicide programs were equally effective at preventing crater rot, but
three fewer fungicide applications were made by initiating the ﬁrst application four
weeks after transplanting compared with the early program initiated one week after
transplanting. The preventive fungicide program reduced fungicide costs by
approximately $40/A compared to the early fungicide program. Symptoms of
phytotoxicity were not observed.

140

 

Crater rot

 

 

Initial No. infected Yield loss
Treatment and rate/A (application sequence‘) applrcatron Incidence (%) petioles (%),
Untreated control ................................................ -- 22.5 b" 6.3 b 5.2 b
Bravo Ultrex 82.5WDG 1.8 lb (1,3,5,7,9)
Amistar 80WG 3 oz (2,6,10)
Tilt 3.6EC 4 ﬂ 02 (4,8) ....................................... early 0.0 a 0.0 a 0.0 a
Bravo Ultrex 82.5WDG 1.8 lb (4,6,8,10)
Amistar 80WG 3 02 (5,9)
Tilt 3.6EC 4 ﬂ oz (7) .......................................... preventive 0.0 a 0.0 a 0.0 a

 

1 Application sequence: 1 = 1 Jul; 2 = 8 Jul; 3 = 15 Jul; 4 = 22 Jul; 5 = 29 Jul; 6 = 5
Aug; 7 = 12 Aug; 8 = 19 Aug; 9 = 26 Aug; 10 = 2 Sep. Kocide 2000 53.8DF (1.5
lb/A) was added to each fungicide treatment to limit the development of bacterial leaf
spot (Pseudomonas syringae pv. apii).

Yield loss was calculated as the % of total yield lost due to crater rot.
Means within a column followed by the same letter are not signiﬁcantly different
according to the Waller-Duncan Bayesian k-ratio t-test (k-ratio = 100).

141

CELERY (Apium graveolens ‘Dutchess’) R.S. Bounds and MK. Hausbeck
Crater rot; Rhizoctonia solani Michigan State University
Department of Plant Pathology
East Lansing, MI 48824

Evaluation of fungicides for managing crater rot of celery, 2004.

This study was conducted at cooperator’s farm in Allegan County, M1 on a
Houghton muck ﬁeld previously planted to celery. Eight-week old celery ‘Dutchess’
transplants were planted 8 in. apart in rows staggered 26-14-26 in. apart on 26 May.
Treatment plots were 20 ft long with 5 ft of unsprayed buffer between plots in the same
row and consisted of two rows spaced 14 in. apart. Two buffer rows were left unsprayed
between each treatment plot. Six treatments were replicated four times in a randomized
complete block design. Weeds, insects, and fertilization were managed according to
standard production practices. Fungicides were applied with a C02 backpack sprayer
equipped with three XR8003 ﬂat fan nozzles spaced 19 in. apart and calibrated to deliver
50 gal/A at a nozzle pressure of 52 psi. Ten applications were made at weekly intervals
on 15, 23, and 29 Jun; 7, 13, 20, and 29 Jul; and 3, 10, and 18 Aug. A directed spray
pattern with three XR8002 nozzles was used for the last four applications by replacing
the outer nozzles with 6 in. drop nozzles directed at 45-degree angles inward. The
incidence of crater rot was assessed and yields were recorded on 24 Aug. Average
monthly minimum and maximrun air temperatures (°F) were: May (48.7 and 69.4); Jun
(55.1 and 74.9); Jul (59.0 and 79.7); and Aug (54.3 and 76.4). Rainfall totals (in.) were
6.8, 2.4, 2.6, and 3.0 for the same respective months.

Few crater rot symptoms were detected during random destructive sampling of
plants in untreated buffer rows in the latter part of the season. Overall, disease pressure
was low and no disease symptoms were found in treatment rows at harvest. There was
no signiﬁcant yield difference among treatments. Early blight (caused by Cercospora
apii) lesions were detected on 29 Jul in one localized area of an untreated buffer row, but
cool weather kept the disease from spreading. No phytotoxicity was observed.

142

 

Treatment and rate/A (application sequence)‘ Yield per 10 trimmed plants (lb)

 

Untreated ............................................................................................... 22.6 "
Bravo Ultrex 82.5WDG 1.8 lb (1-10) .................................................... 23.3
Amistar 80WG 3 oz (1,3,5,7,9)

Bravo Ultrex 82.5WDG 1.8 lb (2,4,6,8,10) ........................................... 23.3
Cabrio 20WG 12 oz (1,3,5,7,9)

Bravo Ultrex 82.5WDG 1.8 lb (2,4,6,8,10) ........................................... 23.1
Scholar 50WP 7 oz (1,3,5,7,9)

Bravo Ultrex 82.5WDG 1.8 lb (2,4,6,8,10) ........................................... ‘ 21.6
Moncut 7ODF 11.4 oz + Bravo Ultrex 82.5WDG 1.8 lb (1,3,5,7,9)

Bravo Ultrex 82.5WDG 1.8 lb (2,4,6,8,10) ........................................... 22.3

 

* Kocide 2000 53.8DF (1.5 lb/A) was added to each fungicide treatment to limit
bacterial blight. Application sequence: 1 = 15 Jun; 2 = 23 Jun; 3 = 29 Jun; 4 = 7 Jul;
5 =13 Jul; 6 = 20 Jul; 7 = 29 Jul; 8 = 3 Aug; 9 =10 Aug; 10 =18 Aug.

** Means within a column followed by the same letter or no letter are not signiﬁcantly
different according to Tukey’s Studentized Range Test (P=0.05).

143

CELERY (Apium graveolens var. dulce ‘Dutchess’) R.S. Bounds and MK. Hausbeck
Late blight; Septoria apiicola Michigan State University
Dept. of Plant Pathology
East Lansing, MI 48824

Evaluation of fungicides for managing late blight of celery, 2005.

This study was conducted at the Michigan State University Muck Soils Research
Farm in Laingsburg, MI on a Houghton muck ﬁeld previously planted to carrot. Seven-
week old celery transplants were planted 7 in. apart in rows spaced 32 in. apart on 25
May. Each plot consisted of a 20 ft-long row with 5 ft of unsprayed row between
adjacent plots in the same row. Two unsprayed buffer rows separated adjacent treatment
rows. Treatments were replicated four times in a randomized complete block design.
Weeds, insects, and fertilization were managed according to standard production
practices. Septoria apiicola inoculum (2.9 x 107 spores/ﬂ 02) was prepared by soaking
dried infected celery leaves for 10 min in water and straining the spore suspension
through two layers of cheesecloth. Inoculum was applied to all plants in the trial, several
hours after fungicide treatments were applied on 6 and 13 Jul, using a hand-pump
backpack sprayer with one hollow cone nozzle that delivered approximately 12 gal/A.
Fungicides were applied with a C02 backpack boom sprayer equipped with three XR8003
ﬂat fan nozzles spaced 18 in. apart and calibrated to deliver 50 gal/A at a boom pressure
of 52 psi. Six applications were made at weekly intervals, except for plots treated
biweekly with Bravo Ultrex 82.5WDG, which received three applications on alternate
dates. Leaf blight severity was evaluated on 11 and 16 Aug, and ten plants from the
middle of each treatment row were hand-harvested and trimmed to fresh market
speciﬁcations (14 in. length) on 17 Aug. Petiole disease incidence and severity were
assessed at harvest, diseased petioles were removed from the plants, and yields were
recorded. Data were analyzed using the general linear models procedure in SAS.
Average monthly minimum and maximum air temperatures (°F) were 39.3 and 65.3 in
May, 57.5 and 82.7 in Jun, 58.4 and 84.4 in Jul, and 56.5 and 81.9 in Aug. Rainfall
totaled 1.2, 2.3, 2.8, and 3.2 in. for the same respective months.

Disease symptoms were ﬁrst observed on leaves throughout the control plots on
18 Jul. Untreated control plants were severely diseased at harvest, which necessitated
extensive trimming and resulted in low yield. Four fungicide programs provided season-
long disease prevention on petioles, including Amistar 80WG alternated with Tilt 3.6EC,
Amistar 80WG alternated with Bravo Ultrex 82.5WDG, Pristine 38WG alternated with
Bravo Ultrex 82.5WDG, and Bravo Ultrex 82.5WDG applied weekly. Trimmed yields of
plants treated with TM 473 4F alternated with Bravo Ultrex 82.5WDG, and those treated
biweekly with Bravo Ultrex 82.5WDG, did not differ signiﬁcantly from yields of plants
treated with fungicide programs that prevented petiole infection. Treatments that
included Endura 70WG or Endorse 11.3DF offered limited protection and were generally
inferior to the standard Bravo Ultrex 82.5WDG weekly treatment. Applications of
Serenade Max 14.6WP alternated with Bravo Ultrex 82.5WDG, Kocide 2000 53.8DF,
and Botran 75-W were ineffective at limiting late blight and resulted in disease levels and

144

yields similar to the untreated control plants. Symptoms of phytotoxicity were not
observed.

 

 

 

Treatment and rate/A (application Leaf blight (%)y P611016 blight (%)x Trimmed
sequence’) 11 Aug 16 Aug Incidence Severity yield (lb)
Untreated control ....................................... 40.0 dw 51.3 de 100.0 c 72.5 cd 3.9 d
Amistar 80WG 5 oz (1,3,5)

Tilt 3.6EC 4 ﬂ oz (2,4,6) ........................... 0.0 a 0.8 a 0.0 a 0.0 a 21.9 a
Amistar 80WG 5 oz (1,3,5)

Bravo Ultrex 82.5WDG 1.8 lb (2,4,6) ....... 0.0 a 0.5 a 0.0 a 0.0 a 20.7 a
Bravo Ultrex 82.5WDG 1.8 lb (1-6) .......... 0.8 a 1.0 a 0.0 a 0.0 a 19.7 a
Pristine 38WG 10.5 oz (1,3,5)

Bravo Ultrex 82.5WDG 1.8 lb (2,4,6) ....... 0.5 a 0.8 a 0.0 a 0.0 a 18.1 ab
TM 473 4F 2.9 ﬂ oz (1,3,5)

Bravo Weather Stik 6SC 1.5 pt (2,4,6) ...... 1.0 a 2.0 a 50.0 b 1.0 a 18.1 ab
Bravo Ultrex 82.5WDG 1.8 lb (1,3,5) ....... 2.0 a 7.5 a 75.0 bc 8.0 ab 17.4 ab
Endura 70WG 4.6 oz (1-6) ........................ 7.5 b 17.5 b 100.0 c 11.3 ab 13.2 bc

Endorse 11.3DF 7 oz (1,3,5)
Bravo Ultrex 82.5WDG 1.8 lb (2,4,6) ....... 13.8 c 27.5 c 100.0 c 23.8 b 8.8 cd

Serenade Max 14.6WP 2 lb (1,3,5)
Bravo Ultrex 82.5WDG 1.8 lb (2,4,6) ....... 36.3 d 43.8 d 100.0 c 56.3 c 6.3 d

Kocide2000 53.8DF1.51b(l-6) ............... 38.8 d 48.8 de 100.0 c 68.8 cd 5.2 d
Botran 75-W21b(l-6) .............................. 40.0 d 55.0 e 100.0 c 75.0 d 4.7 d

 

2 Application sequence: 1 = 6 Jul; 2 = 13 Jul; 3 = 21 Jul; 4 = 27 Jul; 5 = 3 Aug; 6 = 11
Aug.

y Evaluated using a leaf blight assessment key representing the % of diseased leaf area.

x Incidence = % of trimmed plants with one or more petiole lesions; severity = % of
petiole surface area with symptoms.

W

Means within a column followed by the same letter are not signiﬁcantly different
according to Tukey’s Studentized Range Test (P 5 0.05).

145

CELERY (Apium graveolens var. dulce ‘Dutchess’) R.S. Bounds and MK. Hausbeck
Bacterial leaf spot; Pseudomonas syringae pv. apii Michigan State University
Dept. of Plant Pathology
East Lansing, MI 48824

Evaluation of products for managing bacterial leaf spot of celery, 2005.

This study was conducted at the Michigan State University Muck Soils Research
Farm in Laingsburg, M1 on a Houghton muck ﬁeld previously planted to carrot. Seven-
week old celery transplants were planted 7 in. apart in rows spaced 32 in. apart on 25
May. Each plot consisted of a 20 ﬁ-long row with 5 ft of unsprayed row between
adjacent plots in the same row. Two unsprayed buffer rows separated adjacent treatment
rows. Treatments were replicated four times in a randomized complete block design.
Weeds, insects, and fertilization were managed according to standard production
practices. Products were applied with a C02 backpack boom sprayer equipped with three
XR8003 ﬂat fan nozzles spaced 18 in. apart and calibrated to deliver 50 gal/A at a boom
pressure of 52 psi. Eleven applications were made at weekly intervals starting on 1 Jun.
Disease developed from natural infection of the transplants. Ten plants from the middle
of each treatment row were hand-harvested, trimmed to fresh market speciﬁcations (outer
petioles removed and plants cut to l4-in. length), and weighed on 17 Aug. Since little
difference in disease was detected among treatments on 17 Aug (data not shown), four
additional plants were hand-harvested from each treatment row and evaluated for disease
on 30 Aug. Outer petioles were removed but not the upper foliage. Terminal leaves were
removed from all marketable petioles (petioles on which the ﬁrst node was 3 9 in. above
the base of the plant), and leaf spot severity was visually assessed on the detached leaves
by estimating the percentage of leaf area with bacterial leaf spot symptoms. Similarly,
disease severity was assessed for 10 whole plants that were not harvested/plot. Disease
incidence was determined by the total number of marketable petioles and the number of
marketable petioles with leaf spot symptoms on the leaves. Data were analyzed using the
general linear models procedure in SAS. Average monthly minimum and maximum air
temperatures (°F) were 39.3 and 65.3 in May, 57.5 and 82.7 in Jun, 58.4 and 84.4 in Jul,
and 56.5 and 81.9 in Aug. Rainfall totaled 1.2, 2.3, 2.8, and 3.2 in. for the same
respective months.

Disease symptoms were ﬁrst observed on newly expanded leaves on 8 Jun. The
causal agent was conﬁrmed as Pseudomonas syringae pv. apii by BIOLOG analysis
conducted by Diagnostic Services at Michigan State University. Disease levels were
moderate at the time of ﬁnal evaluation but not high enough to warrant additional
trimming of diseased leaves. All treatments, except those that included Serenade Max
14.6WP or Citraplex 20% Copper, reduced leaf spot severity relative to the untreated
control. Applications of Actigard SOWG + Kocide 2000 53.8DF resulted in the lowest
leaf spot severity but did not differ signiﬁcantly from the standard treatment of Kocide
2000 53.8DF (1.5 lb/A) applied alone. No signiﬁcant rate response was observed with
Kocide 2000 53.8DF applied at 1 vs. 1.5 lb/A. Leaf spot severity evaluations on terminal
leaves and whole plants yielded similar results, but the latter required less time for
evaluation. Only Serenade Max 14.6WP + Kocide 2000 53.8DF signiﬁcantly reduced

146

the incidence of marketable petioles with diseased leaves relative to the untreated control,
yet this control was not statistically superior compared to that achieved with the other
treatments. Future studies could quantify P. syringae pv. apii populations on leaves of
the treatments included in this study. Symptoms of phytotoxicity were not observed.

 

Leaf spot severity (%) Incidence of

petioles with Trimmed

 

Treatment and rate/A (application

 

sequence“) Terminal leaves Whole plants diseased leaves (%) yield (lb)
Untreated control ........................................... 26.3 c“ 28.8 c 67.6 b 19.5
Actigard 50WG 0.5 oz

+ Kocide 2000 53.8DF 1.5 lb (1-11) ............. 4.0 a 6.5 a 50.3 ab 19.8
Tanos 50DF 8 02

+ Kocide 2000 53.80F 1.5 lb (1,3,5,7,9,l l)

Kocide 2000 53.8DF 1.5 lb (2,4,6,8,10) ........ 11.3 ab 11.3 a 51.8 ab 20.3
Kocide 2000 53.8DF 1.5 lb (1-11) ................. 10.0 ab 12.5 ab 57.5 ab 20.3
Kocide 2000 53.8DF 1 lb (1-11) .................... 13.8 ab 13.8 ab 51.6 ab 19.2
Messenger STS 3WDG 6 02

+ Kocide 2000 53.8DF 1.5 lb (1-1 1) ............. 13.8 ab 15.0 ab 55.3 ab 19.3
Serenade Max 14.6WP 3 lb

+ Kocide 2000 53.8DF 1.5 lb (1-11) ............. 16.3 be 17.5 abc 47.2 a 20.1
Citraplex 20% Copper 8 oz (1-1 1) ................. 26.3 c 25.0 be 57.2 ab 18.8

 

* Application sequence: 1 = 1 Jun; 2 = 8 Jun; 3 = 16 Jun; 4 = 22 Jun; 5 = 29 Jun; 6 = 6
Jul; 7= 13 Jul; 8 =21 Jul; 9=27 Jul; 10=3 Aug; 11 =11 Aug.

** Means within a column followed by the same letter or no letter are not signiﬁcantly
different according to Tukey’s Studentized Range Test (P _<_ 0.05).

147

CELERY (Apium graveolens ‘Dutchess’) R.S. Bounds and MK. Hausbeck
Early blight; Cercospora apii Michigan State University
Department of Plant Pathology
East Lansing, MI 48824

Evaluation of calendar-based schedules and TOM-CAST DSV thresholds for timing
the initial fungicide treatment to manage early blight of celery, 2005.

This study was conducted at a cooperator’s farm in Ottawa County, M1 on a
Houghton muck ﬁeld previously planted to onion. Eight-week old celery ‘Dutchess’
transplants were planted 7.5 in. apart in rows spaced 20 in. apart on 16 Jun. Temperature
and leaf wetness sensors were placed in the ﬁeld at the time of transplanting to monitor
TOM-CAST DSV accumulation throughout the growing period. Treatment plots
consisted of one row 20 ft long with 5 ft of unsprayed buffer between plots in the same
row. Two buffer rows were leﬁ unsprayed between each treatment row. Seven
treatments were replicated four times in a randomized complete block design. Weeds,
insects, and fertilization were managed according to standard production practices.
Fungicides were applied with a C02 backpack sprayer equipped with two XR8003 ﬂat
fan nozzles spaced 19.5 in. apart and calibrated to deliver 50 gal/A at a boom pressure of
53 psi. Sprays were initially applied to the early initiation and preventive treatments on
23 Jun and 15 Jul, respectively, representing 1 and 4 weeks after transplanting. Sprays
for the remaining treatments were initially applied when 10,20, 30, and 40 TOM-CAST
DSVS had accumulated since transplanting and occurred on 27 Jun, 1 Jul, 11 Jul, and 17
Jul, respectively. Subsequent applications were made to all fungicide treatments
according to TOM-CAST 10-DSV. Leaf blight severity was evaluated and ten plants
from the middle of each treatment row were hand-harvested on 31 Aug. Plants were
trimmed to fresh market speciﬁcations (14 in. length) and weighed.

Early blight lesions were detected on 24 Aug in unsprayed buffer and untreated
control rows. At harvest, untreated plants had very few early blight lesions on the leaves
(covering <1% of the total leaf area) and none on the petioles. There were no yield
differences since the disease did not occur on the petioles. No phytotoxicity was
observed.

148

 

Treatment and rate/A
Timing of ﬁrst application‘ (no. sprays) Leaf blight severity (%)y Total yield (lb)"

Untreated control ............................................. 0.8 17.7

Bravo Ultrex 82.5WDG 1.8 lb applied at
TOM—CAST lO-DSV intervals

Early (10) ................................................... 0.0 17.8
Preventive (8) ............................................ 0.0 21.3
TOM-CAST lO-DSV (10) ......................... 0.0 19.2
TOM-CAST 20-DSV (9) ........................... 0.3 18.5
TOM-CAST 30-DSV (8) ........................... 0.0 17.2
TOM-CAST 40-DSV (7) ........................... 0.0 20.5

 

z The ﬁrst fungicide application for the early and preventive programs were made 1
and 4 weeks after transplanting, respectively, and sprays were applied thereafter
according to the TOM-CAST lO-DSV interval. The ﬁrst fungicide application for the
remaining treatments was applied after 10, 20, 30, or 40 DSVs had accumulated after
transplanting; subsequent sprays were applied thereafter according to the TOM-

CAST lO-DSV interval.
y Evaluated using a leaf blight assessment key representing the percentage of diseased

leaf area.
x Ten plants per plot were trimmed to fresh market speciﬁcation (1 4 in. length) and

weighed on 31 Aug.

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