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This is to certify that the

dissertation entitled

Epidemiology of Mildews of
Floricultural Crops

presented by

Janet M. Byrne

has been accepted towards fulfillment
of the requirements for

Ph . D degree in Plant Pathology

 

Major professor

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Date [71'd5‘0a

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EPIDEMIOLOGY OF MILDEWS ON FLORICULTURAL CROPS
By

Janet M. Byrne

A DISSERTATION
Submitted to
Michigan State University
In partial fulfillment of the requirements
for the degree of
DOCTOR OF PHILOSOPHY

Department of Plant Pathology

2002

ABSTRACT
EPIDEMIOLOGY OF MILDEWS ON FLORICULTURAL CROPS
By

Janet M. Byrne

In the US. tloriculture crops are grown in the greenhouse as potted plants or in
the field as cut flowers. Due to their high value and the consumer‘s low tolerance for
blemishes. foliar diseases are managed intensively. The mildews are especially
troublesome because epidemics can develop rapidly and are difficult to control. In this
study. the epidemiology of powdery mildew (Oia’ium sp.) on greenhouse poinsettias and
downy mildew (Perommpora umirrhini) on field-grown snapdragons was investigated.

Atmospheric conidial concentrations (ACC) of ()idizmz Sp. in research
greenhouses containing infected poinsettias were monitored to investigate the role of
environment in prompting conidial release and dissemination. The influence of
temperature on disease development was studied by placing healthy poinsettias in each
greenhouse for 7-day periods. removing them. and recording the days to the appearance
of the first colony. When averaged over 5 December to 1 June. ACC were greatest during
1000 to 1800 h. Large numbers of conidia were sampled (2100/m3) within l-h periods
indicating conidial release events (CRES). Fluctuations in relative humidity (RH)
prompted CREs. In both greenhouses. CREs (up to 23) occurred following RH
fluctuations of 5 to 15%. When greenhouse temperatures exceeded 25C for 19 days in
March and 21 days in May ACC were reduced by 275% from the previous months.

Conidial germination and infection processes of ()idium Sp. were quantified on

poinsettia foliage. Leafdisks were inoculated with conidia Of ()idizmz sp. and incubated
at 15, 20 and 25°C in chambers with glycerol/water solutions that provided 35. 50. 65, 80
and 92% (21:29/0) relative humidity (RH). Formation of appressoria and primary germ
tubes were favored by 20°C. Haustorium formation appeared to be favored by 20°C and
35. 50. and 65% RH. The average length of germ tubes produced at 25°C (36.2um) was
longer than at 20°C (27.0um) and 15°C (1 1.1um). Effects Oftemperature on sporulation
of Oiclium sp. were quantified on inoculated leafdisks incubated for 14 days at 15 and
20°C under high relative humidity. Temperature had a significant effect on the number of
conidia produced per conidiophore. At 15 and 20°C. a maximum Of4 and 7 conidia were
produced per conidiophore. respectively.

Atmospheric concentrations of Peronospora amirrhini conidia in a commercial
snapdragon production field were monitored over three growing seasons to investigate the
influences environment has on the concentration of airborne conidia. The incidence of
downy mildew among different cultivars was also determined. Atmospheric conidial
concentrations followed a diurnal pattern and were greatest during 0500 to 1200 h.
Minimum daily temperatures below 60°C and maximum daily temperatures above
300°C limited AC C . Long dew periods (.:6 hours.) were associated with relatively large
conidia releases. Consecutive days with short leaf wetness periods suppressed ACC.
Snapdragon cultivar had a significant effect on area under the disease progress curve.

The information gained from this study will further the understanding of the
relationship between the environment and disease development and may be incorporated

into current disease management programs. enhancing their effectiveness.

ACKNOWLEDGEMENTS

I owe thanks to many people who have helped me accomplish this work. My
major professor. Dr. Mary I-lausbeck. has been a tremendous source of both personal and
professional support. It is largely through her encouragement that l was able to complete
my studies while initiating a career in plant pathology.

The assistance provided by undergraduate students who spent many hours at a
microscope with the tedious task Ofcounting spores is greatly appreciated. I also received
a great deal Of technical support from Brian Cortright. Sheila Linderman and Bill
Quackenbush. My committee members Dr. Ray Hammerschmidt. Dr. Andrew Jarosz and
Dr. Joseph Vargas are to be commended for their critical evaluations and flexibility.

Lastly. I would like to thank my family. both immediate and extended. who

supported me in countless ways during this endeavor.

iv

TABLE OF CONTENTS

LIST OF TABLES ...................................................... vi
LIST OF FIGURES .................................................... vii
LITERATURE REVIEW
Powdery Mildew .................................................. 2
Downy Mildew ................................................... 9
Literature Cited .................................................. 16

CHAPTER I.

Factors affecting concentrations of airborne conidia of Oidium sp. among poinsettias in a

greenhouse.

Abstract ........................................................ 23
Introduction ..................................................... 24
Materials and Methods ............................................. 25
Results ......................................................... 28
Discussion ...................................................... 38
Literature Cited .................................................. 42

CHAPTER II.

Influence of environment on infection and sporulation of Oidium sp. on poinsettia foliage.
Abstract ........................................................ 46
Introduction ..................................................... 47
Materials and Methods ............................................. 48
Results ......................................................... 51
Discussion ...................................................... 58
Literature Cited .................................................. 61

CHAPTER III.

Influence of environment on atmospheric concentrations of Peronospora antirrhim‘
conidia in field grown snapdragons.

Abstract ........................................................ 64
Introduction ..................................................... 65
Materials and Methods ............................................. 66
Results ......................................................... 69
Discussion ...................................................... 86
Literature Cited .................................................. 92

LIST OF TABLES

CHAPTER I.

Table 1. Number ofconidial release events associated with relative humidity
fluctuations (positive or negative) occurring between 5 December to 1
June (greenhouse l l) and 5 December to 17 July (greenhouse 2) ...... 32

CHAPTER III.

Table 1. Association of atmospheric conidial concentration of Peronospora
antirrhini with rainfall. leaf wetness and cold temperatures from 2
December 1998 to 13 April 1999. .............................. 71

Table 2. Association Ofatmos heric conidial concentration of Peronos 70m
P l
amirrhini in Sites 1 and 2 with rainfall. leaf wetness and cold
temperatures from 22 November 1999 to 5 May 2000. .............. 72

Table 3. Association of atmospheric conidial concentration of Peronospora
antirrhini with rainfall. leaf wetness and cold temperatures from 14

December 2000 to 18 April 2001. .............................. 73

Table 4. AUDPC values of disease ratings of Peronospora antirrhini on field
grown snapdragon cultivars. .................................. 86

vi

CHAPTER I.

Figure

Figure

Figure

Figure

Figure

CHAPTER 11.

Figure

Figure

Figure

Figure

LIST OF FIGURES

l. Hourly average concentration of airborne conidia of Oidium sp. in
research greenhouses 2 (O) and l 1(I). during 5 December to 1 June. . . . 29

2. Association of greenhouse activity. including watering (V) and

venting (O). and fluctuation of relative humidity with concentration Of
airborne conidia of ()idz‘um sp. in research greenhouses 2 (A) and l 1.

(B) during 13 through 16 March. .............................. 31

3. Extended conidial release event period and associated greenhouse
activity. including watering (V) and venting (O), and fluctuations of
relative humidity in greenhouse 2 during 28 through 31 March ........ 34

4. The A) days to colony development on poinsettias following

exposure to atmospheric conidial concentrations. B) total number of
conidia released during the trap week. and C) number of days the

average hourly temperatures exceeded 25C and 30C in research
greenhouse 2 during 23 January to 10 July. ...................... 36

5. The A) days to colony development on poinsettias following

exposure to atmospheric conidial concentrations. B) total number of
conidia released during the trap week, and C) number of days the

average hourly temperatures exceeded 25C and 30C in research
greenhouse l 1 during 23 January through 10 July .................. 37

1. Effect of temperature and relative humidity on A) shriveling. B)
germination. C) appressorium formation. D) haustorium formation.

E) primary germ tube formation. and F)secondary germ tube formation

of Oidium sp. on poinsettia foliage. ............................. 53

2. Effects of temperature and relative humidity on the length of germ
tubes produced by conidia of Oidium sp. on poinsettia foliage ....... 55

3. The effect of temperature (15 and 20°C) on conidiophores
produced per mm2 of leaf disk. 14 days after inoculation. ........... 56

4. Sporulation of Oidium sp. on poinsettia leaves, incubated at 15
and 20°C. 14 days after inoculation. ............................ 57

vii

CHAPTER III

Figure 1. Hourly average concentration of airborne conidia of Peronospora
umirrhini in research plots from (I) 2 Dec. 1998 to 15 April. 1999.
from (V) 23 Nov.. 1999 to 5 May 2000. and from (I) 14 Dec. 2000 to
18 April. 2001. ............................................. 70

Figure 2. Number of conidia of Peronospora amirrhini trapped per day and
associated minimum (0) and maximum (V) and average temperatures
(0). relative humidity (I). rainfall and leaf wetness (0) during 14 Dec
1998 to 13 April 1999. ....................................... 76

Figure 3. Number of conidia of Peronospora antirrhini trapped per day and
associated minimum ( O) and maximum (V) and average temperatures
(0), relative humidity (I). rainfall and leaf wetness (O) in section 1
during 14 Dec. 1999 to 13 April 2000. .......................... 78

Figure 4. Number of conidia of Peronospora antirrhini trapped per day and
associated minimum (0) and maximum (V) and average temperatures
( 0). relative humidity (I). rainfall and leaf wetness (0) during 14 Dec
2000 to 13 April 2001. ...................................... 80

Figure 5. Association of atmospheric conidial concentration of Peronospora
antirrhini in snapdragon fields with temperature (V), relative humidity
(0), leaf wetness (I) and rainfall during 12 through 18 February 1999. . 83

Figure 6. Association of atmospheric conidial concentration of Peronospora
cmtirrhini in snapdragon fields with temperature (V). relative humidity
(0). leaf wetness (I) and rainfall in sites I and 2 during 7 through 13
March 2000. ............................................... 85

Figure 7. Disease progress curve of Peronospora antirrhini for 7 snapdragon
cultivars from 7 March 2001 (Julian day 66) to 8 May 2001 (Julian day
128). ..................................................... 87

viii

LITERATURE REVIEW

POWDERY MILDEW OF POINSETTIA

Poinsettias (Euphorbia pulcherrinm Willd ex Klotz) have a wholesale value in the
United State of $237 million (14). Michigan growers produce 7% ofthe total U. S. crop
(14). Poinsettias. have become a symbol of Christmas throughout the world. since being
introduced to the United States from Mexico in 1825. In their native habitat poinsettias
are a perennial. Shrub-like plant. but are grown in the US. as a seasonal greenhouse-
grown ornamental. Small flowers. or cyathia. are surrounded by brightly colored bracts
ranging from red to variegated to white: flowering is initiated under Short-day/long-night
photoperiods.

Poinsettia cultivars are developed. maintained and patented by a few large primary
propagators. Desirable cultivars are propagated vegetatively by taking tip or terminal
cuttings from the branches of stock plants and placing them in rooting medium (12).
Primary propagators maintain stock plants all year for cutting production. Secondary
propagators purchase cuttings for stock plant production from primary propagators
beginning in early spring. Cuttings from these stock plants are sold in midsummer tO
finishing growers who produce the flowering crop.

Historically, the most significant diseases of poinsettia have been leaf, stern. and
bract blight caused by Botrytis cinerea Pers.:Fr., and root rot caused by Pythium sp.
Pringsh, nom.cons. Management of both foliar and root rot diseases is a concern
throughout all stages of poinsettia production. Most growers use regular calendar-based
fungicide applications throughout the growing season to control these diseases. High
fungicide costs. increasing energy, heating, and shipping costs and low retail prices set by

large chain stores have greatly reduced the profitability of poinsettia production for

finishing producers. As a result. cost effective disease management strategies are of
interest to many producers.

Powdery mildew occurred on poinsettias in Mexico in 1988 and 1989. The
disease was first reported in the United States in 1990, and was epidemic in Michigan in
1992 (19). Growers in Puerto Rico have reported powdery mildew outbreaks, and the
disease has also been observed on poinsettias in the landscape (9). Although the disease is
not common. it occurs yearly and is an economically significant problem, especially for
poinsettia growers in the midwestem and northern United States.

Powdery mildew fungi are characterized by the hyaline or white mycelium
produced on external host surfaces. Taxonomically these fungi are grouped in the
Erysiphaceae. order Erysiphales. class Ascomycetes. and are defined based on the sexual
(perfect) stage. Although many genera relying on descriptions of conidia and
conidiophores exist in the literature. classification based exclusively on the anamorph form
is discouraged.

In the absence of an Observed teleomorph, the causal agent of this powdery
mildew is referred to simply as ()I'dium sp. and is recognized by its production of conidia
in chains on upright conidiophores characterized by an arched basal cell. The conidia are
oval to cylindrical. 15 to 20 x 25 to 50 um, with a chain of up to nine conidia on a single
conidiophore (26). The genus OI'dium was established by Link in 1809 (51), and has been
widely used for imperfect stages of Erysiphaceae. At least 89 species of Oidium have
been named. and can infect over 1100 host species (51).

Disease signs.

Signs of powdery mildew appear as small, white, talcum-like colonies on bract or

leaf surfaces. C hlorotic spots appear Opposite colonies on abaxial leaf surfaces. Under
favorable conditions colonies enlarge and coalesce. blighting sections of the leaf. Stern
and petiole tissue can also be infected. Severe infection results in defoliation.

In the initial stages of an epidemic initiated in the summer colonies form more
frequently on the abaxial surface. where temperatures are cooler as a result of canopy
shade and close proximity to the moist surface of the growing medium. If colonies first
develop on the undersides of leaves. the disease may go undetected until late in production '
when bracts become infected. making plants unsaleable.

Temperature.

Optimum temperatures reported for powdery mildews (21.0°C) are generally
lower than those for other plant pathogens (2). Kim et al. (26) observed that plants with
active powdery mildew colonies. grown in a greenhouse throughout the summer and
exposed to high temperatures and solar radiation. showed no signs of infection for a
period of time. Controlled studies comparing effects of temperature on Oidz'um sp. on
poinsettia found conidial germination was significantly reduced at 30°C versus 200°C (4).
Temperature also influences conidial germination. haustorium formation, development of
secondary germ tubes, and sporulation. At 300°C, formation of secondary germ tubes
was significantly limited (<1 %) compared with 200°C (253%). Less than 4% of
germinated conidia formed haustoria at 300°C. compared with >53% of those at 200°C
(4).

Relative humidity.
Unlike many other fungal plant pathogens. powdery mildew does not require leaf

wetness for germination or penetration. The level of xerophytism differs depending on the

species of powdery mildew. Several researchers have observed germination of powdery
mildew conidia at very low humidities (<25%) (10,50). This ability has been attributed to
the moisture retained in large water storing vacuoles in the relatively large powdery
mildew conidia (’53). Others have suggested that the great water retaining ability of
powdery mildew condia, created by a gelatinous sheath in the cell wall, is a more
important factor (44). The ability ofa conidium to germinate in low relative humidity
conditions is a factor of temperature, tolerance to low relative humidity decreases with
increasing temperatures. C onidia of even the most xerophytic species will shrivel and lose
viability under low relative humidity and warm temperatures.

Infection process.

Powdery mildews are biotrophic pathogens, requiring live plant tissue to infect,
grow. and reproduce. As a result, these pathogens have specialized infection processes,
which minimize cell death or in some cases delay cell death until the pathogen has
completed reproduction. Exceptions to this are some Phyllactinia species that have
surface mycelia that infect through stomata to the mesophyll tissue where haustoria are
formed. and fungi in the genus Leveillula that form well-developed internal mycelia.

The infection process begins with the production of germ tubes by a conidium on a
host surface. Germ tubes are more sensitive to relative humidity than conidia. Conidia of
Erysiphe gramim's germinate at extremely low relative humidity (< 10%) but germ tube
growth is poor below 98% relative humidity (32). Germ tube length of several Erysiphe
and Phyllactim'a species are affected by low relative humidity (2).

Once cued by the host surface, the germ tube lays down septation and

differentiates into an appressorium. The host cuticle is penetrated with mechanical force

created by the appressorium and an infection peg. Cell wall degrading enzymes are
involved in cell wall penetration by some Erysiphe species (8). In defense. the host
deposits electron dense cytoplasm directly beneath the appressorium. Papilla are then
formed which help prevent infection in incompatible host pathogen interactions.

Most powdery mildew fungi are restricted to the epidermal cell layer of the host.
Powdery mildew are intracellular pathogens, penetrating the host cell wall but do not enter
the cytoplasm of the host cell. The host plasmalemma invaginates allowing room for the
formation ofa haustorium. The haustorium suppresses further defense responses of the
host and absorbs nutrients from the host which are supplied to the hyphae. The shape of
the haustorium varies with the species and ranges from nearly spherical to an ellipsoid
center with many finger-like projections or lobes. Successful formation of a haustorium is
essential for further hyphal growth. Additional hyphae. develop from the appressorium
and contribute to the expanding colony. Additional haustoria are produced in other
epidermal cells.
Reproduction.

Asexual reproduction occurs on the host surface. where conidia are formed on
upright conidiophores. Some species produce a single conidium every 24 hours, while
others produce daily chains of conidia (2). Powdery mildew fungi, with the exception of
E. gramim's (18). produce conidia in a diurnal pattern. The periodicity of powdery mildew
which form conidia in chains is determined by the final stage of sporulation. when the
abstriction of the conidium from the germ cell of the conidiophore occurs (2). This
maturation stage is photosensitive, proceeding faster in light than in dark; therefore.

conidia produced at night remain immature and are released once exposed to light (6).

Condia dispersal.

Conidial release of Oidium sp. of poinsettias is associated with relative humidity
fluctuations. but the mechanism of release is not known. Both passive and active
liberation mechanisms have been proposed for powdery mildew species (28). Mechanical
disruption (18). wind speed (16). and humidity (l) or temperature changes have been
associated with large conidial releases of powdery mildews. In the field, wind plays a
significant role in the release and dispersal of the conidia of many powdery mildew
species, either directly or as a result of leaf movement dislodging conidia (13,17,49). An
alternative method of dispersal is hygroscopic twisting in response to changing relative
humidity. This twisting causes movement capable of dislodging spores of Phytophthora
and Peronospora spp. (28). However. Jarvis (22) concluded that hygroscopic movements
loosen spores of B. cinerea for subsequent rain-splash dispersal. Adams (1) proposed that
a change from high to low relative humidity creates an electrical charge with sufficient
voltage to remove powdery mildew conidia from conidiophores.

Disease management.

The ability of powdery mildew to become epidemic when poinsettias have colored
bracts has made disease management difficult. Scouting the poinsettia crop for early signs
of the disease can ensure timely fungicides applications prior to bract coloration. It is
recommended that growers inspect eight fully expanded leaves of each newly received
plant (19). Growers scout one out of 30 plants weekly thereafter. If powdery mildew is
detected, the infected leaf tissue is removed carefully to deter further spread of conidia, an
appropriate fungicide is applied, and scouting increases to one out of 10 plants weekly.

Once the surrounding plants have been free of disease for three weeks, scouting drops to

one out of 30 plants weekly. Without carefully scouting for colonies that develop on the
undersides of leaves. disease may go undetected until late in production when bracts
become infected. making plants unsaleable. However. frequent scouting is time consuming
and difficult for large greenhouse operations that may have more than an acre of closely-
spaced poinsettias.

All popular poinsettia cultivars are susceptible to powdery mildew. Celio and
Hausbeck (3) observed higher disease incidence among poinsettias with red bracts than
those with pink. variegated. or white bracts. Kim et al.. (25) found that ‘Dark Red Hegg’,
was significantly less susceptible than 'Freedom Red’, both are red bract varieties.

Several fungicides are labeled for use against this disease; Strike (triadimiefon) and
Teraguard 50W (triflumizole) are both effective systemic products. Fungicide costs range
from $0.48 to $2.44 per 1000 square feet of greenhouse area (19). and reduce growers’
profits. Although applications of appropriate fungicides to poinsettia bracts prevent
further colony development. the fungal mycelium remains visible and is commercially
unacceptable. Additionally. residue and phytotoxicity resulting from fungicide

applications to the bracts may result in an unmarketable crop.

DOWNY MILDEW OF SNAPDRAGON

Snapdragons (Antirrhimrm majus L. (Huxley et al.. 1992)) are produced for
several markets. including cut flowers. bedding plants, and potted plants. The cut flower
portion has a wholesale value in the US. of $1 8.5 million (14). Dwarf cultivars are a
common greenhouse bedding plant crop. Cut flowers are produced in greenhouses or in
fields. Taller cultivars (90 to 180 cm) are grown for cut flowers. The majority of the U.
S. snapdragon cut flower production occurs in California and Florida(14). Snapdragons
are increasing in popularity with floral designers and consumers and has become a
profitable crop for growers looking to diversify their production with additional crops.

Snapdragons are seed propagated. Commercial producers buy finished plugs from
specialty propagators and transplant them into production beds. The tall cultivars used for
cut flower production require support. which is provided by mesh wiring or netting, to
produce the tall-straight stalks (29). As the plant height increases the wiring is raised to
provide continued support. In Florida. the second largest production state, planting
occurs weekly from September to December. Planting is scheduled to provide a large
volume of spikes for spring occasions such as Mother’s and Valentine’s days.

Snapdragons are a cool season crop, preferring temperatures between 7 to 10°C
(7). Once flower initiation has occurred, night temperatures greatly influence flowering
time and quality. When grown under cooler temperatures flowering spikes have thick
stems and compact flowers, which are desirable characteristics that increase crop value.
Spikes are harvested by hand when one-third of the florets are open.

Several commercial seed producers conduct plant breeding programs with tall

snapdragons cultivars, most of the commonly grown cultivars are FI hybrids. Cut flower

cultivars are classified into flowering response groups. each with Specific combinations Of
night temperature. light level and photoperiod duration (7.11). Cultivars are available in a
wide variety Ofcolors including, white, red. pink. rose, yellow. orange, and several
bicolors. The coloration of the foliage varies with some cultivars. for example the orange
and red varieties have purplish foliage.

Snapdragons are affected by several different foliar diseases including Botrytis
cinerea (Pers.:F r). rust (Puccinia antirrhini). powdery mildew (OI'dium sp.), and downy
mildew (Peronospara amirrhini Schroet.. Hedwigia). Peronospora antirrhini infects
snapdragon foliage both locally and systemically. causing economically significant damage
(48). Local infections produce pale lesions that are visible on the upper leaf surface.
Systemic infections result in downward curling of foliage, and shortened intemodes. The
disease can kill terminal buds of young transplants. causing undesirable branching, which
decreases the crop value (48).

The downy mildews are obligate pathogens characterized by fungal morphology
and their ability to rapidly reach epidemic proportions. The group contains economically
significant pathogens of several important crops including tobacco, grapes. hops, sugar
beets, sunflowers. soybeans and brassieas (47). Taxonomically these fungi are grouped in
the Peronosporaceae, this group includes the genera Peronospora. Pseudoperonospora,
Plasmopora, Bremia, and Sclerospora.

In general. crop losses caused by downy mildews can vary greatly between years
and seasons, depending on climate and weather (52). Leaf wetness duration and
temperatures are known to be critical environmental factors for sporulation, germination,

and infection of downy mildew that affects tobacco (Peronospora tabacina (D.B. Adam))

10

(27.36) , grape (Plasmopara viticola (Berk. & Curt)) (30.31), and lettuce (Bremia
lactucae (Regel)) (40-43).
History of occurrences of Peronospora antirrhim'.

Peronospora amirrhini was described by Schroeter in 1874 on the wild
Antirrhinum oromium. but was not described on cultivated snapdragons until 1936, when
it caused severe damage to snapdragon seedlings (52). Some discrepancy exists in the
literature as to whether both reports were the same pathogen (15,48). In 1940. P.
antirrhini was found on snapdragons in California. From 1940 tol942. the disease was a
problem throughout southern California. such that it was reportedly difficult to Obtain
healthy seedlings (52). At the same time. extensive losses were also occurring in areas of
Europe. During the 1940's, Yarwood and a few other researchers published details of
field observations and results of limited research projects on P. antirrhini (33,52). Since
that time little has been published concerning this pathogen.

Reproduction.

Asexual reproduction in the Peronosporacea occurs through conidia or sporangia,
which release zoospores. Peronospora species do not produce zoospores. their asexual
reproductive structures are true conidia and germinate directly with a germ tube. Conidia
of P. antirrhim‘ are produced externally on the leaf surface and are therefore sensitive to
environmental factors. Sporulation typically occurs on the abaxial leaf surface but with
severe disease pressure and favorable environmental conditions sporulation can occur on
the adaxial leaf surface (personal observation). Conidiophores are dicotomously
branched, and the tips of the branches are narrowed. C onidia are 23-30 x 14-18um.

ellipsoid to ovoid with a small flat area of attachment (15).

ll

The spore release Ofmany downy mildews (P. Iabacina. P. nzans/zw'ica. P. viciae.
B. lactuca. PseI(doperonospora cubensis. P. Immu/i. etc.) follows a diurnal pattern (38).
Sporangia or conidia are produced at night, mature early in the morning and are released
midmorning. The time at which spore release begins varies among species. Conidia of P.
tabacina and P. Iri/Oliorum are released in response to decreasing relative humidity.
When exposed to dry air conidiophores twisted counter-clockwise instantaneously and
spores were forcibly ejected (37). When the conidiophores were exposed to high relative
humidity the twisting was reversed.

Sexual reproduction of downy mildew pathogens may be either homothallic or
heterothallic depending on the species. P. parasitica is somewhat unique in that both
homothallic and heterothallic isolates have been found (34). Oospore production has been
associated with Chlorotic and necrotic tissue. Therefore. oospore formation by several
downy mildew pathogens has been suggested to be induced by senescence of the host
tissue. However. others observed that the onset of sporulation occurs before chlorosis
and is therefore the cause. rather than the result of sporulation (34). Oospore production
is not as sensitive to the environment as conidial production.

Oospores of P. amirrhini are 24-28 pm in diameter and have a thick ridged wall.
In Ireland, oospores were Observed in the root cortex of infected nursery plants. and were
found less frequently in above ground plant tissue (33.34).

Influence of leaf wetness.

Free moisture on the host surface is considered essential for infection by downy

mildew pathogens (3 8). The duration of leaf wetness needed for infection varies with

species. and is also dependent on the temperature during the leaf wetness period. In field

12

Observations Of both Sclerospora sp. and Bremia lactuca. a leaf wetness period between
spore release and evaporation of leaf wetness in the morning was sufficient for infection to
occur (38.42). Infection periods of B. lactuca occurred mainly on days on which leaf
wetness ended late in the morning (0100 h Pacific standard time or later). thereby
extending nightly leaf wetness durations (40.42).

The impact of leaf wetness on conidial production varies with species. Conidial
production of P. Irifbliorum. which is pathogenic on forage legumes. does not occur in
free water on leaves (46). Pseudoperonospora lmmuli (pathogenic on hops) produces
sporangia on dry leaf tissue. but is not inhibited by leaf wetness (45). Other downy
mildews require leaf wetness for sporangium development. A minimum of 6 h of leaf
wetness is required by P. cubensis (pathogenic to cucurbits) for sporangium formation
(3 8).

Influence of relative humidity.

High relative humidity (>90%) is necessary for asexual reproduction of downy
mildew fungi in general (34). Relative humidity level is most critical at night, when
sporangium development occurs. A mean relative humidity below 64% between the hours
Of 2000 and 0600 curtailed sporulation of P. humuli (24), a relative humidity level
between 95 - 100% is required for abundant sporulation (39). P. destructor (pathogenic
on onions) also requires high relative humidity (295%) for sporulation, the critical time
period for this pathogen appears to be between 0200 and 0600 hours (20). The disease
predictive model proposed by Hildebrand (20) included the parameter of relative humidity

above 95% beginning at 0200 hours or earlier and remaining until 0600 hours.

13

Influence of temperature.

Downy mildew pathogens are generally limited by temperatures over 25 - 30°C.
Peronospora amirrhini sporulated on systemically infected snapdragons incubated at 13
and 19°C but not on those incubated at 7 and 22°C ( 52). Yarwood also observed that
plants inoculated during the winter and early spring months were more successful than
inoculations during the hotter summer months. Sporulation of P. destruclor is inhibited
when mean temperatures of the preceding day exceed 23-24°C (21). Limiting effects of
high temperatures become more severe with increasing durations of warm temperatures
(>30°C) (35).

Minimum temperatures below 5°C are limiting to downy mildew pathogens.
Pseudoperonospora humuli and P. cubensis do not produce sporangia when the minimum
night temperature is s5°C (5.24). An analysis of environmental factors over a 28 year
period of hop production determined that minimum daily temperatures were generally
higher in April and May in years with severe epidemics of P. Immuli. than in years with
mild or no epidemics (23). Average minimum daily temperatures were a better predictor
of epidemics of P. humu/i than the average daily temperature (23).

Management of Peronospora antirrhini.

It has not been determined whether this obligate pathogen survives on weeds and
plant debris, or is annually introduced into the production area via seedling transplants
(48,52). Currently, larger producers use chemical fumigants between the growing
seasons to control soilbome pathogens. This option will likely be limited to ornamental
producers in the long term.

Irrigation is required on a regular. almost daily, basis to maintain the crop.

l4

Irrigation is done in late morning or early afternoon. after harvest has been completed.
Timing irrigation to avoid prolonged durations ofleaf wetness may help limit downy
mildew as well as several other foliar diseases.

Tall snapdragon cultivars vary in their sensitivity to downy mildew (personal
observation). Although thorough scouting for signs of infection is helpful for prompting
fungicide sprays growers can target especially susceptible cultivars for scouting. Growers
may also choose cultivars that are more resistant to downy mildew to ease disease
pressure. Fungicides are applied at l - 3 week intervals throughout the growing season to

control this disease.

15

l\)

La)

10.

LITERATURE CITED
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initiating liberation ofconidia of powdery mildews. Phytopathology 76:1239-1245.
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edited by D. M. Spencer. New York: Academic Press.
Celio. G. J.. and M. K. Hausbeck. 1997. Evaluation ofpoinsettia cultivars for
susceptibility to powdery mildew. HortScience 32:259-261.
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formation. and early colony development of powdery mildew on poinsettia.
Phytopathology 88:105-1 13.
Cohen. Y. 1981. Downy Mildew ot'Cucurbits. In: The Downy Mildews. edited by
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Cole. J .S. 1966. Powdery mildew of tobacco (Erysiphe cichoracearum DC) IV.
Conidial content of the air within infected crops. Annals of Applied Biology
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Corr, B.. and Laughner. L. Antirrhinum (Snapdragon). In: The Ball Red Book.
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Strider. New York: Praeger Publishers.
Daughtrey. ML. and J. Hall. 1992. Powdery mildew: A new threat to your
poinsettia crop. Grower Talks, 23-31.
Delp, C. J. 1954. Effect of temperature and humidity on the grape powdery

mildew fungus. Phytopathology 44:615-626.

16

l2.

14.

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l6.

l7.

18.

19.

Dole. J. M.. and H. F. Wilkins. 1999. Antirrhinum. In: Floriculture Principles and
Species. Upper Saddle River: Prentice Hall.

Ecke. P. . O.A. Matkin. and DE. Hartley. eds. 1990. The Poinsettia Manual.
Encinitas. CA: Paul Ecke Poinsettias.

Fernando. TM. 1971. ()idium leaf disease: the effect of environment and control
measures on incidence of disease and atmospheric spore concentration. [Hevea
brasiliensis]. Quarterly .Ioumal of Rubber Research 48: 100-1 1 l.

Floriculture crops, 2000 Summary.2001.. edited by N. A. S. Service. Washington.
DC: United States Department Of Agriculture.

Francis. S. M. 1981. Peronospora antirrhini. In: C Ml Descriptions of Pathogenic
Fungi and Bacteria NO 685.

Frinking. H.D. 1977. Research on wind dispersion Ofrose-mildew spores
(Sphaerotheca pannosa) in field. glasshouse and climate room. Grava 16:155-158.
Hammet. K.R.W. and Manners. J .G. 1971. Conidium liberation in Erysiphe
graminis 1. Visual and statistical analysis of spore trap records. Transactions of
the British Mycological Society 56:387-401.

Hammett. K. R. W.. and J. G. Manners. 1973. Conidium liberation in Erysiphe
graminis. II. Conidial chain and pustule structure. Transactions of the British
Mycological Society 61 :121-133.

Hausbeck. M.. J. Kalishek, M. Daughtrey. and L. Barnes. 1994. Keep the colonies
at bay (Part 1). Greenhouse Growerz45-48.

Hildebrand, P. D.. and J. C. Sutton. 1982. Weather variables in relation to an

epidemic of onion downy mildew Peronospora destructor, near Bradford, Ontario.

17

22.

23.

24.

25.

26.

27.

28.

29.

Phytopathology 72:219-224.

Hildebrand. P. D.. and J. C. Sutton. 1984. Interactive effects of the dark period,
humid period. temperature. and light on sporulation of Peronospora destructor.
Phytopathology 74: 1444-1449.

Jarvis, W.R. 1962. The dispersal of spores of Botrytis cinerea Fr. in a raspberry
plantation. Transactions of the British Mycological Society 45:549-559.
Johnson. D. A._. C.B. Skotland, and J. R. Alldredge. 1983. Weather factors
affecting downy mildew epidemics of hops in the Yakima Valley of Washington.
Phytopathology 73:490-493.

Johnson. D. A.. and C .B. Skotland. 1985. Effects of temperature and relative
humidity on sporangium production of Pseudoperonospora humuli on hop.
Phytopathology 75:127-129.

Kim. S. H.. and A. H. Micheal. 1995. Susceptibility of poinsettia cultivars to
powdery mildew. Phytopathology (Abstr) 85:1 171.

Kim. S. H. . and T. N. Olson. 1995. Recurrence of powdery mildew on over-
summered asymptomatic poinsettias. Phytopathology (Abstract) 85:1 171.
Kucharek. T. A. 1987. Rainfall. irrigation water, and temperatures associated with
first occurrences of tobacco blue mold in leaf production area of north Florida
from 1979 to 1984. Plant Disease 71:336-339.

Lacey, J. 1996. Spore dispersal-Its role in ecology and disease : The British
contribution to fungal aerobiology. Mycological Research 100:641-660.
Langhans. ed. 1962. Snapdragons: a manual of the culture, insects and diseases

and economics of snapdragon. Ithaca, NY.

18

31.

33.

34.

35.

36.

37.

38.

39.

40.

Madden. L. V.. G. Hughes. and M. A. Ellis. 1995. Spatial heterogeneity ofthe
incidence of grape downy mildew. Phytopathology 85:269-275.

Madden. L. V.. and M. A. Ellis. 2000. Evaluation ofa disease waming system for
downy mildew Of grapes. Plant Disease 84:549-554.

Manners. J.G.. and S.M.M. Hossain. 1963. Effects of temperature and humidity on
conidial germination in Envsip/ze graminis. Transactions of the British Mycological
Society 46:225-234.

McKay. R. 1949. The production ofoospores by the downy mildew of
Antirrhinums. The Gardener's C hronicle:28.

Michelmore. R.W. 1981. Sexual and Asexual Sporulation in the Downy Mildews.
In: The Downy Mildews. edited by D. M. Spencer. London: Academic Press.
Moss. M.A.. and C .E. Main. 1988. The effect of temperature on sporulation and
viability Of isolates OfPeronospora labacina collected in the United States.
Phytopathology 78:110-114.

Nesmith. W. C. 1984. The North American blue mold waming system
[Peronospora Iabacina. tobacco. Canada. USA]. Plant Disease 68:933-936.
Pinckard. J. A. 1942. The mechanism of spore dispersal in Peronospora tabacina
and certain other downy mildew fungi. Phytopathology 32:505-511.

Populer, C. 1981. Epidemiology of Downy Mildews. In: The Downy Mildews.
edited by D. M. Spencer. London: Academic Press.

Royle, DJ. and H. Kremheller. 1981. Downy Mildew of Hop. In: The Downy
Mildews. edited by D. M. Spencer. London: Academic Press.

Scherrn. H.. and A. H. C. van Bruggen. 1994. Weather variables associated with

19

41.

44.

45.

46.

47.

48.

49.

infection of lettuce by downy mildew (Bremia lactucae) in coastal California.
Phytopathology 84:860-865.

Scherm. H.. and A. H. C. van Bruggen . 1995. Concurrent spore release and
infection of lettuce by Bremia lacmcae during mornings with prolonged leaf
wetness. Phytopathology 85:552—555.

Scherm. H.. and A. H. C. Van Bruggen. 1995. Comparative study of microclimate
and downy mildew development in subsurface drip- and furrow-irrigated lettuce
fields in California. Plant disease 79:620-625.

Scherm. 11.. van Bruggen. A.H.C. 1994. Effects of fluctuating temperatures on the
latent Period Of letuce downy mildew (Bremia lactucae). Phytopathology 84:853-
859.

Somers. and Horsfall. 1966. The water content of powdery mildew conidia.
Phytopathology 56: 103 1-1035.

Sonoda. RM. and J .M. Ogawa. 1972. Ecological factors limiting epidemics of
hop downy mildew in arid climates. Hilgardia 41:457-474.

Stuteville. D.L. 1981. Downy mildew of forage legumes. In: The Downy Mildews.
edited by D. M. Spencer. London: Academic Press.

Viennot-Bourgin, G. 1981. History and Importance of Downy Mildews. In: The
Downy Mildews. edited by D. M. Spencer. San Francisco: Academic Press.
Wheeler. B.E.J. 1981. Downy Mildew of Omamentals. In: The Downy Mildews.
edited by D. M. Spencer. San Francisco: Academic Press.

Willocquet. L. , D. Colombet, M. Rougier. J. Fargues, and M. Clerjeau. 1996.

Effect of radiation. especially ultraviolet B, on conidial germination and mycelial

20

growth of grape powdery mildew. European Journal of Plant Pathology 1022441-
449.

Yarwood. C. E. 1936. The tolerance ofErj'siphe polygoni and certain other
powdery mildews to low humidity. Phytopathology 26:845-859.

Yarwood. C. E. 1957. Powdery Mildews. The Botanical Review XXIII:235-300.
Yarwood. CE. 1947. Snapdragon Downy Mildew. Hilgardia 17:241-249.
Yarwood. C .E. 1950. Water content of fungus spores. American Journal of

Botany 37:636-639.

21

CHAPTER I.

FACTORS AFFECTING CONCENTRATIONS OF AIRBORNE CONIDIA OF

OIDIUM SP. AMONG POINSETTIAS IN A GREENHOUSE

22

ABSTRACT

Atmospheric concentrations of Oia’izmz sp. conidia in two research greenhouses
containing infected poinsettias were monitored to investigate the role of environment in
prompting conidial release and dissemination. Hourly concentrations of conidia of Oidium
sp. were estimated using a Burkard volumetric spore sampler. The influence of
temperature on disease development was studied by placing healthy poinsettias in each
greenhouse for 7-day periods. removing them. and recording the days to the appearance of
the first colony. When averaged over 5 December to 1 June. atmospheric conidial
concentrations in greenhouse 2 (GH 2) were greatest during 1000 to 1800 h with a peak
(325 conidia/m3/h) occurring at 1200 h. In greenhouse l 1 (GH 11). peak concentrations
occurred at in 1300 h (65 conidia/m3/h) and 1600 h (75 conidia/m3/h). Large numbers of
conidia were sampled (2100/m3) within l-h periods indicating conidial release events
(CRES). Fluctuations in relative humidity (RH) (either positive or negative) prompted
CREs. In both greenhouses. the highest number of C REs (up to 23) occurred following
RH fluctuations of5 to 15%. Watering resulted in an immediate increase (325%)
followed by a rapid decrease in RH (332%) beginning 1 to 2 h later. Eighty-nine percent
(GH 2) and 48% (GH 11) of the C RES occurred within 3 h following greenhouse
watering. When greenhouse temperatures exceeded 25C for 21 days in May (GH 2) and
19 days in March (GH 1 l) atmospheric conidial concentrations were reduced 80 and 75%

from the previous months. respectively.

23

INTRODUCTION

Poinsettias (Euphorbia pulcherrima Willd. ex Klotzsch) have a wholesale value in
the United States of $212 million with California. F lorida, Ohio. and Michigan leading
production (19). Poinsettias are propagated by taking tip or terminal cuttings from the
branches of vegetative stock plants and placing them in rooting medium (20). Primary
propagators maintain stock plants all year for cutting production. Secondary propagators
purchase cuttings for stock plant production from primary propagators beginning in early
spring. Cuttings from these stock plants are sold in mid-summer to growers who produce
the flowering crop. Management of foliar diseases is a concern throughout all stages of
poinsettia production. The primary foliar disease of poinsettia historically has been leaf,
stem. and bract blight caused by Botrytis cinerea Pers.:Fr. (16).

Powdery mildew on poinsettias in the United States was first reported in 1990 (9)
and was epidemic in Michigan in 1992. While the disease is not common, it occurs yearly
and is an economically significant problem especially for poinsettia growers in the
midwestem and northern United States (M.K. Hausbeck, unpublished data). In absence of
an observed teleomorph, the causal agent of this powdery mildew is referred to simply as
OI'dium sp. and is recognized by its production of conidia in chains on upright
conidiophores characterized by an arched basal cell [Celio, 1998 #143].

Signs of disease appear as small, white, talcum-like colonies on bract and/or leaf
surfaces; colonies can coalesce to cause blighting. If colonies first develop on the
undersides of leaves, the disease may go undetected until late in production when bracts
become infected. making plants unsaleable. While applications of appropriate fungicides

to poinsettia bracts prevent further colony development. the fungal mycelium remains

24

visible and is commercially unacceptable. Additionally, residue and phytotoxicity
resulting from fungicide applications to the bracts may result in an unmarketable crop.

Scouting the poinsettia crop for early Si gns of the disease can ensure timely
fungicide applications prior to bract coloration. However, frequent scouting is time
consuming and difficult for large greenhouse operations that may have more than an acre
of closely-spaced poinsettias. Since popular poinsettia cultivars are susceptible to
powdery mildew (6). growers have relied on preventive applications of systemic
fungicides prior to bract coloration to manage the disease. An improved understanding of
the epidemiology of this pathogen could contribute to development ofa disease
management program that reduces unnecessary fungicide applications without increased
risk of disease. Recent research conducted under controlled environmental conditions
indicates that temperature influences conidial germination, haustorium formation,
development of secondary germ tubes. and sporulation (3,4). The objective of this study
was to determine the role of environment in prompting atmospheric concentrations of
OI'dium sp. conidia in research greenhouses containing infected poinsettias. The influence
of temperature on disease development was also of interest.

MATERIALS AND METHODS

Multi-stemmed, 8- to 10-week Old poinsettias, with colored bracts, grown in 13.2-
cm-diameter plastic pots in commercial soilless potting mix composed of 40% perlite and
60% sphagnum peat moss. were obtained from a commercial Michigan greenhouse in
November 1994. A mixture of cultivars representing a range of disease susceptibility was
used, cultivars included: Eckespoint Red Sails. Peace Jolly Red, GrossTM Supjibi Red,

GutbierTM V-14 Glory, Eckespoint Jingle Bells 3. Annette HeggTM Hot Pink. GutbierTM

25

V-14 Pink. Eckespoint Pink Peppermint. GutbierTM V-l7 Angelika Marble, Annette
HeggTM Topwhite. GutbierTM V-14 White and GutbierTM V-l7 Angelika White. A
random assortment ofcultivars was spaced eight to nine pots per m2 on three benches in
each of two glass research greenhouses located in the same complex, hereafter referred to
as GH2 (9m x 8.5m) and GHI 1 (5.5m x 8.8m). Ideally, this study would have been
conducted in adjacent greenhouses: however. greenhouse availability was a constraint.
Plants were hand watered as needed. taking care to avoid wetting the foliage. Plants were
fertilized during watering with 200 ppm 15-5-25 (N -P-K) poinsettia fertilizer (Grace-
Sierra Horticultural Products Company, Milpitas. CA) at 2- to 3-day intervals. The
irrigation water pH was maintained at 5.8. Glasshouse temperatures were set for 21.1 to
22.0C and vented when temperatures exceeded 26.0C. Temperature and relative humidity
(RH) were monitored using a Neogen EnviroCaster (Neogen, Mason, MI) that recorded
the environmental parameters every 15 min and calculated hourly averages. The number
of days in each spore sampling week having one or more hourly average temperatures
exceed 25.0 or 30.0C was calculated. Light levels were natural photoperiods. When
entering the greenhouses, personnel documented the date, time of day, and greenhouse
activity performed. such as irrigation. vent opening, equipment maintenance and cleaning
of benches. Fungicides were not applied for the duration of the experiment.

In GH 2, inoculation occurred on 18 November by vigorously shaking mildew-
infected plants with actively sporulating colonies above the healthy poinsettias. Colonies
were observed on inoculated plants within 10 days. Although plants in GH 11 were not
intentionally inoculated, colonies were first observed on 29 November. Both bracts and

foliage were infected during the epidemic.

26

Concentrations of airborne conidia were monitored in each greenhouse using a 7-
day volumetric spore sampler (Burkard Mfg. C 0. Ltd.. Rickmansworth. Herfordshire.
England) placed in the center ofa bench from 5 December to 1 June and from 5 December
to 17 July in GH 1 1 and GH 2. respectively. Observations were not continued for an
additional season due to the time-consuming nature of this type of study. The spore
sampler was operated at a flow rate of 10 L/min and the orifice was set level with the top
ofthe plant canopy and fixed in a direction perpendicular to the bench. Conidia were
impacted onto tapes coated with an adhesive mixture of petroleum jelly and paraffin (9:1,
w/w) dissolved in sufficient toluene to give a thick. liquid consistency. Tapes were
removed weekly. cut into 48-mm lengths. marked at 2-mm intervals with a razor blade to
indicate hourly intervals. stained with aniline blue in lactic acid (28 mg of aniline blue, 20
ml of distilled water. 10 mg of glycerol. and 10 ml of 85% lactic acid. diluted with 5 drops
to 25 ml ofdistilled water). and mounted on glass slides beneath 22 x 50 mm coverslips.
Under a compound microscope (x100). conidia were identified as Oidium sp. based on
conidium size. shape, color. surface texture. and translucence. The number of conidia
sampled during each l-h period were recorded. When conidial concentrations were
exceptionally large (>2,000/m3/h), a portion of the 2-mm interval was counted and
multiplied by the appropriate factor to provide an estimate of the concentrations for the 1-
h period. Counts were converted to numbers of conidia per cubic meter air sampled per
hour.

From 23 January to 20 March, nine healthy red-bracted poinsettias (cv. Freedom
Red) grown as previously specified were placed in each greenhouse (three per each of

three benches) for 7-day periods after which they were removed and incubated in a nearby

27

greenhouse under similar environmental conditions. The number ofdays to the
appearance of the first visible colony following the exposure period was recorded for each
plant. Once plants developed visible colonies they were immediately removed to minimize
contamination of the greenhouse atmosphere and nearby poinsettias. Because of limited
numbers of poinsettias available. three plants were used each week (one per bench) from 3
April to 3 July for GH 2.

RESULTS
Atmospheric conidial concentrations. When averaged over the observation period (5
December to 1 June). atmospheric conidial concentrations in GH 2 were greatest during
1000 to 1800 h with a peak (325 conidia/m3/’h) occurring at 1200 h (Fig. 1). Overall,
atmospheric conidial concentrations were much lower in GH 1 1 compared to GH 2 with
peak concentrations occurring at 1300 h (65 conidia/m3/h) and 1600 h (75 conidia/m3/h)
(Fig. 1). During the course of this study, large numbers of conidia were sampled
(2100/m3) within l-h periods indicating conidial release events (CRES). A total of 134
and 50 C RES occurred in GH 2 and GH 1 1. respectively, with maximum CRES occurring
at l 100 and 1200 h (GH2) and 1300 h (GH 1 1) (data not shown). CRES were negligible
(3 1) from 2000 to 0800 h in both greenhouses.
Relative humidity fluctuations. Fluctuations in RH (either positive or negative)
prompted CRES (Fig. 2). In both greenhouses. the highest number of CRES (up to 23)
occurred following RH fluctuations (either positive or negative) of 5 to 15% (Table 1),
although fluctuations in RH were as high as 32% (GH 2) and 27% (GH 1 1). One CRE

occurred in GH 2 without a fluctuation in RH in the 3 h preceding the event.

28

 

00
01
O

 

 

 

.2 Illlll'lldllilllllllllq
'5 300
.r: A r’ —
3005 T T
o E 250— '—
E E r ‘
g :2 200— -
0 g r- ‘
f: 0150- ‘—
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'5 .9
E 2100— '—
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8 8 50r— 7":14 "
to: .. A -
0 0 1111
o o o o o o o o o
o o o o o o o o
co to O) N Lo 00 ‘- st
‘— ‘— x- N N

Figure l. Hourly average concentration of airborne conidia of Oidium sp. in research

greenhouses 2 (O) and 11(l), during 5 December to 1 June.

29

Figure 2. Association of greenhouse activity, including watering (V) and venting (O),
and fluctuation of relative humidity with concentration of airborne conidia of ()ia’ium sp.

in research greenhouses 2 (A) and 1 1. (B) during 13 through 16 March.

2000

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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13

v
‘—

15

31

(O
‘—

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‘-

Table 1. Number of‘conidial release events associated with relative humidity fluctuations
(positive or negative) occurring between 5 December to 1 June (greenhouse l 1) and 5

December to 17 July (greenhouse 2).

 

 

 

Fluctuation in Number ofconidia] release events

relative Greenhouse 2 Greenhouse ll
humidity (96) + - + -
0 l 0 0 0

E
\I
00
N
00

5-10 21 18 4 9
11-15 23 15 10 3
16-20 9 9 6 2
21-25 9 8 2 2
26-30 2 2 O 2
>30 0 2 0 0

 

32

Watering resulted in an immediate increase (325%) followed by a rapid decrease
(332%) in RH beginning 1 to 2 h later. In GH 2. 89% ofthe 134 CRES occurred within 3
h following plant watering (data not shown). In GH 1 1, 48% of the 50 CRES were
observed within 3 h of watering (data not shown). Some C RES were observed that were
not associated with watering. In GH 2, 1 l of these 15 C RES were associated with a
fluctuation of 5 to 23% in RH within 3 h ofthe CRE onset (data not shown). In GH 1 l,
80% of the 25 C RES not associated with watering were preceded by a natural RH
fluctuation ranging from 5 to 28% within 3 h of the CRE onset (data not shown).

The largest C RE in each greenhouse (estimated 8,500 conidia/m3/h) was preceded
by a 10% decrease in RH; in OH 1 l. a 10.0C decrease in temperature also occurred. In
GH 2. C RES lasting 10 or more hours occurred eight times during the observation period.
On 29 March, the CRE began at 1200 h, lasted 55 h, and released a total of 27.625
conidia/m3 (Fig. 3). The onset of this event was associated with a watering event and an
8% increase in RH. Elevated peaks within this extended C RE period were associated with
fluctuations in RH. Similar CRES did not occur in GH 11.

During the study, vents in GH 2 were opened on 50 days. On 34 of these days,
plants were watered as the vents opened, prompting C RES within 3 h on 11 occasions.
When venting occurred without watering, CRES were observed 14% of the time.
Greenhouse 11 was vented 5 8 times during the course of this study. On three of these
days, CRES occurred within 3 h of greenhouse venting or plant watering while venting.
Temperature. Although this study was conducted in greenhouses of similar size,
construction, and orientation located in the same complex, the environmental conditions

varied between greenhouses. In GH 2, temperatures did not routinely exceed 25 .0C

33

Conidialmzlh

Relative humidity

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60 -—
A
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oo 05 O \—
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Time

Figure 3. Extended conidial release event period and associated greenhouse activity,

including watering (V) and venting (O), and fluctuations of relative humidity in

greenhouse 2 during 28 through 31 March.

34

during the first six weeks ( 23 January to 6 March) of the study (Fig. 4). Thereafter.
temperatures exceeded 25.0C three to six days during each week. However. during
weeks 19 to 24 (29 May to 10 July). temperatures exceeded 300C one to seven days per
week and corresponded to the lowest conidial concentrations occurring in this study
(<l.200 m3/week) (Fig. 4).

In GH 1 I. temperatures exceeded 25.0C one to five days per week during the
study (Fig. 5). Temperatures exceeding 30.0C were not observed until week 7 (6 to 13
March). On March 1 l the temperature reached 39.1C and prompted several CRES
(38.500 conidia/1113.01) (data not shown). Total conidial concentrations for weeks one to
seven (23 January to 13 March) were 31.653 conidia/m3. Following the extreme high
temperature occurring in week 7. C RES were reduced for the duration of the study. Total
conidial concentrations for weeks 8 (13 to 20 March) to 18 (22 to 29 May) were 5,832/m3
(Fig. 5). During week 15 (1 to 8 May). temperatures exceeded 30.0C each day of the
week. Subsequently, atmospheric conidial concentrations were not detected for the
remainder of the study.
Disease development. Disease developed on plants placed in GH 2 from week 1 to week
20 (23 January to 12 June). Time to colony development ranged between 0 (colonies
developed during the exposure week) and 4.3 days after the exposure period (Fig. 4).
Atmospheric conidial concentrations during this period ranged from 1.185 to 42,833
conidia/m3/week. No colonies developed on plants placed in GH 2 during weeks 21 to 24
(12 June to 10 July). During this period. fewer than 600 conidia/m3/week were released.

When plants were placed in GH 1 1 during weeks 1 to 6 (23 January to 6 March)

and week 9 (20 to 27 March). colonies developed within an average of three to seven

35

 

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j
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——-44
—1
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Days to colony
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eno colonies
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Trapping week

Figure 4. The A) days to colony development on poinsettias following exposure to
atmospheric conidial concentrations, B) total number of conidia released during the trap
week. and C) number of days the average hourly temperatures exceeded 25C and 30C in

research greenhouse 2 during 23 January to 10 July.

36

 

 

study discontinued 2
l
\v x/ —!
\I

Days to colony
development
A
l
l

 

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.-no colonies

 

30000 -—- w . . 2. w . . z . _
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20000 — .—
.

 

(conidalmalweek)

. study discontinued
10000 ; .7
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4 _._ )2 i- m study discontinued.

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16

Trapping week

Figure 5. The A) days to colony development on poinsettias following exposure to
atmospheric conidial concentrations. B) total number of conidia released during the trap
week, and C) number of days the average hourly temperatures exceeded 25C and 30C in

research greenhouse 11 during 23 January through 10 July.

37

days after the exposure period (Fig. 5). During week 7 (6 to 13 March) when
temperatures reached 39.1C, 19.502 conidia/m3 were released, although plants exposed
during this time did not develop colonies. During week 9 (20 to 27 March). 2,322
conidia/m3 were released and exposed plants developed colonies.

DISCUSSION

Concentrations of airborne conidia of Oidium sp. were present in the greenhouses
throughout the monitored period. The production and release of conidia of many
powdery mildew species is diurnal and we observed conidial release largely between 1000
and 1400 h. The periodicity of powdery mildews which form conidia in chains is
determined by the final stage of sporulation, when the abstriction of the conidium from the
germ cell of the conidiophore occurs (2). This maturation stage is photosensitive,
proceeding faster in light than in dark, therefore conidia produced at night remain
immature and are released once exposed to light (8). Observational field studies of
powdery mildew on grapes (24). rubber (10), cherry (12), apple (22), barley (l3) and
tobacco (7) have also documented diurnal conidial release patterns. Release patterns are
also influenced by wind speed. rainfall. fungicide applications, temperature, RH, and solar
radiation, which also tend to follow diurnal patterns 23, 24).

In our study, a primary factor influencing the occurrence of CRES was fluctuation
of relative humidity, often caused by watering. The association of conidial release in
response to RH fluctuation has been similarly observed with other powdery mildew
species including Sphaerotheca pannosa, Erysiphe pisi and E. graminis (1). Conidial
release by S. pannosa occurs in response to abrupt decreases in RH while release by E.

gramim‘s occurs in response to increasing RH. Butt (2) proposed that the variation in

38

sensitivity to RH may be due to differences in the hygroscopic properties of the spore
wall. Envisphc pisi does not discharge conidia in response to RH fluctuation alone;
conidial liberation occurs when RH change is accompanied by exposure to light and
increasing temperature. In our study C REs occurred in response to both positive and
negative changes in RH. Although many C RES occurred in conjunction with temperature
changes, it was not required for conidial release.

Although we have found an association between RH fluctuations and conidial
release in Oidium sp. of poinsettia. the mechanism of release is not known. Both passive
and active liberation mechanisms have been proposed for powdery mildew species (18).
Mechanical disruption (13). wind speed I I 1). and humidity (1) or temperature changes
have been associated with high conidial releases of powdery mildews. In field situations,
wind plays a significant role in the release and conidial dispersal of many powdery mildew
species either directly or as a result of leaf movement dislodging conidia (10. 14, 23). In
the greenhouse, significant air movement is limited to that caused by cooling fans or open
vents. An alternative method ofdispersal is hygroscopic twisting in response to changing
RH. This twisting causes movement capable ofdislodging spores of Phytophthora and
Peronospora (18). However. Jarvis (15) concluded that hygroscopic movements loosen
spores of Botrytis cinerea for subsequent or rainsplash dispersal. Adams (I) proposed
that a change from high to low RH creates an electrical charge with sufficient voltage to
remove powdery mildew conidia from conidiophores.

Optimum temperatures reported for powdery mildews (21 .OC) are generally lower
than those for other plant pathogens (2). Studies comparing effects of temperature on

Oidium sp. on poinsettia found conidial germination was significantly reduced at 30C

39

(<64%) versus 20.0C ( g 80%). At 30.0C formation of secondary germ tubes was
significantly limited (<1%) compared with 20C (2 53%). Less than 4% of germinated
conidia formed haustoria at 30.0C. compared to greater than 53% of those at 20.0C.

In our study. temperature exceeded 25.0C for 21 days in May in GH2 and the
atmospheric conidial concentration was 20% ofthat occurring in April when only nine
days exceeded 25.0C. Similarly in CH 1 1, when the number of days the temperature
exceeded 25.0C increased from 9 in February to 19 in March, the atmospheric conidial
concentration was reduced by 75%. As a result. the powdery mildew epidemics in the
greenhouses naturally lost intensity in March (GH] 1) and June (GH2). For the last five
weeks of the study. temperatures in GHZ exceeded 30.0C at least two days per week. At
the conclusion of the study in GH2, colonies were no longer developing on exposed plants
and atmospheric conidial concentrations were declining such that fewer than 100
conidia/m3 were released in the final week of monitoring. In GH 11, following the high
temperature of 39.1C in March (week 7), atmospheric conidial concentrations were
reduced (:2 833 conidia/mB/h). Colonies did not develop on plants exposed during weeks 7
and 8. when temperatures exceeded 30.0C two to three days per week. Similar
temperature effects have been observed with other powdery mildew species. The
maximum temperature for germination and appressorium formation of 0. begom'ae is also
30.0C; colonies on mature leaves were eradicated by exposure to 32.0C for three days
(21).

Kim et al. (17) observed that plants with active powdery mildew colonies, grown
in a greenhouse throughout the summer and exposed to high temperatures and solar

radiation showed no signs of infection for a period of time. In two commercial

40

greenhouses in Michigan where powdery mildew was detected in July or August. colonies
were found only on the abaxial sides of the bottom leaves where temperatures would be
cooler as a result ofcanopy shade and close proximity to the moist surface of the growing
medium (Hausbeck. personal observation). Because of the colony formation on leaf
undersurfaces. without thorough and frequent scouting, low levels of disease would thus
escape undetected. However. as the season progresses and temperatures decrease,
conditions will be more favorable for further colony development, with associated high
and frequent C REs prompting rapid onset of an epidemic. At this point, colonies would
likely be evident on the adaxial surface of upper leaves and/or bracts and widespread
enough to be noticed by growers. Since Michigan conditions at this point would not
naturally exceed 25.0C. the epidemic would proceed unhindered and result in entire
poinsettia crops being unsaleable. This has occurred many times in Michigan in the last
several years.

The ability of powdery mildew to become epidemic seemingly at a time when
poinsettias have colored bracts and are especially vulnerable to phytotoxicity and residues
from fungicides has made disease management difficult. While thorough scouting for
signs of infection is optimum for prompting fungicide sprays, many growers do not have
the necessary personnel to accomplish the task. Rather, some growers apply preventive
fungicides during the entire production cycle until just prior to bract coloration. Results
from this study may be helpful in the eventual development of a disease management

strategy that utilizes environmental conditions to prompt fungicide sprays.

41

i\)

LITERATURE CITED
Adams. G. C. Jr.. Gotwald. T. R.. and Leach. C. M. 1986. Environmental factors
initiating liberation ofconidia of powdery mildews. Phytopathology 76: 1239-
1245.
Butt. D. J. 1978. Epidemiology of powdery mildews. Pages 51-81 in: The
Powdery Mildews. D. M. Spencer, ed. Academic Press, New York.
Byrne, J. M.. and Hausbeck. M. K. 1998. Influence of temperature and relative
humidity on infection processes and sporulation on 0idium sp. on poinsettia
foliage. (Abstr) Phytopathology 88:8134.
Byrne. J. M., and Hausbeck. M. K. 1999. The effect of temperature on
sporulation of Oia'um sp. on poinsettia foliage. (Abstr.) Phytopathology 89:810.
Celio. G. J.. and Hausbeck, M. K. 1997. Evaluation of poinsettia cultivars for
susceptibility to powdery mildew. HortScience 32:259-261.
Celio, G. J .. and Hausbeck, M. K. 1998. C onidial germination, infection structure
formation. and early colony development of powdery mildew on poinsettia.
Phytopathology 88:105-113.
Cole, J. S. 1966. Powdery mildew of tobacco (Erysiphe cichoracearum DC) IV.
Conidial content of the air within infected crops. Ann. Appl. Biol. 57:445-450.
Cole, J. S.. and Femandes. D. L. 1970. Effects of light, temperature and humidity
on sporulation of Erysiphe cichoracearum on tobacco. Trans. Br. Mycol. Soc.
55:345-353.
Daughtrey, M. L., and Hall, J. 1992. Powdery mildew: A new threat to your

poinsettia crop. Grower Talks. Sept.:23-31.

42

10.

11.

13.

14.

15.

16.

l7.

l8.

Fernando. T. M. 1971. ()idizmz leaf disease; the effect of environment and control
measures on incidence ofdisease and atmospheric spore concentration. [Hevea
brasilicmis]. Quarterly .1. Rubber Res. Inst. Ceylon 48:100-1 1 1.

Frinking. H. D. 1977. Research on wind dispersion of rose-mildew spores
(Sp/memllzcca pannosa) in field. glasshouse and climate room. Grava 16:155-
158.

Grove. G. G. 1991. Powdery mildew of sweet cherry: influence of temperature
and wetness duration on release and germination of ascospores of Podosphaera
clam/extinct. Phytopathology 81 :1271-1275.

Hammett. K. R. W.. and Manners. J. G. 1971. Conidium liberation in Erysiphe
gramim’s. 1. Visual and statistical analysis of spore trap records. Trans. Br.
Mycol. Soc. 56:387-401.

Hammett. K. R. W., and Manners, J. G. 1974. Conidium liberation in Erysiphe
graminis. 111. Wind tunnel studies. [Wheat]. Trans. Br. Mycol. Soc. 62:267-282.
Jarvis. W. R. 1962. The dispersal of spores of Botryris cinerea Fr. in a raspberry
plantation. Trans. Br. Mycol. Soc. 45:549-559.

Jones, R. K., and Strider. D. L. 1985. Bedding plants. Pages 409-422 in:
Diseases of Floral Crops, Vol. 1. D. L. Strider. ed. Praeger Publishers. New
York.

Kim, S. H.. and Olson, T. N. 1995. Recurrence of powdery mildew on over-
summered asymptomatic poinsettias. (Abstr) Phytopathology 85:1 171.

Lacey, J. 1996. Spore dispersal—its role in ecology and disease: the British

contribution to fungal aerobiology. Mycol. Res. 100:641-660.

43

19.

21.

24.

National Agricultural Statistics Service. 1999. Floriculture crops, 1998 summary.
Agricultural Statistics Board. U. S. Department of Agriculture. Washington. D. C.
Nau. J. 1998. Euphorbia pulcherrima. Pages 482-509 in: Ball Red Book, 16m
ed. V. Ball. ed. Ball Publishing. Batavia. IL.

Quinn. J. A.. and Powell. C. C.. Jr. 1982. Effects oftemperature, light, and
relative humidity on powdery mildew of begonia Oidium begoniae on Begonia x
hiemalis. Phytopathology 72:480-484.

Sutton, T. B.. and Jones. A. L. 1979. Analysis of factors affecting dispersal of
apple powdery mildew caused by Padarphaera leucolricha conidia.
Phytopathology 69:380-383.

Willocquet. L., Berud, F., Raoux. L., and Clerjeau. M. 1998. Effects of wind,
relative humidity, leaf movement and colony age on dispersal of conidia of
Uncinula necator, causal agent of grape powdery mildew. Plant Pathol. 47:234-
242.

Willocquet. L., and Clerjeau. M. 1998. An analysis of the effects of
environmental factors on conidial dispersal of Uncinu/a necator (grape powdery

mildew) in vineyards. Plant Pathol. 47:227-233.

44

CHAPTER II.

INFLUENCE OF ENVIRONMENT ON INFECTION AND SPORULATION OF

OIDIUM SP. ON POINSETTIA F OLIAGE

45

ABSTRACT

The influence of temperature and relative humidity (RH) on conidial germination
and infection processes of Oiclium sp. were quantified on poinsettia foliage. Leaf disks
were inoculation with conidia of Oia’ium sp. and incubated at 15, 20 and 25°C in chambers
with glycerol/water solutions that provided 35. 50. 65, 80 and 92% (i2%) RH. F orty-
eight hours after inoculation 100 conidia per leaf disk were counted and the presence of
appressoria, germ tubes, haustoria. length of germ tubes and shriveled conidia quantified.
Formation of appressoria and primary germ tubes were favored by 20°C. Haustorium
formation appeared to be favored by 20°C and 35. 50. and 65% RH. The development of
secondary germ tubes was favored by 25°C and was not consistently affected by RH. The
average length of germ tubes produced at 25°C (36.2 um) was longer than at 20°C
(27.0um) and 15°C (11.1um). Shriveling ofconidia (18.8 to 23.8%) occurred across the
range of temperatures and RH levels in this study. Effects of temperature on sporulation
of Oidium sp. were quantified on inoculated leaf disks incubated for 14 days at 15 and
20°C under high relative humidity. The number of conidiophores produced per mm2 of
leaf tissue, 80 and 99 at 15 and 20°C, respectively. was not affected by temperature.
Temperature had a significant effect on the number of conidia produced per conidiophore,
at 15 and 20°C a maximum of 4 and 7 conidia were produced per conidiophore,

respectively.

46

INTRODUCTION

Poinsettias (Euphorbiu pulclzcrrima W illd. ex Klotzsch) have a wholesale value of
$237 million in the United States with California. North Carolina. Texas, and Ohio leading
production (6). Powdery mildew on poinsettias in the United States was first reported in
1990 and was epidemic in Michigan in 1992. While the disease is not common, it occurs
yearly and is an economically significant problem especially for poinsettia growers in the
midwestem and northern United States (M.K. Hausbeck, personal communication). In the
absence of an observed teleomorph, the causal agent of this powdery mildew is referred to
simply as Oidium sp. and is recognized by catenate conidia on upright conidiophores
characterized by an arched basal cell ( 3).

Signs of powdery mildew appear as small, white, talcum-like colonies on bract,
leaf, stem, or petiole surfaces that can coalesce to cause blighting. C hlorotic spots appear
opposite colonies on abaxial leaf surfaces (7). If colonies first develop on the undersides
of leaves, the disease may go undetected until later in production when bracts become
infected, making plants unsaleable. While fungicide application to poinsettia bracts limit
further colony development. the fungal mycelium remains visible and is commercially
unacceptable. Additionally, residue and phytotoxicity resulting from fungicide
applications to the bracts may result in an unmarketable crop. Growers who detect
colonies early in production may eliminate the disease by removing infected leaves, and
applying fungicides.

Initial epidemiological studies on this pathogen (Oidium sp.) detailed the temporal
development of infection structures produced by conidia (3). Conidia placed on poinsettia

leaf disks were found to germinate, form secondary germ tubes and haustoria within 24

47

hours at 20C. High temperatures (30C) reduce germination of conidia and the
development of secondary germ tubes; haustorium development is also significantly
limited. This coincides with observations made in both commercial and research
greenhouses (10) where powdery mildew development is limited during growing periods
where average hourly temperatures frequently exceed 30C (2). Powdery mildew vary in
the ranges of relative humidity under which gemination can occur. The effect of relative
humidity on germination and infection of Oidium sp. has not been established on
poinsettia.

The objectives of this study were to examine the effects of relative humidity and
temperature on the infection process. Earlier histological studies of foliar infection by
Oidium sp. were conducted under a fixed RH level (85%) at 20 and 30°C (3). The effect
of temperature on the pathogen’s ability to sporulate was also examined and quantified.

MATERIALS AND METHODS
Plant growth. Multi-stemmed poinsettias (cv. Freedom Red) were grown in a research
greenhouse in 13.2-cm-clay pots containing Baccto soilless potting mix (Michigan Peat
Company, Houston, TX). The greenhouse was maintained at 23°C (day) and 18°C (night).
Plants were watered daily or as needed and fertilized with 200 ppm 15-5-25 (N -P-K)
poinsettia fertilizer (Grace-Sierra Horticultural Products Company, Milipitas, CA), twice
weekly. Pesticides were not applied to the foliage of the plants during the experiment.
Inoculation. Leaf disks were excised from fully expanded leaves with a cork borer,
surface-sterilized in a 0.525% sodium hypochlorite solution for 1 min, rinsed in sterile
distilled water, and dried under a laminar flowhood. Inoculum of Oidium sp. (Michigan

State University Herbarium, East Lansing, accession #361714, Mary Hausbeck 01) was

48

produced on infected poinsettias maintained in a growth chamber (Sherer-Gillett) at 20°C
under high humidity (>70%) and 12 h photoperiod. To ensure uniform age and viability,
conidia were dislodged 3 to 5 days before inoculations by shaking plants at the base of
their stems to promote growth of new conidia. Leaf disks were inoculated by using a
plastic-bristle paintbrush to transfer conidia from the infected leaves to each leaf disk.
Conidial germination and infection process. Inoculated leaf disks (l-cm-diam) were
placed adaxial surface up on agar disks (1.4-cm-diam, 20 g agar/liter) suspended by a
plastic mesh grid (1.7 squares/cm) in a glass humidity chamber (Mason jar, 476 ml)
containing 120 ml of prepared glycerol/water solution. Glycerol/water solutions providing
35, 50, 65, 80 and 92% (i2%) relative humidity (RH) (Fomey and Brandl, 1992) were
made and measured with a hygrometer (VWR Scientific, McGaw Park, IL) which was
accurate to i0.5% RH. Glycerol solutions were allowed to equilibrate for at least 2 hours
before RH measurements were taken. Leaf disks were incubated in tightly sealed humidity
chambers at 15, 20 and 25C (10.2) in a water bath (Precision Scientific, Inc.) under
complete darkness for 48 hours.

Following incubation, leaf disks were fixed with an FAA solution (formalin, acetic
acid, ethanol in 1:18;] v/v/v) for 2 h and cleared in a saturated solution of chloral hydrate
(250g/100ml) for 5 days. Disks were preserved in l—dram vials of lactophenol solution
(20 g phenol, 20 ml lactic acid, 40 g glycerin, 20 ml water) until microscopic observations
were made. Cleared leaf disks were stained with cotton blue in lactophenol solution
(100 ml lactophenol, 1 ml 1% aqueous cotton blue, 20 ml glacial acetic acid) and mounted
in glycerol for observation. One hundred conidia on each disk were counted using light

microscopy (400x) and observed for the presence of germ tubes, appressoria, and

49

haustoria. Conidia that were not germinated but were shriveled were also counted.
Germination was defined as a conidium with either an appressorium or a primary germ
tube equal in length to at least half the width of the conidium (Lacy, 1994). The length of
up to five germ tubes on each disk were also determined. Each experiment included four
leaf disks per humidity jar and two humidity jars per RH, and was conducted three times.
The experiment was analyzed as a split-split plot with RH being nested within
experiment replicate and experiment replicate being nested within temperature.
Experiment replicate. humidity jar and disk were classified as random effects while
temperature and RH were fixed effects. Data was analyzed with the ANOVA procedure
of the Statistical Analysis System (SAS Insititute, Cary, NC), germ tube length was
analyzed with the proc mixed procedure, the remaining data was analyzed with a
generalized mixed linear model (glimmix macro) and a binomial error distribution.
Tukey’s test was used to determine significant differences of least squares means for each
temperature and RH combination. Iteration of the data on percentage of germinated
conidia with germ tubes did not converge with the experimental design model used,
therefore conclusions can not be made as to the significance of either temperature or
relative humidity on this aspect of the infection process.
Sporulation. Inoculated leaf disks (1.7-cm-diam) were placed on agar disks (2.0-cm-
diam) amended with 30 mg/l benzimidazole and were weighed down with sterile metal
washers (1.3-cm-diam hole, 7/8 inch outer) and incubated in double petri dishes (4 leaf
disks/dish). Double petri dishes were comprised of a lower petri dish that contained sterile
distilled water and an upper compartment that housed the washer-leaf disk-agar stacks.

The compartments were connected by an opening through which four filter paper wicks

50

were passed, to keep the agar disks hydrated throughout the experiment.

Leaf disks were incubated at 15 and 20C under 12-h photoperiods for 14 days.
Initially, observations were made daily to determine the time to conidiophore and conidia
development. Fourteen days after inoculation the number of conidiophores and conidia on
two sites per disk were quantified with a compound dissecting microscope (100x). Each
temperature was replicated two times, four disks per double petri dish and four double
petri dish chambers per temperature replicate. The experiment was analyzed with the
genmod procedure of the Statistical Analysis System with a multinomial distribution,
experiment replicate was nested within temperature.

RESULTS
Germination and infection. There was a significant interaction between temperature and
RH for conidial germination (P=0.0529), haustorium formation (P=0.0001), germ tube
length (P=0.0028) and Shriveling of conidia (P=0.0001). In general, the lowest
temperature (15°C) was less conducive than the higher temperatures of 20 and 25°C for
germination, appressorium and haustorium development, primary and secondary germ
tube formation and germ tube elongation. (Fig. 1). Formation of appressoria and primary
germ tubes were favored by 20°C.

Conidial germination was favored by warm temperatures (220°C) and high RH
(280%). With one exception (3 5%RH), 15°C was not favorable for conidial germination.
Appressorium formation was favored by 20°C and high RH (2 80%) with a minimum of
17.6 % of the germinated conidia with an appressorium. Fewer than 10.1% of the

germinated conidia developed an appressorium at 15°C regardless of RH.

51

Figure 1. Effect of temperature and relative humidity on A) shrivelling, B) germination,
C) appressorium formation, D) haustorium formation, E) primary germ tube formation,

and F)secondary germ tube formation of OIdium sp. on poinsettia foliage.

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53

Haustorium formation appeared to be favored by 20°C and RH levels of 35, 50,
and 65%, where the percentage of germinated conidia with a haustorium ranged from 55.1
- 61.7. In contrast. haustorium development never exceeded 28% at 15C across all RH
levels.

The 20°C treatment was conducive to the development of a primary germ tube,
although 80% RH was more favorable than 35% RH. At 15 and 25°C there was little
apparent impact of RH on the development of a single germ tube. The 15°C treatment
was least favorable.

The development of secondary germ tubes was favored by 25°C. Limited
development of secondary germ tubes occurred at 20°C and secondary germ tubes were
not formed at 15°C. There was no consistent trend in the effect of RH on development of
secondary germ tubes at either 20 or 25 °C . The average length of germ tubes produced at
25°C (36.2 um) was longer than at 20°C (27.0um) and 15°C (11.1um) (Fig. 2). There
was no consistent trend in RH effects on germ tube length. Shriveling of conidia (18.8 to
23.8%) occurred across the range of temperature and RH levels included in this study.
Sporulation.

Sporulation was initiated 9 days after inoculation regardless of temperature.
Chains of 3 - 6 conidia developed 11 days after inoculation. Fourteen days after
inoculation the maximum chain length was 7 conidia per conidiophore.

The number of conidiophores produced per mm2 of leaf tissue, 80 and 99 at 15 and
20°C, respectively, was not affected by temperature (Fig. 3). Temperature did have a
significant effect on the number of conidia produced per conidiophore (P<0.0001) (Fig.

4). The effect of the experiment replicate was also significant. At 15°C, a maximum of

54

601 I ' 7 i

 

 

 

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501 1
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(D

 

 

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O
_. "_‘T‘ _.___ ""t"“'""“ .
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Relative Humidity (%)

Figure 2. Effects of temperature and relative humidity on the length of germ tubes

produced by conidia of Oidium sp. on poinsettia foliage.

55

120

 

2

1:315C
ZOC
100 ~

1———I

80

I

60

 

 

Conidiophore produced I mm

 

 

4O

 

Temperature

Figure 3. The effect of temperature (15 and 20°C) on conidiophores produced per mm2

of leaf disk, 14 days after inoculation.

56

60
50 _ 1- 150

 

30
20 ~-

Conidiophores (%)

 

 

 

 

 

 

I 'll'lT
01234567

Conidia l conidiophore

Figure 4. Sporulation of Oia’ium sp. on poinsettia leaves. incubated at 15 and 20°C, 14

days after inoculation.

57

4 conidia were produced per conidiophore. At 20°C up to 7 conidia were produced per
conidiophore.
DISCUSSION

Effects of temperature and relative humidity on the conidial germination and
infection processes of Oidium sp. were quantified on poinsettia foliage. Optimum
temperatures reported for powdery mildews (210°C) are generally lower than those for
other plant pathogens (1). C elio found that many of the infection processes were inhibited
at 30°C. The lowest temperature used in our study (15C) was generally less conducive to
aspects of the infection process than the higher temperatures of 20 and 25°C. This
observation coincides with results of similar studies with other powdery mildews (1).
With an extended incubation period some powdery mildews are able to germinate at lower
temperatures. At 4°C, both 0. begoniae and Podosphaera leucotricha conidia germinate
on host tissue (5.12). Low temperatures lengthen the period between infection and
symptom development (1).

Haustorium development appeared to be severely limited at 15°C. Additionally,
secondary germ tubes did not develop at 15°C. The formation of a haustorium is
considered a visible measure of successful infection. The main function of a haustorium is
to absorb nutrients from the host which are then supplied to the hyphae. Since haustoria
are considered essential for additional hyphal growth it is not surprising that development
of secondary germ tubes did not occur at 15C (9).

The formation of secondary germ tubes and their elongation were favored by the
warmest temperature (25°C). (1 1). Manners and Hussain (1 1) found that the optimum

temperature for germ tube development of Erysiphe graminis was higher than the

58

temperature that favored optimum germination. In this study. germ tubes of Oidium sp.
elongated regardless of RH and differs from E. graminis where germ tubes did not
elongate below 98% RH (1 1).

Powdery mildews in general are able to germinate and infect under conditions of ,
low RH. High amounts of moisture within the relatively large conidia provide a resource,
allowing germination and infection under low RH.. Powdery mildews are actually
inhibited by free moisture on host surfaces. conditions that favor germination of many
other fungal pathogens. Conidia of some powdery mildew species are susceptible to
bursting in the presence of free water due to the additional water that spores contain (13).
Even under controlled environmental conditions it is difficult to maintain a level of very
high RH (>90%) without small fluctuations that promote condensation and subsequent
leaf wetness. The reduced efficiency of the infection processes at the 92% RH level may
be due to temporary formation ofcondensation on the leaf disk surface. The moderate
temperature (20°C) and lower RH levels of 35, 50, and 65 were most favorable to the
formation of haustoria. This stage of the infection process appears less dependent on high
RH than germination and appressorium formation.

The density of conidiophores on the leaf tissue was not affected by the incubation
temperature; however. the number of conidia produced on a conidiophore was affected.
Sporulation of Oidium sp. began nine days after inoculation of the poinsettia foliage.
Conidiophores of E. polygonia differentiate from mycelium five days after infection, and
the conidium at the tip of the conidiophore subsequently matures with each additional 24
hour period (4). The lower temperature (15°C) used in this study apparently slowed the

development and maturation of conidia. The optimum temperature for sporulation of E.

59

graminis is 20°C. conidial production was significantly decreased at 25°C (14). Relative
humidity was not controlled in the chambers used for these sporulation studies. but high
RH is also considered an important influence of sporulation of powdery mildews.

Viability ofthe conidia produced in this study was not assessed in this study but it
is reportedly affected by the environmental conditions during their production ( l ).
Hossain (8) found that humid conditions were necessary for the production of conidia with
high germinability. Ward and Manners ( 14) concluded that the development of epidemics
is more likely to be affected by the environmental conditions that impact sporulation, than
by those that influence conidial germination and infection.

Temperature manipulation may be a useful tool in managing powdery mildew on
poinsettia. Based on results of high temperature studies Celio (3) and Quinn (12)
suggested warm temperatures could be used to help manage powdery mildews on
greenhouse grown poinsettias and begonias. Depending on the time of year and the
climate it may be more feasible and cost effective to reduce greenhouse temperatures.
Reducing greenhouse temperatures to 15°C would create an environment less favorable
for sporulation and infection of poinsettia foliage by ()idium sp. Low temperatures could
be helpful when trying to suppress disease development in the time period between
diagnosis and fungicide application. For horticultural reasons this low temperature could
not be maintained for extended periods of time. and therefore could not be used alone in a
disease management program. There may be the potential to incorporate temperature
manipulations with scouting and fungicide applications to enhance powdery mildew

management.

60

IR)

DJ

LITERATURE CITED
Butt. D. J. 1978. Epidemiology of Powdery Mildews. In: The Powdery Mildews.
edited by D. M. Spencer. New York: Academic Press.
Byrne. J.M.. M. K. Hausbeck. and ED. Shaw. 2000. Factors affecting
concentrations ofairborne conidia of Oidz'um sp. among poinsettias in a
greenhouse. Plant disease 84:1089-1095.
Celio. G. J.. and M. K. Hausbeck. 1998. Conidial germination, infection structure
formation. and early colony development of powdery mildew on poinsettia.
Phytopathology 88: 105-1 13.
Cole, .1. S. 1976. The formation and dispersal of Erysiphe conidia. In:
Microbiology of Aerial Plant Surfaces. edited by C. H. Dickinson and T. H.
Preece. London: Academic Press. Inc.
C oyier. D.L. 1968. Effect of temperature on germination of Podosphaera
leucotricha conidia. Phytopathology 58:1047-1048.
Floriculture crops. 2000 Summary.2001.. edited by N. A. 8. Service. Washington.
DC: United States Department of Agriculture.
Hausbeck. M.. J. Kalishek, M. Daughtrey, and L. Barnes. 1994. Keep the colonies
at bay (Part 1). Greenhouse Growerz45-48.
Husain. 8.1.. and M. Akram. 1995. Effect of temperature and relative humidity on
conidial germination and germ tube elongation of .S‘phaerotheca fuliginea
(Schlecht ex Fr.) Poll. on sunflower. Journal of Plant Diseases and Protection
1022509-513.

Jenkyn, J. F., and A. Bainbridge. 1978. Biology and pathology of cereal powdery

61

10.

ll.

14.

mildews. In: The Powdery l\»'lildews. edited by D. M. Spencer. London Press:
Academic Press.

Kim. S. H. . and T. N. Olson. 1995. Recurrence of pewdery mildew on over-
summered asymptomatic poinsettias. Phytopathology (Abstract) 85:1 171.
Manners. J.G.. and S.M.M. Hossain. 1963. Effects of temperature and humidity on
conidial germination in Erys'iphe graminis. Transactions of the British Mycological
Society 46:225-234.

Quinn. J. A.. and C. C. Powell. Jr. 1982. Effects oftemperature. light, and relative
humidity on powdery mildew of begonia ()idium hegoniae on Begonia X hiemalis.
Phytopathology 72:480-484.

Smith. R. F.. K. Subbarao. S. T. Koike. F. Laemmien. and M. Davis. 1999. Several
fungicides control powdery mildew in peppers. California Agriculture. 40-43.
Ward. S. V.. and J. G. Manners. 1974. Environmental effects on the quantity and
viability ofconidia produced by Ei'jxs‘iphe graminis. Transactions of the British

Mycological Society 62:119-128.

62

CHAPTER III.
INFLUENCE OF ENVIRONMENT ON ATMOSPHERIC CONCENTRATIONS OF
PERONOSPORA ANTIRRHIN] CONIDIA IN FIELD GROWN SNAPDRAGONS

63

ABSTRACT

Atmospheric concentrations of Peronospora antirrlzini conidia in a commercial
snapdragon production field were monitored over three growing seasons to investigate the
influences environment has on the concentration of airborne conidia. Hourly
concentrations ofconidia of P. amirrhini were estimated using a Burkhard volumetric
spore sampler. The incidence of downy mildew among different cultivars was also of
interest. Atmospheric conidial concentrations followed a diurnal pattern and were greatest
during 0500 to 1200 hours. Peak conidial concentrations occurred between 0700 and
0900 hours. Minimum daily temperatures below 100°C appeared to have a moderate
limiting effect on atmospheric conidia concentrations. while temperatures below 60°C had
more severe limiting effects. Maximum daily temperatures higher than 300°C limited
concentrations of atmospheric conidia. Long dew periods ( 26 hours) were associated
with relatively large conidia releases. On 69 days (1999-2001), the daily total of conidia
trapped was >100 and the average length of leaf wetness duration prior to these releases
was 1 1 hours. Consecutive days with short leaf wetness periods suppressed atmospheric
conidial concentrations. Snapdragon cultivar had a statistically significant effect on area

under the disease progress curve.

64

INTRODUCTION

Snapdragons (Antirrhinmn mains L. (Huxley et al.. 1992) are grown commercially
as a cut flower crop with production in the United States located mainly in Florida and
California. Snapdragons are an annual crop established in the field from transplants that
are propagated by seed. Flowers are cut by hand one to two times during the growing
season with the longer stemmed and therefore more valuable blooms harvested first.
Snapdragons have a high value and strict requirements for floret size and stem length, and
no tolerance for blemishes.

Downy mildew incited by Peronospora antirrhini (Schroet.. I-Iedwigia) infects
snapdragon foliage both locally and systemically. causing economically significant damage
(16). Local infections produce pale lesions that are visible on the upper leaf surface.
Systemic infections result in downward curling of foliage, and shortened intemodes. The
disease can kill terminal buds of young transplants. causing undesirable branching, which
decreases the crop value (16). Fungicides are applied every 1 to 3 weeks throughout the
growing season to control this disease.

Depending on climate and weather crop losses caused by downy mildews can vary
greatly between years and seasons (1 1). Leaf wetness duration and temperatures are
known to be critical environmental factors for sporulation. germination and infection of
downy mildew that affect tobacco (Peronospora nicotianae (Speg).) (5,10) , grape
(Plasmopara viticola (Berk. & C urt)) (6.7). and lettuce (Bremia lactucae (Regel)) (12-
15).

It has not been determined whether this obligate pathogen survives on weeds and

plant debris. or if it is annually introduced into the production area via seedling transplants

65

(16.18). Research on P. umirrhini is limited to symptomology. pathogen morphology,
preliminary studies of temperature effects on sporulation. and observations of oospores
within plant tissue (8.18). Incident inoculum is an important consideration in control
strategies. however. airborne concentrations of conidia of P. amirrhini have not. to our
knowledge. been studied. Therefore. hourly concentrations of airborne conidia of P.
antirrhini among snapdragons in a commercial production field were estimated to
determined: 1)ifconidia are present throughout the growing season and 2)what influences
environment has on the concentration ofairborne conidia. The incidences of downy
mildew among different cultivars was also of interest.
MATERIALS AND METHODS

This study was conducted in 1999 (2 December 1998 to 13 April 1999), 2000 (22
November 1999 to 5 May 2000). and 2001 (14 December 2000 to 18 April 2001) on a
74.1 hectare commercial cut flower farm in Palm Beach county, Florida. Research sites
were established within the snapdragon production areas of the farm. In 1999 and 2001
there was only one research site each year. In 2000. there were two research sites
(designated 1 and 2) located in different areas of the farm, located 106.7m apart. Plants
were grown on raised (17.8 cm) plant beds (104 cm x 30.5 m) established on 1.5m
centers. In 1999 there were 176 beds in the research site. In 2000 there were 176 and 88
beds in sites 1 and 2, respectively. In 2001 there were 124 beds in the research site.

Snapdragon seedlings (grown in 288-cell flats) were planted from 20 Sept. to 30
Nov. 1998, 1 1 Oct. to 6 Dec 1999 and 9 Oct. to 26 Oct. 2000. All seedlings within a bed
were planted on the same date. For the 1999 and 2000 growing seasons seventeen

cultivars were grown and included the following: Allure Pink. Allure Red. Attraction

66

Pink. Attraction Rose. Attraction White. Potomac Red. Potomac Apple Blossom.
Potomac Dark Orange. Potomac Early Pink. Potomac Ivory. Potomac Pink. Potomac
Red. Potomac Rose. Potomac Royal. Potomac Soft Yellow. Rocket Lemon. and Rocket
White. Each plant bed was established with one cultivar and cultivars were arbitrarily
arranged within each section.

Plant beds were prepared and maintained according to standard commercial
production practices including annual fumigation with methyl bromide. and amendment
with a granular application of metalaxyl (Subdue 2E. Novartis. Greensboro. NC) prior to
planting. Plantings were maintained with foliar insecticide and fungicide applications. The
fungicides copper sulphate pentahydrate (Phyton 27. Source Technology. Edna. MN)
fosety-al (Alliette. Rhone-Poulenc. Research Triangle Park. NC) chlorothalonil (Daconil.
Zeneca. lnc.. Wilmington. DE). azoxystrobin (Heritage, Zeneca lnc.). EBDC (Manzate,
Griffen. Valdosta. GA). mancozeb (Dithane. Rohm and Haas. Philadelphia. PA). iprodione
(Rhone-Poulenc). fenhexamid (Decree. SePRO C orp.. Carmel. IN) were applied during
the growing season. Plots were watered frequently ( 3 to 6 times/week) with overhead
irrigation. heads were set 1.32m above the ground. Research sites remained in
commercial production throughout the experiment and were harvested regularly.

Temperature. rainfall. relative humidity. and leaf wetness were recorded every 15
min with averages calculated hourly using a Neogen EnviroCaster (Neogen, Mason. MI).
A leaf wetness sensor was placed within a plant bed and was set at a 45° angle. The
sensor was kept within the plant canopy and raised as the plants grew and the canopy
thickened.

Disease was assessed on 5 Jan.. 2 and 25 Feb.. 18 Mar. 14 April. and 5 May 2000

67

and on 7 and 22 Mar. 3 and 17 Apr. and 8 May 2001. Ratings ofdiscase incidence were
based on estimates of the percentage of infected plants within each plant bed. Each plant
bed was divided into ten 3.04m plots. Disease ratings were taken from alternating plots
within each bed and were not taken from the end plots. Disease ratings were used to
calculate area under the disease progress curves (AUDPC). In 2000 there were 5 (sitel)
and 4 (site 2) replicates. In 2001. there were 4 replicates. A replicate consisted of four
3.04 m plots within a single 30.5 m bed. Area under the disease progress curve data from
sites 1 and 2 in 2000 were pooled and analyzed using a split plot design after meeting the
assumptions of Bartlett's test for homogeneity of variances. The significance ofcultivar
effects in each year were determined through analysis of variance (ANOVA). C ultivar
means were compared using F isher‘s least significant difference (LSD) test with
significance at P < 0.05.

Concentrations of airborne conidia were monitored in each field using a 7-day
volumetric spore sampler (Burkard Mfg. C 0. Ltd., Rickmansworth, Herfordshire.
England) placed between the two sections of each site from 2 Dec. 1998 to 13 Apr. 1999.
and 22 November 1999 to 5 May 2000. 14 Dec. 2000 to 18 Apr. 2001. The spore
samplers were operated at a flow rate of 10 liters/min and the orifice was free to move
with changing wind direction. C onidia were impacted onto tapes coated with an adhesive
mixture of petroleum jelly and paraffin (9:1. wt/wt) dissolved in sufficient toluene to give a
thick, liquid consistency. Tapes were removed weekly, cut into 48-mm lengths. marked at
2-mm intervals with a razor blade to indicate hourly intervals, stained with aniline blue in
lactic acid (28 mg of aniline blue, 20 ml of distilled water, 10 mg of glycerol, and 10 ml of

85% lactic acid. diluted with 5 drops to 25 ml of distilled water), and mounted on glass

68

slides beneath 22-by-50-mm coverslips. C onidia were identified as P. amirrhini based on
size (23-30 x 14-1 8pm) and the ellipsoid to ovoid shape using a compound microscope
(x200) (1). The number ofconidia sampled during each l-h period were recorded. When
conidial concentrations were exceptionally large (>2000/m3/h), a portion ofthe 2-mm
interval was counted and multiplied by the appropriate factor to provide an estimate of the
concentrations for the 1-h period. Counts were converted to numbers of conidia per cubic

meter of air sampled per hour.

RESULTS
Atmospheric conidial concentrations. Over the three years. atmospheric conidial
concentrations were greatest during 500 to 1200 hours (Fig. 1). Overall. atmospheric
conidial concentrations were much lower in 1999 and 2001 compared with 2000. Within
each year peak concentrations occurred at 0700 in 2000 (94 conidia/m3/hr) and 2001 (7
conidia/m3/hr). and at 0900 (13 conidia/m3/hr) in 1999 (Fig. 1). Between 1500 and 0300
hours fewer than 4 conidia/m3 were trapped each hour. irrespective of the year.
Temperature effects. Minimum daily temperatures (MinDT) below 100°C appeared to
have a moderate effect of limiting atmospheric conidial concentrations, while temperatures
below 60° had more severe limiting effects (Tables 1-3). Temperatures were 60°C or
lower 6, 9, and 22 days in the 1999, 2000, and 2001 growing seasons respectively. In the
1999 growing season when MinDTs below 60°C occurred in January (weeks 5—6), the
total conidia trapped on these days was <25. Minimum daily temperatures (MinDT)
below 60°C also occurred in February 1999 (weeks 12 -14), and a decrease in

atmospheric conidia occurred in weeks 12 and 13. In 2001. a MinDT

69

100 I

I 1999
8.1 A +
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Figure l. Hourly average concentration of airborne conidia of Peronospora antirrhini in

research plots from (O) 2 Dec. 1998 to 15 April, 1999, from (v) 23 Nov., 1999 to 5 May

2000, and from (I) 14 Dec. 2000 to 18 April, 2001.

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below 100°C occurred in each month ofthe observation period for a total of 39 days,
temperatures were as low as -1.7°C in January. With only one exception. atmospheric
conidia in the 2001 growing season did not exceed 400 conidia per day, significantly less
than daily totals in the 1999 and 2000 growing seasons (Figs 2-4). When temperatures
below 60°C occurred for 13 consecutive days in January 2001 (weeks 5 - 6). total conidia
trapped per day did not exceed 10 Four days of MinDTs below 100°C in March 2001
(week 14) were accompanied by daily atmospheric conidia totals ofless than 10
Maximum daily temperatures (MaxDT) higher than 300°C limited concentrations
of atmospheric conidia. The durations of leaf wetness that occurred on these warm days
were adequate (>8 hours) for sporangium development. Temperatures were 300°C or
higher 4. 5, and 16 days in 1999. 2000 and 2001, respectively. In 1999 high temperatures
occurred in the last 4 days of monitoring and no conidia were trapped on these days.
Maximum daily temperatures of 30.7 and 305°C and average daily temperatures of 23.5
and 253°C occurred on March 30 and 31. 2000 (JD 90-91) (week 18). For the six days
prior to these high temperatures daily atmospheric conidia totals were greater than 250,
conidia totals did not reach this level again for five days. despite moderate rainfall (3.05 -
5.58 mm) and adequate leaf wetness (6 - 13 hours). In 2001, MaxDTs above 300°C
occurred sporadically throughout the growing season (Feb. March, April) and on the last
eight days of monitoring. Atmospheric conidial concentrations were reduced on days
immediately following 14 ofthe 16 days with MaxDT above 300°C.
Leaf wetness and rain. Leaf wetness was often detected beginning at 1900 hours,

persisted throughout the night. and usually dried by 0700 hours. Long dew periods (26

74

Figure 2. Number ofconidia of Pw'mzmporu cmtirrhini trapped per day and associated
minimum (0) and maximum (v) and average temperatures (0). relative humidity (I).

rainfall and leaf wetness (0) during 14 Dec 1998 to 13 April 1999.

75

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Figure 3. Number of conidia of Pcronospora umirrhini trapped per day and associated
minimum ( O) and maximum (V) and average temperatures (0). relative humidity (I),

rainfall and leaf wetness (O) in section] duringl4 Dec. 1999 to 13 April 2000.

77

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Figure 4. Number of conidia of Peronosporu amirrhini trapped per day and associated
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rainfall and leaf wetness (0) during 14 Dec 2000 to 13 April 2001.

79

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hours) were associated with relatively large (>25 conidia/day) conidia releases at the end
of the leaf wetness period (Figs. 5-6). Occasionally, conidia releases were observed after
a brief (:4 hours) leaf wetness period. When leaf wetness duration was short (average
2.7 hours) from 26 Jan. to 23 Feb.. 2000 (weeks 8 - 12), the daily totals of conidia were
low (5 42). Atmospheric conidial concentrations were also suppressed immediately
following consecutive days (2-3) with short leaf wetness periods. Short, mid-day leaf
wetness periods. frequently caused by irrigation. did not impact subsequent conidia
releases.

Irrigation. which was applied almost daily or daytime rainfall did not appear to
have an affect on atmospheric conidia concentrations. Rainfall occurring between 0200
hours and 0600 hours also did not appear to have an effect on subsequent conidial
concentrations
Disease incidence and severity. In 2000 the average disease incidence was less than
10% at the first four disease ratings. Disease incidence was highest at the April 14 (JD
105) rating when the average disease incidence on the most susceptible cultivar, was
44.4%. Disease severity (data not shown) was also highest on this rating. Both the older
foliage that was enclosed within the canopy as well as foliage higher on the stalks were
infected on the more susceptible cultivars. Sporulation was very heavy on both the
abaxial and adaxial sides of the foliage.

Between Julian days 78 and 109. the 4m and 5th disease ratings in the 2000
growing season, the duration of leaf wetness consistently averaged 12 hours each night.
These extended leaf wetness periods were also associated with a significant increase in

disease incidence that occurred over this same time period. In 2000. the most significant

81

Figure 5. Association of atmospheric conidial concentration of Pcromzspora amirrhini in
snapdragon fields with temperature (V). relative humidity (0). leaf wetness (I) and rainfall

during 12 through 18 February 1999.

82

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Figure 6. Association of atmospheric conidial concentration of Pemnm‘pm'a cmtirrhini in
snapdragon fields with temperature (V). relative humidity (0). leaf wetness (I) and rainfall

in sites 1 and 2 during 7 through 13 March 2000.

84

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increase in disease incidence occurred in the midst of the highest concentrations of conidia
during the season. Disease incidence decreased after the fifth rating.

Cultivar susceptibility. The snapdragon cultivar had a statistically significant (P <
0.0001) effect on AUDPC in both growing seasons. ‘Rocket White’ had significantly
higher AUDPC values (Table 4) than the other six cultivars grown in both the 2000 and
2001 growing seasons. In both growing seasons ‘Potomac Ivory‘ had the second highest
AUDPC values. C ultivars Potomac Rose. Potomac Apple. and Potomac Dark Orange

had significantly lower AUDPC values than the remaining cultivars grown in each of the
two seasons.

Table 4. AUDPC values ofdiscase ratings of Peronospora (mthirrini on field grown

snapdragon cultivars.

 

 

 

Cultivar 2000 2001
Rocket White 95.5 a 187.75 a
Potomac Ivory 55.93 b 140.16 b
Potomac Royal 50.6 bc 131.03 be
Potomac Pink 34.4 cd 87.47 cd
Potomac Rose 29.4 d 17.13 e
Potomac Apple 27.5 d 67.66 d
Potomac Dark Orange 27.8 d 52.72 de
DISCUSSION

In this study, concentrations of atmospheric conidia of P. antirrhim‘ followed a
diurnal cycle. Sporulation of many downy mildews is dependent on cycling of light and
darkness; continuous light or darkness prevents the sporulation of Bremia Iactucae, P.

destructor. Plasmopara vilicola and Pseudoperonospora humuli (17). The timing of

86

 

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66 81 93 107 128
Julian Day

Figure 7. Disease progress curve of Peronospora amirrhini for 7 snapdragon cultivars

from 7 March 2001 (Julian day 66) to 8 May 2001 (Julian day 128).

87

the sporulation process (sporangiophore emergence. differentiation. conidial formation.
maturation and liberation) including spore liberation varies with species. The sporulation
process of P. unrirr/zini appeared to be completed earlier and perhaps more quickly than
other downy mildew pathogens. C onidial release in our study began at 0500 h and in
1999 and 2001 peak conidial concentrations occurred at 0700 hours. Field observations
of P. humuli determined that sporangiophores emerged from stomata by midnight.
differentiation was complete and small conidia formed by 0300 h. conidia were full-sized
at 0600 h and were mature and being liberated at 0900 h (17).

Several mechanisms may be involved in the liberation ofconidia. however the
timing of peak daily releases in our study appears to be associated with decreasing relative
humidity and increasing temperature. These factors tend to follow diurnal patterns and
cannot be evaluated independently in this field study. Hildebrand and Sutton (2) found
peak releases of P. dcs'n'ucim' occurred as relative humidity dropped. leaf wetness
evaporated. and wind speeds increased. However. in field studies of Bremia lactuca low
numbers of spores were released on mornings with prolonged leaf wetness and high
relative humidity ( 13). Red-infrared radiation and solar radiation have been implicated as
triggers of spore release under such conditions (1 1.13).

Maximum daily temperatures above 300°C limited atmospheric conidial
concentrations. This coincides with Yarwood‘s (18) earlier observations that sporulation
of P. antirrhini occurred on systemically infected plants incubated at 13°C and 19°C but
not on those incubated at 7°C or 22°C. Additionally. Yarwood observed that inoculations
during the winter and early spring months were more successful than inoculations during

the hotter summer months (18). The average daily temperatures (20 - 25°C) on days that

88

MaxDT exceeded 300°C were tempered by cooler night temperatures. Sporulation was
affected by high day-time temperatures. although sporangiophores are likely not initiated
until evening.

In this study. the greatest number ofconsecutive days with high temperatures
occurred at the end of each monitoring period and atmospheric conidial concentrations
decreased progressively at these times. The limiting effects of high temperatures were
more severe with increasing durations of warm temperatures. The same trend has been
reported with P. Iubacina. which has a higher temperature tolerance (9). Increasing
temperature. maturing plant tissue. and less available tissue (due to harvesting) are all
thought to be involved in the decrease in disease incidence that occurred at the end of the
growing seasons.

Minimum daily temperatures below 6.0 °C occurred periodically in all three
growing seasons. temporarily reducing atmospheric conidial concentrations. Average
MinDT is an important component in disease prediction of other downy mildew
pathogens. An analysis of environmental factors over a 28 year period of hop production
determined that MinDTs were generally higher in April and May of years with severe
epidemics of P. lmmu/i, than in years with mild or no epidemics (3).

Effects of rainfall or irrigation did not have a notable effect on the atmospheric
conidial concentrations. Yarwood also concluded that rain did not play an important role
in the disease progress of P. amirrhini based on observations of more significant
epidemics in the greenhouse than on outdoor plantings (18). Rainy weather is unfavorable
for the sporulation of P. destructor: sporulation failed to occur on 8 of 10 nights that were

otherwise considered favorable based on the relative humidity and temperature (2).

89

 

The duration of leaf wetness at night affected the number of atmospheric conidia
trapped on the subsequent day. Among the three growing seasons there were 69 days
where the daily total ofconidia was 2 100. The average length of leaf wetness durations
on the evenings prior to these large releases was 1 1 hours. There were several occasions
where limited leaf wetness (0-3 h) directly preceded sizable (20-100 conidia/day) releases.
However. it was noted that these days followed an extended leaf wetness period (1 1-16 h)
that occurred on the prior day. A dry night in the middle of a series of days with high
concentrations of atmospheric conidia resulted in a decreased spore release on the
following day. This has also been observed with epidemics caused by P. tabacina (1 1).

When leaf wetness is prolonged and overlaps with conidia release. infection is
likely favored. Field studies of B. luctucae found nightly leaf wetness durations were
consistent predictors of infection days (12). Infection periods occurred mainly on days on
which leaf wetness ended late in the morning (0100 h Pacific standard time or later) (14).

Disease incidence differed significantly with cultivar. Although thorough scouting
for signs of infection is optimum for prompting fungicide sprays growers can use ‘Rocket
White‘ and other especially susceptible cultivars for targeted and more time efficient
scouting. Other especially susceptible cultivars are no longer being grown by this grower
cooperator based on the high disease incidence ratings obtained in the first year of this
cultivar susceptibility study (data not shown). The variability in disease susceptibility
complicates development of a disease predictive model that prompts fungicide application
and has been a problem with other downy mildew predictive models. For example. in
grape production the relationship between temperature. wetness duration. and relative

humidity, and the resulting infection and sporulation depends on the grape species or

90

hybrid (7).

While MinDT. relative humidity and leaf wetness are important factors in downy
mildew development. a favorable level of one may compensate for a marginal level in the
other (4). This compensatory ability is one of the factors that has prevented the
development of accurate and effective disease prediction models for other downy mildew
pathogens. Differing climates in various production areas of the world also prevent wide
scale adoption ofexisting disease prediction systems. For example monitoring systems
used in European grape production are too conservative for use in the mid-western US.
(7). and models for hop downy mildew used in Europe are not used in the western US.
because the environmental factors that restrict disease development differ in the two
locations. US. cut flower snapdragon production is primarily located in Florida and
southern California. It is our intent that information gained from this study will help
growers understand the relationship between environment and disease development and
encourage the implementation ofdiscase resistant cultivars. These tools may be

incorporated into current disease management programs. enhancing their effectiveness.

91

 

I.)

b)

0.

LITERATURE CITED
Francis. S. M. 1981. Pcmnospum umirrhini. In: CMI Descriptions of Pathogenic
Fungi and Bacteria No 685.
Hildebrand. P. D.. and J. C. Sutton. 1982. Weather variables in relation to an
epidemic of onion downy mildew Perrmospora destruclor. near Bradford. Ontario.
Phytopathology 72:219-224.
Johnson. D. A” CB. Skotland. and J. R. Alldredge. 1983. Weather factors
affecting downy mildew epidemics of hops in the Yakima Valley of Washington.
Phytopathology 73 :490-493.
Johnson. D. A.. J. R. Alldredge. and J. R. Allen. 1994. Weather and downy mildew
epidemics of hop in Washington state. Phytopathology 84:524-527.
Kucharek. T. A. 1987. Rainfall. irrigation water. and temperatures associated with
first occurrences of tobacco blue mold in leaf production area of north Florida
from 1979 to 1984. Plant Disease 71:336-339.
Madden. L. \ G. Hughes. and M. A. Ellis. 1995. Spatial heterogeneity ofthe
incidence ofgrape downy mildew. Phytopathology 85:269-275.
Madden. L. V.. and M. A. Ellis. 2000. Evaluation ofa disease warning system for
downy mildew of grapes. Plant Disease 84:549-554.
McKay. R. 1949. The production of oospores by the downy mildew of
Antirrhinums. The Gardener's Chronicle:28.
Moss. M.A.. and C .E. Main. 1988. The effect of temperature on sporulation and
viability of isolates of Peronospora tabacina collected in the United States.

Phytopathology 78:110-1 l4.

ll.

14.

15.

16.

17.

18.

Nesmith. W. C. 1984. The North American blue mold warning system
[Peronmpuru Iahueina. tobacco. Canada. USA]. Plant disease 68:933-936.
Populer. C. 1981. Epidemiology of Downy Mildews. In: The Downy Mildews.
edited by D. M. Spencer. London: Academic Press.

Scherm. 11.. and A. H. C. van Bruggen. 1994. Weather variables associated with
infection of lettuce by downy mildew (Bremiu luctucae) in coastal California.
Phytopathology 84:860-865.

Scherm. H.. and A. H. C. van Bruggen . 1995. Concurrent spore release and
infection of lettuce by Bremiu laclucue during mornings with prolonged leaf
wetness. Phytopathology 85:552-555.

Scherm. H.. and A. H. C. Van Bruggen. 1995. Comparative study of microclimate
and downy mildew development in subsurface drip- and furrow-irrigated lettuce
fields in California. Plant Disease 79:620-625.

Scherm. 11.. van Bruggen. A.H.C. 1994. Effects of fluctuating temperatures on the
latent Period of letuce downy mildew (Bremia lactucae). Phytopathology 84:853-
859.

Wheeler. B.E.J. 1981. Downy Mildew ofOrnamentals. In: The Downy Mildews.
edited by D. M. Spencer. San Francisco: Academic Press.

Yarwood. CE. 193 7. Relation oflight to the diurnal cycle of sporulation of ceratin
downy mildew. Journal ongricultural Research 54:365-373.

Yarwood. CE. 1947. Snapdragon Downy Mildew. Hilgardia 17:241-249.

93

     

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