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This is to certify that the

thesis entitled

USING SCOUTING AND DISEASE
FORECASTERS TO MANAGE
FOLIAR BLIGHTS 0F CARROTS

presented by

Ryan Scott Bounds

has been accepted towards fulﬁllment
of the requirements for

M.S. _ Plant Pathology
ﬁegree 1n

 

 

 

Major professor

Date 6/1! 1/03

0.7639 MS U is an Afﬁrmative Action/Equal Opportunity Institution

 

PLACE IN RETURN Box to remove this checkout from your record.
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6/01 c-JCIFICIDatoDuopes-pJ 5

USING SCOUTING AND DISEASE FORECASTERS TO MANAGE FOLIAR
BLIGHTS OF CARROTS

By

Ryan Scott Bounds

A THESIS

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

MASTER OF SCIENCE
Department of Plant Pathology

2003

ABSTRACT

USING SCOUTING AND DISEASE FORECASTERS TO MANAGE FOLIAR
BLIGHTS OF CARROTS

By

Ryan Scott Bounds

Fungal foliar blights of carrots, caused by Alternaria dauci and Cercospora
carotae, result in necrotic lesions on leaves and petioles that may cause defoliation,
decreasing the efﬁciency of mechanical harvest. Fungicides are applied every 7 to 10
days, regardless of weather conditions or disease pressure. The objectives of these
studies were to evaluate Tom-Cast and other disease forecasters for timing sprays and to
determine when to apply the ﬁrst spray based on ﬁeld scouting and disease incidence
thresholds. The Tom-Cast system was the most effective and reliable disease forecaster
tested, resulting in a fungicide savings of $47.25 and $54.88 per acre in 2001 and 2002,
respectively, compared with the 7-day schedule, while providing similar blight control.
Chlorothalonil alternated with azoxystrobin was applied every 10 days or according to
Tom-Cast with a threshold of 15, 20, or 25 disease severity values (DSVs). Sprays for
these programs were initiated prior to symptom development, or when foliage was
infected at a trace, 5%, or 10% level. Up to four sprays were omitted saving $46.05 and
$41.85 per acre in 2001 and 2002, respectively, and comparable disease control was
achieved by initiating applications when a trace amount of the foliage was blighted and
applying subsequent sprays according to Tom-Cast 15 DSV, compared with the 10-day

spray schedule initiated prior to disease development.

DEDICATION

In memory of Louis Willard Harper (1936-2001), former Crops Club advisor and
professor of Crop Science at California Polytechnic State University, San Luis Obispo.
Thank you for all your inspiration.

iii

ACKNOWLEDGEMENTS

I would like to thank the people who contributed to the success and completion of
these studies. Dr. Mary Hausbeck, my major professor, provided guidance and numerous
opportunities for me to sink my teeth into applied plant pathology. Brian Cortright and
Sheila Linderman offered technical assistance.

To my family and loved ones — I wouldn’t be where I am today without your love
and support. You have been the most supportive people during my graduate education

and every day of my life.

iv

TABLE OF CONTENTS

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

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

LITERATURE REVIEW
Introduction
Alternaria dauci
Cercospora carotae
Disease Control Strategies
Disease Scouting
Disease Forecasting
Literature Cited

CHAPTER I. COMPARING DISEASE FORECASTING SYSTEMS FOR

TIMING FUNGICIDE APPLICATIONS TO CONTROL
ALTERN ARIA AND CERCOSPORA BLIGHTS OF CARROTS. .

Abstract

Introduction

Materials and Methods

Results

Discussion

Literature Cited

CHAPTER II. TIMING FUNGICIDE APPLICATIONS ACCORDING TO FIELD

SCOUTING AND TOM-CAST TO CONTROL ALTERNARIA
AND CERCOSPORA BLIGHTS OF CARROTS .........................

Abstract

Introduction

Materials and Methods

Results

Discussion

Literature Cited

APPENDIX A. SUMMARY OF WEATHER DATA, FUNGICIDE
APPLICATION DATES, AND FINAL DISEASE
ASSESSMENTS FOR FIELD STUDIES IN 2001 AND 2002 ..........

APPENDIX B. PESTICIDE APPLICATION EQUIPMENT STUDIES IN 2001 ...... 1

.vi

xii

OWNF‘

00

11

16

20
21
22
24
30
35
38

39
40
41
43
5 1
92
96

98

13

LIST OF TABLES

Table Page

CHAPTER I. COMPARING DISEASE FORECASTING SYSTEMS FOR
TIMING FUNGICIDE APPLICATIONS TO CONTROL
ALTERNARIA AND CERCOSPORA BLIGHTS OF CARROTS

1. Effect of disease caused by A. dauci and C. carotae on yield of ‘Cellobunch’
carrots left untreated or sprayed with the fungicide Chlorothalonil (1.29 kg a.i./ha)
applied every seven days or according to disease forecasters and number of
fungicide applications and cost of fungicide programs in 2001 and 2002 ............... 28

2. Effect of disease caused by A. dauci and C. carotae on the area under the disease
progress curve for petiole and leaf blight on ‘Cellobunch’ carrots left untreated or
sprayed with the fungicide Chlorothalonil (1.29 kg a.i.lha) applied every seven
days or according to disease forecasters in 2001 and 2002 ................................ 31

3. Effect of disease caused by A. dauci and C. carotae on petiole health of
‘Cellobunch’ carrots left untreated or sprayed with the fungicide Chlorothalonil
(1.29 kg a.i./ha) applied every seven days or according to disease forecasters in
2001 and 2002 ................................................................................... 34

CHAPTER II. TIMING FUNGICIDE APPLICATIONS ACCORDING TO
FIELD SCOUTING AND TOM-CAST TO CONTROL
ALTERNARIA AND CERCOSPORA BLIGHTS OF CARROTS

4. Summary of disease assessment variables that were not normally distributed
and accompanying transformations used to normalize variables ......................... 50

5. Number of fungicide applications and cost of fungicide programs per acre for
‘Early Gold’, ‘Cellobunch’, and ‘Prime Cut’ carrots left untreated or sprayed
with Chlorothalonil (1.29 kg a.i.fha) alternated with azoxystrobin (0.11 kg a.i./ha)
initially applied based on disease incidence thresholds and reapplied every 7
or 10 days or according to Tom—Cast in 2001 and 2002 ................................... 52

6. Effects of spray initiation timings and application intervals on yield of ‘Prime
Cut’ carrots treated with the fungicides Chlorothalonil (1.29 kg a.i.lha) alternated
with azoxystrobin (0.11 kg a.i./ha) when initiated prior to disease development
(0%) or at disease levels of trace, 5%, or 10% and reapplied at 7 or 10 day
intervals or according to Tom-Cast at 15, 20, or 25 disease severity values (DSV)
used to control foliar blights caused by A. dauci and C. carotae in 2002 ............... 53

vi

LIST OF TABLES (cont’d)

Table Page

7.

10.

11.

12.

Effect of the fungicides Chlorothalonil (1 .29 kg a.i./ha) alternated with

azoxystrobin (0.11 kg a.i.lha) when initiated prior to disease development (0%)

or at disease levels of trace, 5%, or 10% and reapplied at lO-day intervals or
according to Tom-Cast at 15, 20, or 25 disease severity values (DSV) on yield

of ‘Early Gold’ carrots infected with A. dauci and C. carotae in 2001 and 2002. . 54

Effect of the fungicides Chlorothalonil (1.29 kg a.i.lha) alternated with

azoxystrobin (0.11 kg a.i.lha) when initiated prior to disease development (0%)

or at disease levels of trace, 5%, or 10% and reapplied at lO-day intervals or
according to Tom-Cast at 15, 20, or 25 disease severity values (DSV) on yield

of ‘Cellobunch’ carrots infected with A. dauci and C. carotae in 2001 and 2002. . ....55

Effect of the fungicides Chlorothalonil (1.29 kg a.i./ha) alternated with

azoxystrobin (0.11 kg a.i.lha) when initiated prior to disease development (0%)

or at disease levels of trace, 5%, or 10% and reapplied at lO-day intervals or
according to Tom-Cast at 15, 20, or 25 disease severity values (DSV) on yield

of ‘Prime Cut’ carrots infected with A. dauci and C. carotae in 2001 ................... 56

Effects of spray initiation timings and application intervals on the area under the
disease progress curve for petiole and leaf blights caused by A. dauci and C.

carotae on ‘Early Gold’ carrots treated with the fungicides Chlorothalonil (1.29

kg a.i./ha) alternated with azoxystrobin (0.11 kg a.i.lha) when initiated prior to
disease development (0%) or at disease levels of trace, 5%, or 10% and reapplied

at 10 day intervals or according to Tom-Cast at 15, 20, or 25 disease severity

values (DSV) in 2001 and 2002 ............................................................... 60

Effect of spray initiation timings for each application interval on the area under

the disease progress curve of petiole blight caused by A. dauci and C. carotae

on ‘Early Gold’ carrots treated with the fungicides Chlorothalonil (1.29 kg a.i.lha)
alternated with azoxystrobin (0.11 kg a.i./ha) when initiated prior to disease
development (0%) or at disease levels of trace, 5%, or 10% and reapplied at

lO-day intervals or according to Tom-Cast at 15, 20, or 25 disease severity

values (DSV) in 2001 ........................................................................... 63

Effects of spray initiation timings and application intervals on the area under the
disease progress curve and ﬁnal ratings for petiole health of ‘Early Gold’ carrots
treated with the fungicides Chlorothalonil (1.29 kg a.i.lha) alternated with
azoxystrobin (0.11 kg a.i./ha) when initiated prior to disease development (0%)

or at disease levels of trace, 5%, or 10% and reapplied at 10 day intervals or
according to Tom-Cast at 15, 20, or 25 disease severity values (DSV) for

control of A. dauci and C. carotae in 2001 and 2002 ...................................... 64

vii

LIST OF TABLES (cont’d)
Table

13. Effect of spray initiation timings for each application interval on the ﬁnal
petiole health evaluation assessing disease caused by A. dauci and C. carotae
on ‘Early Gold’ carrots treated with the fungicides chlorothalonil (1.29 kg
a.i.lha) alternated with azoxystrobin (0.11 kg a.i./ha) when initiated prior to
disease development (0%) or at disease levels of trace, 5%, or 10% and
reapplied at lO-day intervals or according to Tom-Cast at 15, 20, or 25

disease severity values (DSV) in 2001 ...................................................... 67

14. Effects of spray initiation timings and application intervals on the area under
the disease progress curve for petiole and leaf blights caused by A. dauci and
C. carotae on ‘Cellobunch’ carrots treated with the fungicides chlorothalonil
(1 .29 kg a.i.lha) alternated with azoxystrobin (0.11 kg a.i.fha) when initiated
prior to disease development (0%) or at disease levels of trace, 5%, or 10%
and reapplied at 10 day intervals or according to Tom-Cast at 15, 20, or 25

disease severity values (DSV) in 2001 and 2002 ........................................... 71

15. Effects of spray initiation timings and application intervals on the area under
the disease progress curve and ﬁnal ratings for petiole health of ‘Cellobunch’
carrots treated with the fungicides chlorothalonil (1.29 kg a.i.lha) alternated
with azoxystrobin (0.11 kg a.i./ha) when initiated prior to disease development
(0%) or at disease levels of trace, 5%, or 10% and reapplied at 10 day intervals
or according to Tom-Cast at 15, 20, or 25 disease severity values (DSV) for

control of A. dauci and C. carotae in 2001 and 2002 ...................................... 73

16. Effects of spray initiation timings and application intervals on the area under
the disease progress curve for petiole and leaf blights caused by A. dauci and
C. carotae on ‘Prime Cut’ carrots treated with the fungicides chlorothalonil
(1.29 kg a.i./ha) alternated with azoxystrobin (0.11 kg a.i./ha) when initiated
prior to disease development (0%) or at disease levels of trace, 5%, or 10%
and reapplied at 10 day intervals or according to Tom-Cast at 15, 20, or 25

disease severity values (DSV) in 2001 ....................................................... 79

17. Effect of spray initiation timings for each application interval on the area under

the disease progress curve of petiole blight caused by A. dauci and C. carotae on

‘Prime Cut’ carrots treated with the fungicides chlorothalonil (1.29 kg a.i.lha)
alternated with azoxystrobin (0.11 kg a.i./ha) when initiated prior to disease
development (0%) or at disease levels of trace, 5%, or 10% and reapplied at
lO-day intervals or according to Tom-Cast at 15, 20, or 25 disease severity

values (DSV) in 2001 ........................................................................... 81

viii

LIST OF TABLES (cont’d)

Table

18.

19.

20.

21.

Effects of spray initiation timings and application intervals on the area under
the disease progress curve for petiole and leaf blights caused by A. dauci and
C. carotae on ‘Prime Cut’ carrots treated with the fungicides chlorothalonil
(1.29 kg a.i./ha) alternated with azoxystrobin (0.11 kg a.i./ha) when initiated
prior to disease development (0%) or at disease levels of trace, 5%, or 10%
and reapplied at 7 or 10 day intervals or according to Tom-Cast at 15, 20,

or 25 disease severity values (DSV) in 2002 ................................................ 83

Effect of spray initiation timings for each application interval on the area under
the disease progress curve of petiole blight caused by A. dauci and C. carotae
on ‘Prime Cut’ carrots treated with the fungicides chlorothalonil (1.29 kg a.i./ha)
alternated with azoxystrobin (0.11 kg a.i.lha) when initiated prior to disease
development (0%) or at disease levels of trace, 5%, or 10% and reapplied at 7

or 10 day intervals or according to Tom-Cast at 15, 20, or 25 disease severity

values (DSV) in 2002 ........................................................................... 85

Effect of spray initiation timings for each application interval on the area under
the disease progress curve of leaf blight caused by A. dauci and C. carotae on
‘Prime Cut’ carrots treated with the fungicides chlorothalonil (1.29 kg a.i./ha)
alternated with azoxystrobin (0.1 1 kg a.i./ha) when initiated prior to disease
development (0%) or at disease levels of trace, 5%, or 10% and reapplied at 7
or 10 day intervals or according to Tom-Cast at 15, 20, or 25 disease severity

values (DSV) in 2002 ........................................................................... 88

Effects of spray initiation timings and application intervals on the area under
the disease progress curve and ﬁnal ratings for petiole health of ‘Prime Cut’
carrots treated with the fungicides chlorothalonil (1.29 kg a.i.lha) alternated
with azoxystrobin (0.11 kg a.i./ha) when initiated prior to disease development
(0%) or at disease levels of trace, 5%, or 10% and reapplied at 7 or 10 day
intervals or according to Tom-Cast at 15, 20, or 25 disease severity values

(DSV) for control of A. dauci and C. carotae in 2002 .................................... 90

APPENDIX A. SUMMARY OF WEATHER DATA, FUNGICIDE

22.

23.

APPLICATION DATES, AND FINAL DISEASE
ASSESSMENTS FOR FIELD STUDIES IN 2001 AND 2002

Weather data and Disease Severity Values (DSV) from the Michigan State
University Muck Soils Experimental Farm during the 2001 and 2002 growing

seasons ........................................................................................... 99

Dates of chlorothalonil (1 .29 kg a.i.lha) applications on ‘Cellobunch’ carrots

in 2001 ........................................................................................... 100

ix

Table

24.

25.

26.

27.

28.

29.

30.

31.

32.

LIST OF TABLES (cont’d)

13

Dates of chlorothalonil (1.29 kg a.i.lha) applications on ‘Cellobunch’ carrots
in 2002 .......................................................................................... 101

Summary of the effects of disease caused by A. dauci and C. carotae on

petiole blight, leaf blight, and yield of ‘Cellobunch’ carrots left untreated

or sprayed with the ﬁmgicide chlorothalonil (1 .29 kg a.i.lha) applied every

seven days or according to disease forecasters in 2001 and 2002 ...................... 102

Weather data and Disease Severity Values (DSV) from the trial site near
Fremont, MI during the 2001 and 2002 growing seasons ............................... 103

Dates of fungicide applications on ‘Early Gold’ and ‘Cellobunch’ carrots
using chlorothalonil (1 .29 kg a.i.lha) alternated with azoxystrobin (0.11 kg
a.i.lha) in 2001 .................................................................................. 104

Dates of fungicide applications on ‘Early Gold’ and ‘Cellobunch’ carrots
using chlorothalonil (1.29 kg a.i./ha) alternated with azoxystrobin (0.11 kg
a.i.lha) in 2002 ................................................................................. 105

Dates of fungicide applications on ‘Prime Cut’ carrots using chlorothalonil
(1 .29 kg a.i.fha) alternated with azoxystrobin (0.11 kg a.i./ha) in 2001 ............... 106

Dates of fungicide applications on ‘Prime Cut’ carrots using chlorothalonil
(1.29 kg a.i.lha) alternated with azoxystrobin (0.11 kg a.i.lha) in 2002 ............... 107

Summary of the effects of disease caused by A. dauci and C. carotae on petiole
blight, leaf blight, and yield of ‘Early Gold’ carrots left untreated or sprayed

with the fungicides chlorothalonil (1.29 kg a.i./ha) alternated with azoxystrobin

(0.11 kg a.i./ha) when initiated prior to disease development (0%) or at disease

levels of trace, 5%, or 10% leaf blight and reapplied at 10 day intervals or

according to Tom-Cast at 15, 20, or 25 disease severity values (DSV) in 2001

and 2002 ........................................................................................ 109

Summary of the effects of disease caused by A. dauci and C. carotae on petiole
blight, leaf blight, and yield of ‘Cellobunch’ carrots left untreated or sprayed

with the fungicides chlorothalonil (1.29 kg a.i./ha) alternated with azoxystrobin

(0.11 kg a.i.fha) when initiated prior to disease development (0%) or at disease

levels of trace, 5%, or 10% leaf blight and reapplied at 10 day intervals or

according to Tom-Cast at 15, 20, or 25 disease severity values (DSV) in 2001

and 2002 ........................................................................................ 1 10

LIST OF TABLES (cont’d)

Table Page

33. Summary of the effects of disease caused by A. dauci and C. carotae on petiole
blight, leaf blight, and yield of ‘Prime Cut’ carrots left untreated or sprayed
with the fungicides chlorothalonil (1.29 kg a.i.lha) alternated with azoxystrobin
(0.11 kg a.i./ha) when initiated prior to disease development (0%) or at disease
levels of trace, 5%, or 10% leaf blight and reapplied at 7 or 10 day intervals or
according to Tom-Cast at 15, 20, or 25 disease severity values (DSV) in 2001
and 2002 ........................................................................................ 11 1

APPENDIX B. PESTICIDE APPLICATION EQUIPMENT STUDIES IN 2001

34. Effect of nozzle type on foliar blights caused by Alternaria dauci and
Cercospora carotae on ‘Goliath’ carrots in 2001 ......................................... 116

35. Effect of fungicides on foliar blights caused by Alternaria dauci and
Cercospora carotae on ‘Goliath’ carrots in 2001 ......................................... 117

36. Effect of nozzle type on foliar blights caused by Altemaria dauci and
Cercospora carotae on ‘Goliath’ carrots in 2001 ......................................... 120

37. Effect of fungicides on foliar blights caused by Alternaria dauci and
Cercospora carotae on ‘Goliath’ carrots in 2001 ......................................... 121

xi

LIST OF FIGURES

Figure Page

CHAPTER I. COMPARING DISEASE FORECASTING SYSTEMS FOR
TIMING FUNGICIDE APPLICATIONS TO CONTROL
ALTERNARIA AND CERCOSPORA BLIGHTS OF CARROTS

1. Disease progress curves for petiole and leaf blight caused by A. dauci and
C. carotae on ‘Cellobunch’ carrots left untreated or treated with the fungicide
chlorothalonil (1.29 kg a.i.lha) applied every 7 days or according to disease
forecasters in 2001 and 2002 .................................................................. 33

CHAPTER II. TIMING FUNGICIDE APPLICATIONS ACCORDING TO
FIELD SCOUTING AND TOM-CAST TO CONTROL
ALTERNARIA AND CERCOSPORA BLIGHTS OF CARROTS

2. Disease progress curves for leaf blight caused by A. dauci and C. carotae on
‘Early Gold’ carrots left untreated or treated with chlorothalonil (1.29 kg a.i./ha)
alternated with azoxystrobin (0.11 kg a.i./ha) applied prior to blight symptom
development (0%) or at disease levels of trace, 5%, 10% and reapplied every
10 days or according to Tom-Cast using intervals of 15, 20, or 25 DSVs in 2001
and 2002 .......................................................................................... 58

3. Disease progress curves for petiole blight caused by A. dauci and C. carotae on
‘Early Gold’ carrots left untreated or treated with chlorothalonil (1.29 kg a.i.lha)
alternated with azoxystrobin (0.11 kg a.i./ha) applied prior to blight symptom
development (0%) or at disease levels of trace, 5%, 10% and reapplied every
10 days or according to Tom-Cast using intervals of 15, 20, or 25 DSVs in 2001
and 2002 .......................................................................................... 59

4. Disease progress curves for petiole blight caused by A. dauci and C. carotae on
‘Cellobunch’ carrots left untreated or treated with chlorothalonil (1.29 kg a.i.fha)
alternated with azoxystrobin (0.11 kg a.i./ha) applied prior to blight symptom
development (0%) or at disease levels of trace, 5%, 10% and reapplied every
10 days or according to Tom-Cast using intervals of 15, 20, or 25 DSVs in 2001
and 2002 .......................................................................................... 68

5. Disease progress curves for leaf blight caused by A. dauci and C. carotae on
‘Cellobunch’ carrots left untreated or treated with chlorothalonil (1.29 kg a.i./ha)
alternated with azoxystrobin (0.11 kg a.i.lha) applied prior to blight symptom
development (0%) or at disease levels of trace, 5%, 10% and reapplied every
10 days or according to Tom-Cast using intervals of 15, 20, or 25 DSVs in 2001
and 2002 .......................................................................................... 69

xii

LIST OF FIGURES (cont’d)

'6

Figure age

6. Disease progress curves for leaf blight caused by A. dauci and C. carotae on
‘Prime Cut’ carrots left untreated or treated with chlorothalonil (1 .29 kg a.i./ha)
alternated with azoxystrobin (0.11 kg a.i./ha) applied prior to blight symptom
development (0%) or at disease levels of trace, 5%, 10% and reapplied every
10 days or according to Tom-Cast using intervals of 15, 20, or 25 DSVs in 2001
and 2002 .......................................................................................... 76

7. Disease progress curves for petiole blight caused by A. dauci and C. carotae on
‘Prime Cut’ carrots leﬁ untreated or treated with chlorothalonil (1.29 kg a.i.lha)
alternated with azoxystrobin (0.11 kg a.i.lha) applied prior to blight symptom
development (0%) or at disease levels of trace, 5%, 10% and reapplied every
10 days or according to Tom-Cast using intervals of 15, 20, or 25 DSVs in 2001
and 2002 .......................................................................................... 77

8. Disease progress curves for petiole and leaf blight caused by A. dauci and C.
carotae on ‘Prime Cut’ carrots leﬁ untreated or treated with chlorothalonil
(1.29 kg a.i./ha) alternated with azoxystrobin (0.11 kg a.i.lha) applied prior to
blight symptom development (0%) or at disease levels of trace, 5%, 10% and
reapplied every 7 days in 2002 ............................................................... 78

xiii

LITERATURE REVIEW

Introduction

In 2001, Michigan harvested 6,300 acres of carrots (Daucus carota L. var. sativa
DC.) that yielded 115,500 tons for use in fresh market and processing (NASS, 2003).
Carrots grown for the fresh market were grown on 4,800 acres and valued at $23.2
million. Carrots for processing were grown on 1,500 acres and valued at $2.2 million.
Michigan ranks third and ﬁfth for production of fresh market and processing carrots,
respectively, and the state ranks third in total carrot production. California, Washington,
Colorado, Texas, and Wisconsin rank ﬁrst, second, fourth, ﬁﬁh, and sixth, respectively,
in total carrot production (NASS, 2003).

Michigan carrots are grown in deep, well-drained mineral and muck soils.
Carrots are planted during mid-April to mid-June and are harvested from the beginning of
August through November. Carrots are harvested by a machine that loosens the soil and
simultaneously grips the foliage, pulling the roots from the soil (Zandstra et al., 1986).

In Michigan, Alternaria leaf blight and Cercospora leaf blight are the most
common pathogens affecting carrot foliage. Alternaria dauci (Kuhn) Groves & Skolko
(Groves and Skolko, 1944) is favored by periods of warm, wet environmental conditions
(Doran and Guba, 1928). Similarly, Cercospora carotae (Passerini) Solheim is favored
by warm periods and high relative humidity. Environmental conditions that enhance the
grth of these pathogens frequently occur during the carrot-growing season in
Michigan. Both pathogens cause foliar blight resulting in a reduction of photosynthetic
capacity and weakening the petioles and foliage needed for mechanical harvest. Carrot
tops weakened by blight are easily detached from the root when gripped by mechanical

harvesters, resulting in unharvested carrots (Gillespie and Sutton, 1979; Strider, 1963).

Traditionally, blights are controlled through the use of protectant fungicides
applied every 7 to 10 days, from July through September (Gillespie and Sutton, 1979).
Calendar-based spray schedules typically require 5 to 9 applications and may result in
unneeded sprays, increasing production costs (Gillespie and Sutton, 1979), and the
likelihood of pathogens developing chemical insensitivity (Bolkan and Reinert, 1994).
Alternaria dauci

Alternaria dauci is distributed worldwide (Rotem, 1994) and is considered a
preeminent foliar pathogen in most carrot-growing regions (Hooker, 1944; Maude, 1966;
Netzer and Kenneth, 1969; Scott and Wenham, 1973). The fungus was ﬁrst reported in
the United States as Macrosporium carotae Ellis & Langlois (Ellis and Langlois, 1890).
The pathogen has a limited host range, infecting only wild carrot (Daucus carota L.) and
cultivated carrot (Daucus carota L. var. sativa DC.), both in the family Apiaceae (Doran
and Guba, 1928). The fungus is classiﬁed in the phylum Deuteromycota, order
Hyphomycetes, and family Dematiaceae (Rotem, 1994).

Conidiophores of A. dauci are olive-brown, septate, simple or branched, variable
in length, and 6-10u in diameter. Conidia are dark olive-brown, obclavate, with long
beaks, having 7-10 transverse septa and few longitudinal septa, born singly or in short
chains, and 150-25011 long by 15-25u in diameter (Groves and Skolko, 1944). In general,
A. dauci is capable of producing spores at all temperatures that occur during the growing
season (Strandberg, 1977). C onidia germinate over a wide temperature range from 16° to
28° C, but germinate best at 22° to 24° C (Doran and Guba, 1928). Hooker (1944)

concluded that maximum mycelial growth occurs at 28° C, and most conidia germinate

between 20° and 37° C. Sporulation is favored by darkness, a minimum leaf wetness
duration of 10 h, and temperatures from 11 to 23° C (Langenberg, 1975).

Though often confused with C. carotae, A. dauci generally infects older foliage
(Doran and Guba, 1928; Hooker, 1944; Maude, 1966), producing dark brown to black
irregularly shaped lesions along leaf margins. Under favorable conditions, blight
symptoms develop four to six days after inoculation and Sporulation occurs within eight
to ten days (Strandberg, 1977). Chlorosis surrounding the Alternaria lesion is more
pronounced than that caused by C. carotae (Hooker, 1944). The lesions expand and join
together, producing a blighted or burned appearance on the foliage. Severely infected
petioles may become girdled and disengage from the root.

A. dauci can infect all parts of the carrot inﬂorescence (Strandberg, 1983),
resulting in contaminated seed. Mycelium may penetrate the inner layer of the pericarp
tissue, and conidiophores protrude through the pericarp (N etzer and Kenneth, 1969).
Conidia adhering to the surface of seeds can germinate after 9 months (Strandberg, 1983)
or up to 30 months (Netzer and Kenneth, 1969) when stored at ambient conditions; thus,
infected seeds serve as a source of long-tenn primary inoculum (Strandberg, 1983).
When seeds are infected with A. dauci, pre or post-emergence damping-off may occur
(Maude, 1966; Neergaard, 1945; Netzer and Kenneth, 1969). Infected seedlings may
increase inoculum levels and contribute to early season leaf blight epidemics (Strandberg,
1984).

Concentrations of airborne A. dauci conidia and weather variables were recorded
in an area of intensive carrot production in Ontario (Langenberg et al., 1977). Peak

numbers of conidia were trapped between 1300 hours and 1500 hours. The declining

number of trapped conidia after 1500 hours were not correlated with weather variables,
but were assumed to be an indication of the number of mature conidia available for
dispersal. Release of A. dauci conidia was correlated with increasing temperatures,
increasing wind, decreasing RH, and drying of the foliage. Rain removed airborne
conidia from the air and contributed to prolonged periods of leaf wetness, which reduced
the number of airborne conidia by restricting spore liberation. Although wind
temporarily increased spore liberation and numbers of airborne conidia, extensive high
winds damaged conidiophores and resulted in few trapped conidia following the wind
period (Langenberg et al., 1977).

Other ﬁeld studies indicate that plentiful amounts of conidia are produced
following nights of 95—1 00% RH or leaf wetness for 8-12 hours (Strandberg, 1977).
Spore abundance is not well correlated with hours of RH above 95%, but hours of leaf
wetness are correlated with number of conidia trapped. Following daybreak, and when
the RH fell below 80%, conidia are liberated and disseminated by wind. Wind speeds 2-
3 m/sec are needed to dislodge large numbers of conidia (Strandberg, 1977).

Seasonal differences in the onset and development of Alternaria blight epidemics
have been observed. Doran and Guba (1928) found that young carrot plants (up to eight
weeks) were moderately resistant to infection, but susceptibility increased with plant age.
Hooker (1944) observed that older leaves were more susceptible to infection, causing the
disease to progress more rapidly at the end of the season. Zimmer and McKeen (1969)
suggested that the interaction of photoperiod and temperature was the cause of seasonal
differences. Strandberg (1977) attributed late season epidemics to the increased duration

of leaf wetness, typical during the cool, longer nights of late summer and early fall.

Cercospora carotae

Cercospora carotae occurs in temperate regions and is considered an important
foliar pathogen in most carrot-producing areas (Arcelin and Kushalappa, 1991; Thomas,
1943). C. carotae has a wide host range in the genus Daucus, infecting wild and
cultivated carrots (Daucus carota L. and Daucus carota L. var. sativa DC.), D.
hispanicus Gouan, D. maritimus Lam., D. pulcherrimus Koch ex DC., D. maximus Desf.,
D. gingidium L., and D. pusillus Mich. (Thomas, 1943). The fungus is classiﬁed in the
phylum Deuteromycota, order Hyphomycetes, and family Dematiaceae (Solheim, 1929).

Conidiophores of C. carotae are light yellow-brown, amphigenous, simple,
straight to subﬂexous, non-stromatic, with minute conidial scars near the tip, and 15-45p.
long by 3-5u in diameter. Conidia are at ﬁrst cylindrical then become narrowly
obclavate, bacilliforrn, hyaline to subhyline, continuous or obscurely 1-8 septate, and 30-
115111 long by 2-3u in diameter (Solheim, 1929).

The life cycle of C. carotae is similar to that of A. dauci. C. carotae grows
rapidly between 19° and 28° C, and maximum germination is observed at 16° to 28° C.
Sporulation occurs between 13° and 28° C but requires free moisture (Thomas, 1943).
Hooker (1944) made similar ﬁndings; the majority of conidia germinated at 20° to 32° C,
and the optimum temperature for mycelial development was 28° C. Wind is assumed as
the primary disseminating agent of C. carotae. Thomas (1943) collected conidia on agar
plates, exposed for 3 min, located 3, 30, and 92 m downwind from severely blighted
carrot ﬁelds.

Infection occurs only through stomata via a germ tube (Thomas, 1943). Lesions

may occur on the foliage and ﬁrst appear as pinpoint chlorotic spots that expand and

generate necrotic centers. Generally, lesions located away from the edge of the leaf are
circular, and lesions along leaf margins and petioles are elongate. A light gray or silvery
mass of conidia may be observed macroscopically on lower surfaces of the lesion during
periods of high humidity. Lesion expansion and Sporulation continues until spots
coalesce and the leaﬂet is killed (Thomas, 1943).

The relationship of temperature and leaf wetness duration on infection of C.
carotae was examined using a quantitative model developed under controlled
experimental conditions (Carisse and Kushalappa, 1990). In general, infection occurs
following 12 h of leaf wetness at temperatures 16, 20, 24, 28, and 32° C, and increases
with wetness duration, except at 32° C where infection decreases with increasing periods
of leaf wetness. Leaf wetness duration of 24 h and temperatures between 20 and 28° C
are required to promote extensive infection of C. carotae. Maximum numbers of lesions
were produced in growth chambers held at 16, 20, 24, and 28° C with 96 h of leaf
wetness (Carisse and Kushalappa, 1990).

Carisse and Kushalappa (1992) examined the influence of interrupted wet periods
and relative humidity on infection by C. carotae. An interrupted wet period consisted of
24 h of initial leaf wetness and 12 h of ﬁnal leaf wetness, separated by a dry period of 3,
6, 12, 18, 24, 30, or 36 h. Continuous leaf wetness durations were 36, 39, 42, 48, 54, 60,
66, and 72 h. In general, fewer lesions per plant were produced during interrupted wet
periods than continuous wet periods. However, plants subjected to the 3 to 24 h dry
periods, except for the 12 h dry period, produced more lesions than plants held at
continuous leaf wetness for 36 h. Therefore, germinated spores can survive dry periods

and resume infection when subjected to an additional wetness period. The number of

lesions per plant increased with temperatures of 16 to 28° and with an increase in
humidity level (Carisse and Kushalappa, 1992).

Carisse et a1. (1993) investigated the effect of temperature and duration of
different moisture conditions on sporulation of C. carotae. No sporulation occurred at a
relative humidity g 92%, but abundant conidia were produced under leaf wetness, 96%
RH, and 96% RH preceded by 12 h of leaf wetness. Sporulation increased with
increasing duration of moisture period or leaf wetness and increasing temperature from
16 to 28° C. Maximum sporulation was observed at 28° C after 96 h of leaf wetness,
although all temperature-moist conditions sporulated after 48 h. In general, numerous
conidia were produced after 48 h of leaf wetness at 20 to 28° C. The temperature range
for conidial infection is similar to that for sporulation, therefore, temperatures between 16
and 32° C accompanied by leaf wetness or RH 196% for 24 h are considered favorable
periods for sporulation of C. carotae (Carisse et al., 1993).

Disease Control Strategies

Alternaria and Cercospora leaf blights are managed through crop rotation,
disease-free seed, tolerant cultivars, and fungicide applications. Crop rotation was
recommended by Doran & Guba (1928) to obtain partial control of A. dauci and C.
carotae because these fungi overwinter in the soil and on infected carrot debris and may
infect subsequent carrot plantings. Deep plowing or the destruction of infected carrot
tops and planting of non-host crops following carrot production is encouraged to prevent
high levels of inoculum deposition in the soil. However, C. carotae was recovered ten
months after infected carrot leaves were placed in wire containers and buried 10 and 15

cm below the soil surface (Thomas, 1943). A. dauci survived longer on petioles on the

surface of the soil than on petioles buried at depths of 10 and 20 cm, although the
experiment duration was only ﬁve months (N etzer and Kenneth, 1969). Survival of A.
dauci is negatively correlated with increasing soil moisture (Pryor et al., 2002). Fallow
carrot ﬁelds in a warm, dry carrot production area in California allowed A. dauci to
survive on infected foliage on and below the soil surface for up to one year; reduced
survival was observed in irrigated alfalfa and rose ﬁelds and in a warm, moist carrot ﬁeld
in Florida. Plowing under of infected carrot residue hastens decomposition thereby
reducing survival, and may be more beneﬁcial for cooler carrot production areas where
decomposition rates are slower (Pryor et al., 2002). Volunteer carrot plants (Pryor et al.,
2002) and alternate hosts provide a source of inoculum for present and future carrot
plantings and should be removed from carrot-growing areas (Doran and Guba, 1928).
Seed treatments reduce damping-off of carrot plants, but these measures may not
entirely eradicate A. dauci from carrot seed. Maude (1966) claimed complete eradication
of A. dauci from carrot seeds by a 24-hour soak at 30° C in a 0.2% thirarn suspension,
without compromising seed germination. Strandberg (1984) repeated Maude’s
experiment with a larger number of seeds and a more sensitive assay method and
observed Alternaria blight symptoms, indicating that the seed treatment did not entirely
eradicate the pathogen. The use of 0.5% iprodione seed-soak for 24 hours at 30° C
apparently eradicated A. dauci from medium sized seed samples, but 0.01% infected
seedlings were detected in a larger seed sample using a similar assay method (Strandberg,
1984). Although hot water seed treatments have been tested (Strandberg, 1988), this
application alone is incapable of eliminating A. dauci without banning the seed

(Strandberg and White, 1989). In commercial carrot production, planting large quantities

of treated carrot seeds may result in several hundred infected plants per hectare, sufﬁcient
to cause an Alternaria blight epidemic (Strandberg, 1984).

Cultivar resistance to A. dauci and C. carotae was examined for 50 unsprayed
carrot varieties and breeding selections in 1999 (James et al., 1999). Forty percent of the
cultivars examined produced AUDPC (Area Under the Disease Progress Curve) values
that were signiﬁcantly less than the susceptible standard varieties. All 50 cultivars
showed symptoms of blight, and no cultivars displayed complete resistance to both A.
dauci and C. carotae. In another study, Strandberg et a1. (1972) evaluated 331 carrot
varieties from 31 countries for resistance to A. dauci and found nine varieties capable of
containing blight symptoms for the entire growing season without any fungicidal
application. Cercospora-tolerant plants exhibit fewer and smaller lesions than susceptible
cultivars (Angell and Gabelman, 1968). Cultivars with complete resistance to A. dauci or
C. carotae have not been identiﬁed, but blight tolerant cultivars are available.

Santos et a1. (2000) examined the use of gibberellic acid (GA) applications to
control Alternaria blight. Applications of (GA) or iprodione decreased the severity of
blight and increased the top and root weights compared to the untreated control. Disease
severity decreased linearly with an increase in GA concentration, although the high GA
concentration (250 mg/L) increased the amount of foliage at the expense of root mass.
The reduction in disease severity of the GA treated plants may be a result of the increase
in leaf length and more upright appearance of plants, allowing greater air circulation
through the crop canopy (Santos et al., 2000).

Fungicides are relied on as disease control tools. Traditionally, blights are

controlled through the use of protectant fungicides applied every 7 to 10 days, from July

10

through September (Gillespie and Sutton, 1979). Currently, two protectant active
ingredients (chlorothalonil and copper-based fungicides) and two systemic active
ingredients (iprodione and azoxystrobin) are registered to control Alternaria and
Cercospora (iprodione not registered) leaf blights in Michigan (Bird et al., 2002).

F ungicide resistance of A. dauci to iprodione has been reported (Fancelli and Kimati,
1991). In addition, iprodione and chlorothalonil are pesticides classiﬁed as B2
carcinogens and are scheduled for review by the EPA under the Food Quality Protection
Act (FQPA). The future availability of these products is uncertain. One Michigan food
processor does not allow the use of iprodione on carrots because it is a B2 carcinogen and
residues are a concern. Azoxystrobin (Quadris, Syngenta Crop Protection, Greensboro,
NC), a recently registered reduced risk systemic fungicide, is an effective tool to control
Alternaria and Cercospora blights in rotation with protectant fungicides (Hausbeck et al.,
2000; James and Stevenson, 1999). Use of this product in rotation with protectant
fungicides already used by Michigan growers may reduce the number of B2 carcinogenic
fungicide applications.

Most growers employ disease management practices to reduce the occurrence and
development of Alternaria and Cercospora blights, otherwise, harvesting losses are often
incurred. For example, a Wisconsin study reported the standard weekly fungicide
program yielded 15.7 tons/A while the untreated plot yielded 11.7 tons/A and resulted in
23.8% unmarketable carrots (James and Stevenson, 1999).

Disease Scouting
Growers and agricultural consultants utilize ﬁeld scouting to monitor pests, soil

moisture, projected yield, and other various aspect of a ﬁeld or the crop grown in the

11

ﬁeld. Such observations often contribute to the decision making process for
implementing management strategies. One disease forecasting system used to time
sprays for controlling A. dauci on carrots incorporates the use of ﬁeld scouting to
determine when the ﬁrst fungicide application should be made (Gillespie and Sutton,
1979). Fields are scouted and the ﬁrst ﬁmgicide spray is applied when blight symptoms
developed on 1 to 2% of the foliage. This level of disease was selected because it was
the ﬁrst observable stage of Alternaria blight. In each of four years, one to three spays
were saved by delaying the initial fungicide application until blight symptoms were
detected by scouting compared with the standard fungicide schedule (Gillespie and
Suuon,l979)

A tomato Integrated Pest Management (IPM) program advised that ﬁelds should
be scouted one or two times per week for diseases and insects (Keinath et al., 1996).
Fungicide sprays to control early blight (Alternaria solani) were initiated when 3 to 6%
of the leaf area showed symptoms of disease. The IPM scouted plots were initially
sprayed 42 days after the standard weekly fungicide schedule commenced. The delay in
initiating the IPM treatment resulted in higher disease severity and a reduction in yield of
extra large fruit compared to the weekly fungicide schedule. Management of early blight
using scouting in the tomato IPM program was unsuccessﬁrl, but may be improved by
lowering the spray initiation threshold to 1 to 3% leaf area blighted (Keinath et al., 1996).
Disease Forecasting

European studies in the early to mid-19005 on infection periods for Plasmopara
viticola and Phytophthora infestans represented the beginning of disease forecasting.

Other terms describing the concept of disease forecasting include disease predictive

l2

schemes and disease warning systems, but the term “disease forecasting” will be used
here. Most disease forecasting systems rely on weather variables (Zadoks, 1984), in
conjunction with the biology and epidemiology of the pathogen, to predict infection or
developmental periods of the disease on a particular host (Krause and Massie, 1975).
Forecasting systems are appropriate for diseases that are economically signiﬁcant,
somewhat variable between seasons, controlled by available and economical methods,
and known to depend on speciﬁc weather factors, as investigated by laboratory or other
experiments (Bourke, 1970). Furthermore, disease-forecasting systems are suited for
IPM systems. The potential beneﬁts from using disease-forecasting systems include cost
effective disease control, increased attention of farmers to the biology of the cropping
system, and reduced environmental contamination (Johnson, 1987).

A disease forecasting model was created to 1) identify environmental conditions
that favor the development of early blight on tomato, and 2) enhance the efﬁciency of
fungicide use (Madden et al., 1978). FAST (Forecaster of Alternaria solani on Tomato)
uses daily values for maximum and minimum air temperature, hours of leaf wetness,
maximum and minimum air temperatures during the leaf wetness period, hours of relative
humidity greater than 90%, and rainfall (Madden et al., 1978). Field studies proved the
weather-prompted FAST system to be as effective as standard spray regimes for
controlling early blight (Madden et al., 1978). FAST was subsequently evaluated for
predicting infection periods and timing fungicide applications for control of Stemphylium
vesicarium on pear (Montesinos, 1992). Commercial orchard studies showed that FAST

resulted in 28-3 8% fewer fungicide applications, compared with the standard 7-day

l3

commercial schedule, while maintaining the same level of disease control (Montesinos,
1992).

The FAST system was modiﬁed by RE. Pitblado to meet the needs of Ontario
tomato growers for controlling early blight (Alternaria solani Sorauer), septoria leaf spot
(Septoria lycopersici), and fruit anthracnose (Collectotricum coccodes) (Pitblado, 1988,
1992). The reﬁned system, Tom-Cast (TOMato disease foreCASTer), calculates daily
disease severity values (DSVs) based on hours of leaf wetness and the average air
temperature during the wetness period. Sprays are initiated when the DSV reaches a
threshold value, and, after spraying, the DSV is reset to zero (Pitblado, 1992).

Tom-Cast was evaluated as a disease management tool for timing fungicide
applications to control purple spot (Stemphylium vesicarium) on asparagus (Meyer et al.,
2000). The Tom-Cast spray program prompted an equal or fewer number of sprays and
provided better disease control than the 14-day standard program. Additionally, some
newly established asparagus plots managed according to Tom-Cast resulted in increased
fern stands (Meyer et al., 2000).

Gillespie and Sutton (1979) developed a predictive system for timing fungicide
applications for the control of Alternaria leaf blight on carrots. The criteria of the system
were to: 1) apply the initial fungicide application after 1 to 2% of the foliage showed
symptoms of blight; 2) apply subsequent fungicide applications only when the 36 hour
predicted weather favored the development of blight; and 3) apply ﬁmgicide at no more
than a minimum of 7 or 10 days (Gillespie and Sutton, 1979). Commercial ﬁeld
experiments showed that two to four weather-timed sprays proved the predictive system

controlled blight as effectively as six or seven weekly applications. In addition, this

14

program eliminated the need for one to three sprays before the blight symptoms reached 1
to 2% (Gillespie and Sutton, 1979).

A disease forecasting system has been developed for timing sprays to control
Cercospora apii (Fres.) on celery (Berger, 1969a, 1969b). A hygrothennograph and
spore trap were used to identify sporulation periods. Sporulation increased following
nights when temperatures ranged from 15 to 30° C and the relative humidity was near
100% for 8 or more hours. Fewer spores were caught following nights when the
temperature dropped below 15° C regardless of the duration of high humidity. If
temperatures fell below 12° C, two consecutive nights with humidity levels near 100%
and temperatures ranging from 15 to 30° C were required for C. apii to resume
signiﬁcant sporulation. Five to ﬁfteen sprays were saved during the 1968 winter growing
season by utilizing the disease forecaster to time fungicide applications (Berger, 1969b).

There is substantial evidence that disease forecasting systems can reduce the
number of fungicide applications per season or increase the efﬁcacy of sprays by
prompting sprays only when the environment is conducive for disease development.
Disease forecasting systems may be appropriate for managing Alternaria and Cercospora

blights in Michigan.

15

LITERATURE CITED

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carota var. sativa, to the leaf spot fungus, Cercospora carotae. Proceedings of the
American Society for Horticultural Science 93:434-437.

Arcelin, R., and Kushalappa, AC. 1991. A survey of carrot diseases on muck soils in
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Berger, R.D. 1969a. A celery Early Blight spray program based on disease forecasting.
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Bird. G., Bishop, B., Graﬁus, E.J., Hausbeck, M.K., Jess, L.J., Kirk, W., and Pett, W.
2002. Insect, disease and nematode control for commercial vegetables. East Lansing,
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Bolkan, HA, and Reinert, W.R. 1994. Developing and implementing IPM strategies to
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Bourke, P.M.A. 1970. Use of weather information in the prediction of plant disease
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Carisse, 0., and Kushalappa, AC. 1990. Development of an infection model for
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Phytopathology 80: 1233-1238.

 

. 1992. Inﬂuence of interrupted wet periods, relative humidity, and temperature
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Carisse, 0., Kushalappa, A.C., and Cloutier, DC. 1993. Inﬂuence of temperature, leaf
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Ellis, J .B., and Langlois, AB. 1890. New species of Louisiana fungi. Journal of
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Fancelli, M.I., and Kimati, H. 1991. Occurence of iprodione resistant strains of
Alternaria dauci. Summa Phytopathologica 17:135-146.

16

Gillespie, T.J., and Sutton, J .C. 1979. A predictive scheme for timing fungicide
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Pathology 1:95-99.

Groves, J .W., and Skolko, A.J. 1944. Notes on seed-bome fungi. ll. Alternaria.
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Hausbeck, M.K., Cortright, RD, and Lindennan, SD. 2000. Chemical control of
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Hooker, J.W. 1944. Comparative studies of two carrot leaf diseases. Phytopathology
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James, R.V., and Stevenson, W.R. 1999. Evaluation of selected fungicides to control
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James, R.V., Stevenson, W.R., and Rand, RE. 1999. Evaluation of carrot cultivars and
breeding selections to identify resistance to foliar blights, 1999. Madison, University
of Wisconsin. Wisconsin vegetable disease control trials, 1999, 73-75.

Johnson, KB. 1987. The role of predictive systems in disease management. Edited by P.
S. Teng., Crop loss assessment and pest management. APS Press, St. Paul, MN,
pp. 1 76-190.

Keinath, A.P., DuBose, V.B., and Rathwell, PI. 1996. Efﬁcacy and economics of three
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tomato. Plant Disease 80: 1277-1282.

Krause, RA, and Massie, LB. 1975. Predictive systems: modern approaches to disease
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Langenberg, W.J., Sutton, J .C ., and Gillespie, T.J. 1977. Relation of weather variables
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17

Meyer, M.P., Hausbeck, M.K., and Podolsky, R. 2000. Optimal fungicide management
of purple spot of asparagus and impact on yield. Plant Disease 84:525-530.

Montesinos, E. 1992. Evaluation of FAST as a forecasting system for scheduling
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18

 

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19

CHAPTER I.
COMPARING DISEASE FORECASTING SYSTEMS FOR TIMING FUNGICIDE

APPLICATIONS TO CONTROL ALTERNARIA AND CERCOSPORA BLIGHTS
OF CARROTS

20

ABSTRACT

Alternaria dauci and Cercospora carotae cause foliar blight of carrots and can
reduce the harvestable yield in severely blighted ﬁelds. Traditionally, fungicides are
applied every 7 to 10 days, regardless of weather conditions or disease pressure. The
objective of this study was to evaluate available disease forecasting systems for timing
fungicide sprays to limit foliar blights, including 1) a modiﬁed disease forecaster
previously tested for timing sprays to control Cercospora apii on celery, 2) an Alternaria
disease forecaster designed to time sprays for controlling A. dauci on carrots but not yet
tested in Michigan, and 3) Tom-Cast, originally developed to predict the occurrence of
diseases on tomatoes. Chlorothalonil was applied every seven days or according to the
forecasting systems in 2001 and 2002. Sprays applied according to Tom-Cast 15 DSV
resulted in a fungicide savings of $47.25 and $54.88 per acre in 2001 and 2002,
respectively, compared with the 7-day schedule, while providing similar blight control.
The number of sprays was reduced when fungicides were applied according to modiﬁed
predictive systems for Alternaria and Cercospora compared with the 7-day schedule, but
acceptable blight control was not always achieved. The Tom-Cast disease forecaster was
easy to use and reliable for determining the appropriate timing of fungicide applications

on CEII'TOIS.

21

INTRODUCTION

Alternaria dauci (Kiihn) Groves & Skolko and Cercospora carotae (Passerini)
Solheim infect the leaves and petioles of carrots (Daucus carota L. var. sativa DC.)
causing a foliar blight that contributes to harvesting losses. Alternaria blight symptoms
initially appear as irregularly shaped necrotic lesions along leaf margins (Hooker, 1944).
Symptoms of Cercospora blight are more distinct and appear as small, pinpoint necrotic
lesions surrounded by a chlorotic halo (Thomas, 1943). Both fungi are capable of
infecting carrot petioles and cause tan to black colored lesions surrounded by a light tan
or gray halo. Petiole infection occurs when favorable conditions are maintained for long
durations inside the dense crop canopy. If favorable conditions persist, entire leaﬂets and
petioles become blighted and will not withstand the pull of mechanical harvesters. An
increase in harvesting difﬁculty and yield reduction occurs when severely blighted
foliage detaches from the root during mechanical harvest or when plants are defoliated by
blights (Gillespie and Sutton, 1979; Strider, 1963).

Carrot growers in Michigan are advised to apply registered fungicides at 7 to 14
day intervals following crop emergence (Bird et al., 2002). However, calendar based
spray schedule do not take into account when environmental conditions are unfavorable
for blight development and needless sprays can be applied.

Numerous disease forecasting systems exist that alert growers when a fungicide
spray is needed based on environmental conditions. One such system has been developed
for timing sprays to control Cercospora apii (Fres.) on celery (Berger, 1969a, 1969b). A
hygrothennograph and spore trap were used to identify sporulation periods. Sporulation

increased following nights when temperatures ranged from 15 to 30° C and the relative

22

humidity (RH) was near 100% for eight or more hours. Fewer spores were caught
following nights when the temperature dropped below 15° C regardless of the duration of
high humidity. If temperatures fell below 12° C, two consecutive nights with humidity
levels near 100% and temperatures ranging from 15 to 30° C were required for C. apii to
resume signiﬁcant sporulation. Five to ﬁfteen sprays were saved during the 1968 winter
growing season by utilizing the disease forecaster to time fungicide applications (Berger,
1969b)

Gillespie and Sutton (1979) developed a disease forecasting system to time
fungicide applications for controlling Alternaria blight of carrots. The criteria of the
system included the following: 1) apply the initial fungicide application after 1 to 2% of
the foliage showed symptoms of blight; 2) apply subsequent fungicide applications only
when the 36 hour predicted weather favored the development of blight; and 3) apply
fungicide at no more than a minimum of 7 or 10 days. To determine if the upcoming 36
hours were favorable for blight development, the system used forecasted weather
information to produce an infection index that was calculated by comparing forecasted
temperatures and estimated leaf wetness durations. Regional forecasts of rain, cloud
cover, and wind speeds were used to derive the leaf wetness duration. Commercial ﬁeld
experiments showed that two to four weather-timed sprays controlled blight as effectively
as six or seven weekly applications. In addition, this program eliminated the need for one
to three sprays before the blight symptoms reached 1 to 2% compared with the standard
ﬁmgicide schedule (Gillespie and Sutton, 1979).

The Tom-Cast disease forecasting system is a modiﬁed version of F .A.S.T.

(Forecaster for Alternaria solam’ Sorauer on Tomato). Tom-Cast was designed to include

23

control for anthracnose fruit rot (Collectotrichum coccodes (Wallr.) Hughes) and Septoria
leaf spot (Septoria lycopersici (Speg)) in addition to early blight (Alternaria solam'
Sorauer) (Pitblado, 1988). For each 24-hour period (11:00 AM to 11:00 AM), Tom-Cast
uses the hours of leaf wetness and the average temperature during the wetness periods to
calculate a Disease Severity Value (DSV) ranging from O to 4, corresponding to
environmental conditions unfavorable to highly favorable for disease development.
Daily DSV values are summed and accumulate until a threshold value is reached, a
fungicide spray is applied, and the DSV total is reset to zero (Pitblado, 1988, 1992).

With increases in production costs and public concerns regarding pesticide use, it
is desirable to evaluate Integrated Pest Management (IPM) methods for managing
Alternaria and Cercospora blights of carrots. Improved methods for determining the
appropriate timing of fungicide applications are needed to make blight control more cost
effective without compromising quality and yield. The objective of this study was to
evaluate available disease forecasting systems for timing sprays to limit foliar blights,
including 1) a modiﬁed disease forecaster previously tested for timing sprays to control
Cercospora apii on celery, 2) an Alternaria disease forecaster designed to time sprays for
controlling A. dauci on carrots but not yet tested in Michigan, and 3) Tom-Cast,
originally developed to time sprays for controlling diseases on tomatoes.

MATERIALS AND METHODS

Plot establishment. Plots were established at the Michigan State University
Muck Soils Experimental Farm in Bath, Michigan in 2001 and 2002. Beds were formed
and ‘Cellobunch’ carrot seeds were planted on 14 May 2001 and 21 May 2002 in

I-Ioughton Muck soil, previously planted with potato. Seeds were spaced 1.43 cm apart in

24

rows spaced 45.7 cm apart on three-row raised beds measuring 162.6 cm from center to
center. Plant populations were 631,583/ha (2001) and 484,97l/ha (2002). All treatment
plots were 6.1 m long. One and a half meter sections of unsprayed carrots separated
treatment plots within beds, and one bed of carrots was left untreated between treatment
beds. Natural inoculum was relied on for infecting plants. Weeds, insects, and
fertilization requirements were managed according to standard production practices (Bird
et al., 2002; Wamcke et al., 1992; Zandstra, 2002). Plots were sprinkler irrigated as
needed.

Weather monitoring. Hourly measurements of temperature, relative humidity,
and leaf wetness duration were obtained using a digital data recorder (WatchDog
Temperature and Relative Humidity Logger 3684; Spectrum Technologies, Inc.,
Plainﬁeld, Illinois) placed in the ﬁeld prior to row closure in mid-June. The external leaf
wetness sensor (WatchDog Leaf Wetness Sensor 3666; Spectrum Technologies, Inc.,
Plainﬁeld, Illinois) was located in the upper 75% of the crop canopy in the center of an
unsprayed bed at a 45° angle facing north. Data were downloaded every other day to a
laptop computer using a computer program (Specware 6.01; Spectrum Technologies,
Inc., Plainﬁeld, Illinois) equipped to calculate DSVs for the Tom-Cast system. The
program was set to record temperatures from 0 to 100° C and to detect leaf wetness
whenever moisture was present on the leaf wetness grid. A summary of weather data and
DSV accumulation from the trial site in 2001 and 2002 is listed in Appendix A (Table
22).

Disease forecasting programs and fungicide treatments. A modiﬁed version

of the C. apii disease forecasting system was tested in this study. The modiﬁed system

25

omitted the spore trap and relied solely on hourly measurements of temperature and
relative humidity. A fungicide spray was applied if all of the following criteria were met:
1) no fungicide was applied during the past seven days; 2) 3 12 hours of R.H. 2 90%
were recorded the previous day (0700 yesterday to 0600 today); 3) temperatures ranged
15-27° C during previous day; 4) temperatures during the past three days were 3 12° C
or, if the temperature was below 12° C, the night temperatures (2200 to 0700) on the two
succeeding nights (yesterday and the day before yesterday; yesterday being 2200 last
night until 0700 this morning) were 2 15° C and had R.H. 3 95%. Modiﬁcations to the
original forecasting system were made according to recommendations (M.L. Lacy,
unpublished data).

Sprays were applied according to a modiﬁed disease forecaster designed to time
sprays for controlling A. dauci on carrots (Gillespie and Sutton, 1979). In the present
study, fungicides were applied only after 1 to 2% of the foliage displayed disease
symptoms, with a minimum reapplication interval of seven days. Subsequent sprays
were applied the day before forecasted rain or before nights when the forecasted
minimum temperature was 2 16° C.

The Tom-Cast disease forecasting systems was tested for timing sprays to control
Alternaria and Cercospora blights. For each 24-hour period (1 1 :00 AM to 11:00 AM),
Tom-Cast used the hours of leaf wetness and the average temperature during the wetness
periods to calculate a DSV ranging from 0 to 4, corresponding to environmental
conditions unfavorable to highly favorable for disease development (Pitblado, 1992).
Daily DSVs were summed and accumulated until a threshold value of 15 DSV was

reached, a fungicide spray was applied, and the DSV total was reset to zero.

26

Chlorothalonil (Bravo Ultrex 82.5WDG at 1.29 kg a.i./ha, Zeneca Ag Products,
Wilmington, DE) was applied to all treatment plots, excluding the control. Fungicides
were applied with a C02 backpack sprayer (R & D Sprayers, Opelousas, LA) equipped
with three Teejet XR11002VS ﬂat-fan nozzles (Spraying Systems Co., Wheaton, IL)
spaced 45.7 cm apart, operating at 344.8 kPa, and delivering 467.6 liters/ha. Sprays were
applied every seven days or according to the modiﬁed Cercospora predictor, the modiﬁed
Alternaria predictor, or Tom-Cast using a threshold of 15 DSVs. Treatment plots were
arranged in a randomized complete block design and replicated four times.

Initial sprays for the weekly schedule and Tom-Cast were applied prior to blight
symptom development on 2 July 2001 and 8 July 2002. The modiﬁed Cercospora
predictor prompted the ﬁrst application on 24 July 2001 and 11 July 2002. Initial sprays
for the Alternaria predictor occurred when the 1 to 2% blight threshold was reached on
14 August 2001 and 4 August 2002. In 2001, the 7-day schedule, modiﬁed Cercospora
predictor, modiﬁed Alternaria predictor, and Tom-Cast treatments received 13, 4, 5, and
8 chlorothalonil applications, respectively. In 2002, the 7-day schedule, modiﬁed
Cercospora predictor, modiﬁed Alternaria predictor, and Tom-Cast treatments received
13, 7, 6, and 6 chlorothalonil applications, respectively. The total cost of each fungicide
program was calculated by multiplying the number of applications by the cost of
chlorothalonil used (Table 1). Dates of fungicide applications are listed in Appendix A
(Tables 23 and 24).

Disease assessment. All disease assessments were made from the center 3.05 m
of the middle row of each plot. Leaf blight severity was determined biweekly (2001) or

weekly (2002) using an expanded Alternaria leaf blight assessment key (Strandberg

27

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28

1988). Plots were visually assessed as having 0, 1, 5, 10, 20, or 40% leaf blight, and the
key was expanded to estimate 60, 80, and 100% leaf blight. Petiole disease incidence
was determined biweekly (2001) or weekly (2002) by counting the number of plants with
one or more lesions. Petiole disease severity was evaluated concurrently with petiole
disease incidence using the following scale: 1 = average of 0 to 5 lesions per plant; 2 = 6
to 20 lesions; 3 = 21 to 50 lesions; 4 = more than 50 lesions; 5 = dead. An additional
petiole rating was conducted to estimate the overall health, amount of dead and living
petioles, and condition of petioles for harvest. Petiole health was determined on the day
before harvest in 2001 and weekly in 2002 starting 2 September and continuing with
other disease evaluations until harvest. The following scale (l-lO) was used to assess
petiole health: where l = petioles healthy and vigorous to 10 = petioles unhealthy, weak,
or dead. The area under the disease progress curve (AUDPC) was calculated to express
the cumulative disease incidence on petioles, severity of disease on petioles, leaf blight,

and petiole health (2002 only) by the calculation described by Shaner and Finney (1977):
n

A UDPC -: §1[(Yi+m+ Yr )/2][xi+1- xi ]
where Y,- = percent foliar blight, percent petiole blight, or petiole health rating at the ith
observation, X,- = time (days) at the ith observation, and n = total number of observations.
Carrots in the center 3.05 m of the middle row of each plot were hand-harvested, the
foliage was removed at the crown. and roots were weighed to determine yield on 2
October 2001 and 2002. Leaves showing blight symptoms were periodically removed

from untreated buffer rows and examined under magnification (ZOOX) in the laboratory to

confirm the presence of A. dauci and/or C. carotae.

29

Statistical analysis. Data for all disease assessments and yield were analyzed
with analysis of variance (ANOVA) using the Proc GLM procedure of the Statistical
Analysis System (SAS Institute, Cory, NC) in which a linear model including treatment,
year, year by treatment, and replicate nested within year were factors. The assumptions
of normality and equal variances were examined using the residuals from the ANOVA.
Normality was examined using the Proc Univariate procedure of SAS, and the equal
variance assumption was assessed by plotting the residuals. The AUDPC data for petiole
blight incidence were not normally distributed and were transformed to normality using:
square root (AUDPC). There were signiﬁcant year by treatment interactions for the
AUDPC data for petiole blight severity and the petiole health data from the ﬁnal rating,
so analyses were done separately for each year using an analysis of a randomized
complete block experiment. Data were analyzed using a linear model that included
treatment and replicate as factors using the Proc GLM procedure of SAS. Treatment
effects were examined using Tukey’s Studentized Range (HSD) test.

RESULTS

The 7-day schedule was the most costly fungicide program (Table 1), since it
required 13 applications. In 2001, the modified Cercospora predictor, modified
Alternaria predictor, and Tom-Cast disease forecaster eliminated nine, eight, and ﬁve
applications, respectively, saving $85.05, $75.60, and $47.25 per acre compared with the
7-day schedule. In 2002, the modified Cercospora predictor, modiﬁed Alternaria
predictor, and Tom-Cast disease forecaster eliminated six, seven, and seven applications,
respectively, saving $47.04, $54.88, and $54.88 per acre compared with the 7-day

schedule.

30

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Leaf blights were detected in plots during the ﬁrst week of August in 2001 and
2002 (Figure 1). Petiole blight progressed rapidly during August 2002 when the
incidence of petiole blight in untreated plots went from 4% to 97% in a 29-day period
from 4 August through 2 September. The incidence of petiole blight in untreated plots
reached 84.2% and 100% on 22 September 2001 and 8 September 2002, respectively
(Figure l). A summary of ﬁnal disease assessments is listed in Appendix A (Table 25).

According to the AUDPC data, all fungicide treatments were equally effective in
signiﬁcantly reducing petiole blight severity and leaf blight compared with the untreated
(Table 2). The AUDPC data of petiole blight incidence indicated that the Tom-Cast
system was comparable to the 7-day schedule and the modiﬁed Cercospora predictor.
The 7-day schedule signiﬁcantly reduced the incidence of petiole blight compared with
the modiﬁed Cercospora predictor. Disease in the untreated plots and the modiﬁed
Alternaria predictor plots did not differ (Table 2).

Untreated plots reached ﬁnal petiole health ratings of 8.25 and 8.50 in 2001 and
2002, respectively and were signiﬁcantly different from all of the fungicide treatments
(Table 3). The petiole health data suggest that all fungicide treatments were similar, and
according to the AUDPC data reduced disease compared with the untreated plots. In
2001, the 7-day schedule and Tom-Cast improved petiole health compared with the
modiﬁed Alternaria predictor. Tom-Cast was also similar to the modiﬁed Cercospora
predictor, which had a signiﬁcantly higher petiole health rating compared to the 7-day
schedule. In 2002, the modiﬁed Cercospora predictor and Tom-Cast application
schedules were not signiﬁcantly different from the 7-day schedule. Although the

modiﬁed Cercospora predictor and Tom-Cast treatments were comparable to the

32

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(1.29 kg a.i.fha) applied every 7 days or according to disease forecasters in 2001 and
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33

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34

modiﬁed Alternaria predictor, the Alternaria predictor resulted in signiﬁcantly more
disease than the 7-day schedule (Table 3).

The analysis of yield indicated a signiﬁcant treatment effect (Table 1). All
fungicide-treated plots were similar to one another, but the untreated had signiﬁcantly
lower yields than the modiﬁed Cercospora predictor and Tom-Cast treatments.

All disease forecasters provided similar disease control compared with the 7-day
schedule by limiting the AUDPC for petiole blight severity, petiole health, and leaf
blight. However, the modiﬁed Cercospora and Alternaria predictors were not always as
effective as Tom-Cast in limiting the AUDPC for petiole blight incidence and improving
the ﬁnal petiole health rating.

DISCUSSION

Carrot growers in Michigan have been concerned that they may be applying
fungicides more frequently than needed and when environmental conditions do not favor
blight development. An alternative method of scheduling fungicide applications was
needed. The goal of this study was to determine if disease forecasting systems could be
used to effectively time fungicide applications to control Alternaria and Cercospora
blights without compromising yield.

Although the Tom-Cast disease forecaster generally eliminated the least number
of sprays compared with the other forecasting systems tested, it was the most effective in
consistently limiting disease when compared to the 7-day schedule. In addition, Tom-
Cast was the most reliable and simple disease forecaster tested in this study. The hourly
measurements of leaf wetness and temperature during the wetness periods were

downloaded to a laptop computer equipped with the Specware program that calculated

35

the DSV accumulation each day. The automated system eliminated the tedious
calculations required to time sprays according to the modiﬁed Cercospora predictor. The
use of in-ﬁeld environmental measurements was a beneﬁt of Tom-Cast disease forecaster
when compared with the modiﬁed Alternaria predictor. For example, the leaf wetness
sensor detected the moisture generated by irrigation events, thereby extending the hours
of leaf wetness used in the DSV calculation.

The modiﬁed Cercospora predictor required daily monitoring once the 7-day
spray interval expired. If at least 12 hours with RH. 390% had occurred and
temperature requirements were met, several other measurements from the previous two
and three days had to be considered before a spray was prompted. The calculation
process was not automated using a computer program and required a considerable
amount of time that growers would likely avoid.

The modiﬁed Alternaria predictor required a spray the day before forecasted rain
assuming seven days had passed since the last fungicide application. Plots at the muck
farm were irrigated, but irrigation was not considered as a rain event in this study. If
irrigation was considered as a rain event, the modiﬁed Alternaria predictor would have
prompted several more sprays because plots were irrigated once and sometime twice a
week during July and August. The projected number of sprays applied according to the
modiﬁed Alternaria predictor would have been similar to that of the 7-day schedule if
overhead irrigations were considered as rain events.

The yield measurements recorded in this study does not reﬂect yields that may be
recorded in a commercial production situation where carrots are mechanically harvested.

All carrots in 3.05 m of the center row were hand harvested, whereas yield losses may

36

have become increasingly evident if plots were harvested mechanically. Based on
observations from mechanically harvested commercial carrot ﬁelds, yield reduction
occurs frequently when the ﬁnal petiole health is a rating of ﬁve or above.

Disease forecasting is a viable and economical alternative to the calendar-based
fungicide schedule currently used in Michigan. The disease forecasters tested in this
study prompted fewer sprays than the 7-day treatment, but did not always provide disease
control similar to the 7-day schedule. The Tom-Cast disease forecasting system
controlled disease as effectively as the 7-day schedule while reducing the number of
sprays by 38 and 54% in 2001 and 2002, respectively. The adoption of disease
forecasting systems will likely depend on the reliability and simplicity of the particular
system. Tom—Cast was a dependable and economical system for timing fungicide sprays

to control Alternaria and Cercospora blights of carrots.

37

LITERATURE CITED

Berger, R.D. 1969a. A celery Early Blight spray program based on disease forecasting.
Proceeding from Florida State Horticultural Society Meeting 82:107-111.

 

. 1969b. Forecasting Cercospora blight of celery in Florida. (Abstr.)
Phytopathology 5921018.

Bird, 6., Bishop, B., Graﬁus, E.J., Hausbeck, M.K., Jess, L.J., Kirk, W., and Pett, W.
2002. Insect, disease and nematode control for commercial vegetables. East
Lansing, Michigan State University. E - 312, 44-47.

Gillespie, T.J., and Sutton, J .C. 1979. A predictive scheme for timing fungicide
applications to control Alternaria leaf blight in carrots. Canadian Journal of Plant
Pathology 1295-99.

Hooker, J .W. 1944. Comparative studies of two carrot leaf diseases. Phytopathology
34:606-612.

Pitblado, RE. 1988. Development of a weather-timed fungicide spray program for ﬁeld
tomatoes. (Abstr.) Canadian Journal of Plant Pathology 10:371.

 

. 1992. Development and implementation of Tom-Cast. Ontario Ministry of
Agriculture and Food.

Shaner, G., and Finney, RR. 1977. The effect of nitrogen fertilization on the expression
of slow-mildewing resistance in Knox wheat. Phytopathology 67:1051-1056.

Strandberg, J.O. 1988. Establishment of Alternaria leaf blight on carrots in controlled
environments. Plant Disease 72:522-526.

Strider, D.L. 1963. Control of Alternaria blight of carrot. Plant Disease Reporter 47:66-
69.

Thomas, HR. 1943. Cercospora blight of carrot. Phytopathology 33:114-125.
Wamcke, D.D., Christenson, D.R., Jacobs, L.W., Vitosh, M.L., and Zandstra, EH. 1992.
Fertilizer recommendations for vegetable crops in Michigan. East Lansing, MI,

Michigan State University. E - SSOB, 20-24.

Zandstra, EH. 2002. Weed control guide for vegetable crops. East Lansing, MI,
Michigan State University. E - 433, 16.

38

CHAPTER II.
TIMING FUNGICIDE APPLICATIONS ACCORDING TO FIELD SCOUTING

AND TOM-CAST TO CONTROL ALTERNARIA AND CERCOSPORA
BLIGHTS OF CARROTS

39

ABSTRACT

Fungal foliar blights of carrots, caused by Alternaria dauci and Cercospora
carotae, result in necrotic lesions on leaves and petioles that may cause defoliation,
decreasing the efﬁciency of mechanical harvest. Traditionally, fungicides are applied
every 7 to 10 days, regardless of weather conditions or disease pressure. The primary
objectives of this study were to evaluate the Tom-Cast disease forecasting system for
timing ﬁmgicide sprays to control foliar blights, and to determine when to apply the ﬁrst
spray based on ﬁeld scouting and disease incidence. Chlorothalonil alternated with
azoxystrobin was applied every 10 days or according to Tom-Cast with a threshold of 15,
20, or 25 disease severity values (DSVs). Sprays for these programs were initiated prior
to symptom development, or when foliage was infected at a trace, 5%, or 10% level. Up
to four sprays were omitted saving $46.05 and $41.85 per acre in 2001 and 2002,
respectively, and comparable disease control was achieved by initiating applications
when a trace amount of the foliage was blighted and applying subsequent sprays
according to Tom-Cast 15 DSV, compared with calendar-based sprays initiated prior to
blight symptom development. Field scouting and the Tom-Cast disease forecaster appear
to be valuable tools for determining the appropriate timing of fungicide applications on

carrots while making blight control more cost effective.

40

INTRODUCTION

Alternaria dauci (Kuhn) Groves & Skolko and Cercospora carotae (Passerini)
Solheim, the fungi causing Alternaria blight and Cercospora blight, are the two prominent
foliar pathogens that affect carrots (Daucus carota L. var. sativa DC.) in Michigan
(Hausbeck and Harlan, 2003) and other areas (Arcelin and Kushalappa, 1991; James and
Stevenson, 1999). Symptoms of A. dauci normally develop along leaf margins but can
also affect the petioles and are characterized by irregularly shaped necrotic lesions.
Small, pinpoint necrotic lesions caused by C. carotae are initially surrounded by a
chlorotic halo and develop on the leaves or petioles. A dense crop canopy provides
conditions favorable for disease development prompting Alternaria and Cercospora
lesions to coalesce. If entire leaﬂets and petioles become blighted, they may not be able
to withstand the pull of mechanical harvesters, and unharvested carrots will remain in the
soil.

Carrots in Michigan are grown for fresh market, processing, and the cut and peel
market. The three production systems use speciﬁc cultivars, plant spacing, and plant
populations for their respective uses (Zandstra et al., 1986). Although no disease
resistant cultivars are available, there are cultivars that exhibit levels of genetic tolerance
to foliar blights (James et al., 1999; Strandberg etal., 1972).

Recommendations for controlling Alternaria and Cercospora blights in Michigan
are to apply registered fungicides at 7 to 14 day intervals after carrot seedlings have
emerged (Bird et al., 2002). Typically, growers will follow the recommended spray
interval, but they will rarely apply their ﬁrst fungicide spray before the plants are large

enough to touch within the rows. Michigan carrot growers rely on two B2 carcinogenic

41

fungicides, chlorothalonil and iprodione, to control Alternaria and Cercospora blights.
Pesticides classiﬁed as B2 carcinogens are scheduled for review by the EPA under the
Food Quality Protection Act (FQPA), and the future availability of these products is
uncertain. One Michigan food processor does not allow the use of iprodione on carrots
because it is a B2 carcinogen and residues are a concern. In addition, fungicide
resistance of A. dauci to iprodione has been reported (Fancelli and Kimati, 1991 ).
Azoxystrobin (Quadris, Syngenta Crop Protection, Greensboro, NC), a recently
registered reduced risk systemic fungicide, is an effective tool to control Alternaria and
Cercospora blights in rotation with protectant fungicides (Hausbeck et al., 2000; James
and Stevenson, 1999). Use of this product in rotation with protectant fungicides already
used by Michigan growers may reduce the number of B2 carcinogenic fungicide
applications.

A disease forecasting system (FAST) was developed for timing fungicide
applications to control early blight on tomato caused by Alternaria solani (Ellis & G.
Martin) Sorauer (Madden et al., 1978). The forecaster uses a series of environmental
inputs to alert growers as to when these conditions favor early blight development, and to
subsequently prompt fungicide applications. Tom-Cast, a modiﬁcation of FAST, warns
growers when to control anthracnose fruit rot (Collectotrichum coccodes (Wallr.)
Hughes) and Septoria leaf spot (Septoria lycopersici (Speg)) as well as A. solani. For
each 24-hour period (11:00 AM to 11:00 AM), Tom-Cast uses the hours of leaf wetness
and the average temperature during the wetness periods to calculate a Disease Severity
Value (DSV) ranging from 0 to 4, corresponding to environmental conditions

unfavorable to highly favorable for disease development. Daily DSV values are summed

42

and accumulated until a threshold value is reached, a fungicide spray is applied, and the
DSV total is reset to zero (Pitblado, 1992).

The objectives of this study were to 1) determine whether the Tom-Cast disease
forecasting system can be used to time fungicide applications to control Alternaria and
Cercospora blights of carrots, 2) set the critical DSV threshold for timing fungicide
applications according to Tom-Cast, and 3) determine whether ﬁeld scouting and disease
incidence thresholds can be used to initiate spray programs.

MATERIALS AND METHODS

Plot establishment. Plots were established at the Michigan State University
Muck Soils Experimental Farm in Bath, Michigan and at a commercial carrot ﬁeld in
Fremont, Michigan in 2001 and 2002. Beds at the Muck Soils Experimental Farm
(hereafter referred to as the research farm) were formed and carrot seeds of the fresh
market cultivar ‘Cellobunch’ and the processing cultivar ‘Early Gold’ were planted on 14
May 2001 and 21 May 2002 in Houghton Muck soil, previously planted with potato.
Seeds were spaced 2.54 cm (‘Early Gold’) and 1.43 cm (‘Cellobunch’) apart in rows
spaced 45.7 cm apart on three-row raised beds measuring 162.6 cm from center to center.
Plant populations of ‘Early Gold’ were 431,657/ha (2001) and 418,775/ha (2002), and
populations of ‘Cellobunch’ were 609,047/ha (2001) and 510,043/ha (2002). The study
located in Fremont (hereafter referred to as the commercial ﬁeld) was established with
carrot seeds of the cut and peel cultivar ‘Prime Cut’ planted on 28 May 2001 and 6 June
2002 in Granby Mucky Sand, previously planted with corn. Seeds were placed 3.25 cm
apart in four seed lines per row with rows spanning 15.2 cm and centered 43.2 cm apart

in three-row raised beds measuring 172.7 cm from center to center. Plant populations

43

were 1,444,156/ha (2001) and 1,328,887/ha (2002). All treatment plots were 6.1 m long.
One and a half meter sections of unsprayed carrots separated treatment plots within beds,
and one bed of carrots was left untreated between treatment beds. Natural inoculum was
relied on for infecting plants. Weeds, insects, and fertilization requirements were
managed according to standard production practices (Bird et al., 2002; Wamcke et al.,
1992; Zandstra, 2002). Plots were sprinkler irrigated as needed.

Weather monitoring and disease forecasting. Hourly measurements of
temperature and leaf wetness duration were obtained using a digital data recorder
(WatchDog Leaf Wetness and Temperature Logger 3610TWD; Spectrum Technologies,
Inc., Plainﬁeld, Illinois) located in the upper 75% of the crop canopy in the center of an
unsprayed bed at a 45° angle facing north. Data recorders were place in the plots prior to
row closure in mid-June. Data were downloaded every other day to a laptop computer
using a computer program (Specware 6.01; Spectrum Technologies, Inc., Plainﬁeld,
Illinois) equipped to calculate DSVs for the Tom-Cast system. The program was set to
record temperatures from 0 to 100° C and to detect leaf wetness whenever moisture was
present on the leaf wetness grid. Summaries of weather data and DSV accumulation
from the trial sites are listed in Appendix A (Tables 22 and 26).

Chlorothalonil (Bravo Ultrex 82.5WDG at 1.29 kg a.i.lha, Zeneca Ag Products,
Wihnington, DE) alternated with azoxystrobin (Quadris 2.08F at 0.11 kg a.i.lha,
Syngenta Crop Protection, Greensboro, NC) were applied to all treatment plots,
excluding the control. Fungicides were applied with a CO; backpack sprayer (R & D
Sprayers, Opelousas, LA) equipped with three Teejet XRl 1002VS ﬂat-fan nozzles

(Spraying Systems Co., Wheaton, IL) spaced 45.7 cm apart, operating at 344.8 kPa,

44

and delivering 467.6 liters/ha.

Fields were scouted for disease symptoms using a disease damage index key
(Strandberg, 1988), and spray programs were initiated prior to symptom development
(0%), or when disease was evident on a trace amount, 5%, or 10% of the foliage. Initial
sprays at the research farm occurred on 2 July (0%), 30 July (trace), 15 August (5%), and
25 August (10%) in 2001 and 8 July (0%), 19 July (trace), 1 August (5%), and 13 August
(10%) in 2002. Initial sprays were applied at the commercial ﬁeld on 8 July (0%), 20
July (trace), 25 July (5%), and 1 August (10%) in 2001 and 15 July (0%), 22 July (trace),
5 August (5%), and 19 August (10%) in 2002. Subsequent sprays were applied every 10
days or according to Tom-Cast at intervals of 15, 20, or 25 DSVs. In 2002, a 7-day
fungicide schedule was included in the commercial ﬁeld study to reﬂect the typical
application schedule followed by commercial (cut and peel) carrot growers in the area.
Treatment plots were assigned to each variety and replicated four times in a randomized
complete block design.

At the research farm, Tom-Cast prompted eight, six, and ﬁve sprays in 2001 and
six, ﬁve, and four sprays in 2002 for the 15 DSV, 20 DSV, and 25 DSV thresholds,
respectively, for spray programs initiated prior to disease symptom development. Five,
four, and three sprays (2001 and 2002) were applied for the Tom-Cast 15, 20, and 25
DSV thresholds, respectively, for sprays programs initiated when a trace amount of blight
symptoms developed. Fungicide sprays initiated when 5% of the foliage was blighted
resulted in three, two, and two sprays in 2001 and four, three, and two sprays in 2002 for
the Tom-Cast 15, 20, and 25 DSV thresholds, respectively. Two, one, and one spray(s)

in 2001 and three, two, and two sprays in 2002 were applied for the Tom-Cast 15, 20, and

45

25 DSV thresholds, respectively, for treatments initiated when 10% blight symptoms
developed. Ten-day spray interval treatments initiated prior to disease symptom
development scheduled nine applications in 2001 and 2002. Ten-day spray interval
treatments initiated when a trace amount of blight symptoms developed resulted in six
and eight applications in 2001 and 2002, respectively. Ten-day spray interval treatments
initiated when 5% of the foliage was blighted prompted four and seven applications in
2001 and 2002, respectively. Ten-day spray interval treatments initiated when 10%
blight symptoms developed scheduled three and ﬁve applications in 2001 and 2002,
respectively. Dates of fungicide applications are listed in Appendix A (Tables 27 and
28).

At the commercial ﬁeld, Tom-Cast prompted nine, seven, and ﬁve sprays in 2001
and eight, six, and ﬁve sprays in 2002 for the 15 DSV, 20 DSV, and 25 DSV thresholds,
respectively, for spray programs initiated prior to disease symptom development. Eight,
six, and ﬁve sprays in 2001 and seven, six, and four sprays in 2002 were applied for the
Tom-Cast 15, 20, and 25 DSV thresholds, respectively, for sprays programs initiated
when a trace amount of blight symptoms developed. F ungicide sprays initiated when 5%
of the foliage was blighted resulted in seven, ﬁve, and four sprays in 2001 and ﬁve, four,
and three sprays in 2002 for the Tom-Cast 15, 20, and 25 DSV thresholds, respectively.
Six, ﬁve, and four sprays in 2001 and four, three, and three sprays in 2002 were applied
when sprays were initiated when 10% blight symptoms developed for the Tom-Cast 15,
20, and 25 DSV thresholds, respectively, for treatments initiated when 10% blight
symptoms developed. Ten-day spray interval treatments initiated prior to disease

development scheduled nine and eight applications in 2001 and 2002. Ten-day spray

46

interval treatments initiated when a trace amount of blight symptoms developed resulted
in seven and eight applications in 2001 and 2002, respectively. Ten-day spray interval
treatments initiated when 5% of the foliage was blighted prompted seven and six
applications in 2001 and 2002. Ten-day spray interval treatments initiated when 10% of
the foliage was blighted scheduled six and ﬁve applications in 2001 and 2002,
respectively. Sprays were applied on a 7-day schedule at the commercial ﬁeld in 2002
only. Twelve, eleven, nine, and seven sprays were applied on a 7-day schedule for
treatments that started prior to blight occurrence or when a trace amount, 5%, or 10% of
the foliage was diseased, respectively. Dates of fungicide applications are listed in
Appendix A (Tables 29 and 30).

Disease assessment. All disease assessments were made from the center 3.05 m
of the middle row of each plot at the research farm and from carrots in the four seed lines
of the middle 1.52 m of the center row in the commercial ﬁeld. Leaf blight severity was
assessed biweekly (research farm only, 2001) or weekly using an expanded Alternaria
leaf blight assessment key (Strandberg, 1988). Plots were visually assessed as having 0,
l, 5, 10, 20, or 40% leaf blight, and the key was expanded to estimate 60, 80, and 100%
leaf blight. Petiole disease incidence was determined biweekly (research farm only,
2001) or weekly by counting the number of plants with one or more lesions. Petiole
disease severity was evaluated concurrently with petiole disease incidence using the
following scale: 1 = average of 0 to 5 lesions per plant; 2 = 6 to 20 lesions; 3 = 21 to 50
lesions; 4 = more than 50 lesions; 5 = dead. An additional petiole rating was conducted
to estimate the overall health, amount of dead and living petioles, and condition of

petioles for harvest; petiole health was not recorded in 2001 at the commercial ﬁeld.

47

Petiole health was determined on the day before harvest at the muck farm in 2001 and
weekly at the research farm and commercial ﬁeld in 2002 starting on 27 August and 26
August, respectively and continuing with the other disease evaluations until harvest. The
following scale (1-10) was used to assess petiole health: from 1 = petioles healthy and
vigorous to 10 = petioles unhealthy, weak, or dead. The area under the disease progress
curve (AUDPC) was calculated to express the cumulative disease incidence on petioles,
severity of disease on petioles, leaf blight, and petiole health (2002 only) by the

calculation described by Shaner and Finney (1977):

n
AUDPC = Z [(Y + v )/2][x - x ]
i=1 i+nl i i+l i

where Y, = percent foliar blight, percent petiole blight, or petiole health rating at the ith
observation, X,- = time (days) at the ith observation, and n = total number of observations.
Carrots in the center 3.05 m of the middle row of each plot were hand-harvested, the
foliage was removed at the crown. and roots were weighed to determine yield on 2
October 2001 and 3 October 2002 at the research farm and 27 September 2001 and 8
October 2002 in the commercial ﬁeld. Leaves showing blight symptoms were
periodically removed from untreated buffer rows and examined under magniﬁcation
(200X) in the laboratory to conﬁrm the presence of A. dauci and/or C. carotae.
Statistical and economic analysis. ‘Early Gold’ and ‘Cellobunch’ varieties were
individually analyzed as single experiments with replicates across years, and the
experiment was designed as a split-plot in time, with each year’s data representing a
randomized complete block design. Data for all variables were initially analyzed using
analysis of variance (ANOVA) in which a linear model including treatment, year, year by

treatment, and replicate nested within year as factors was analyzed using the Proc GLM

48

procedure of the Statistical Analysis System (SAS Institute, Cory, NC). The assumptions
of normality and equal variances were examined using the residuals from the ANOVA.
Normality was examined using the Proc Univariate procedure of SAS, and the equal
variance assumption was assessed by plotting the residuals. Data that did not pass
normality tests were transformed (Table 4).

When the year by treatment interaction was signiﬁcant for a variable, analyses
were done separately for each year using an analysis of a randomized complete block
experiment. Data were analyzed using a linear model that included treatment and
replicate as factors using the Proc GLM procedure of SAS (SAS Institute, Cory, NC).

The 17 treatments examined in the ‘Early Gold’, ‘Cellobunch’, and ‘Prime Cut’
(2001 only) experiments represent a four (initiation timings) by four (application
intervals) factorial with an untreated control as the 17th treatment. Twenty-one treatments
were tested on ‘Prime Cut’ in 2002, due to the addition of a 7-day application interval,
representing a four (initiation timings) by ﬁve (application intervals) factorial with an
untreated control as the 21St treatment. The ‘Prime Cut’ experiments during 2001 and
2002 were analyzed separately using a linear model that included treatment and replicate
as factors using the Proc GLM procedure of SAS.

When the ANOVA indicated a signiﬁcant difference among the treatments, the
differences among the treatments were examined by decomposing the treatment sum of
squares into four component sum of squares: (1) the difference between the average of
the spray treatments and the untreated control; (2) differences between initiation timings;
(3) differences between application intervals; (4) an interaction between initiation timings

and application intervals. When the analyses did not detect a signiﬁcant interaction

49

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between initiation timings and application intervals, the main effects of initiation timing
and application interval were examined using the Waller-Duncan Bayesian k-ratio t-test
(Steel et al., 1997) to determine which initiation timing or application interval had the
best mean. When a signiﬁcant initiation timing by application interval interaction
occurred, the effect of initiation timings for each application interval were determined
using the Waller-Duncan Bayesian k-ratio t-test.

The total cost of each fungicide program was calculated by multiplying the
number of applications by the cost of the fungicide used (Table 5). Fungicide costs for
one application of chlorothalonil were $9.45 and $7.84 per acre in 2001 and 2002,
respectively; one application of azoxystrobin cost $13.58 and $13.08 per acre in 2001 and
2002, respectively.

RESULTS

The effect of treatment was highly signiﬁcant for all disease assessments on the
three cultivars examined. Treatments signiﬁcantly affected yield of ‘Prime Cut’ carrots
in 2002 (Table 6), but did not affect yield of ‘Early Gold’ (Table 7), ‘Cellobunch’ (Table
8), or ‘Prime Cut’ carrots in 2001 (Table 9). For each cultivar, all fungicide-treated plots
had signiﬁcantly less disease on leaves and petioles compared with the untreated plots.
Timing the initial fungicide application based on scouting for leaf blight thresholds of
0%, trace, 5%, and 10% had a highly signiﬁcant effect on all disease assessments for the
three cultivars examined. Application interval signiﬁcantly affected most disease
assessments, but did not affect: ‘Early Gold’ AUDPC data for petiole blight incidence in
2002; ‘Cellobunch’ AUDPC data for petiole blight incidence; ‘Cellobunch’ AUDPC data

for petiole blight severity; ‘Prime Cut’ AUDPC data for petiole blight severity in 2002.

51

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Table 6. Effects of spray initiation timings and application intervals on yield of ‘Prime
Cut’ carrots treated with the fungicides chlorothalonil (1.29 kg a.i.lha) alternated with
azoxystrobin (0.11 kg a.i.lha) when initiated prior to disease development (0%) or at
disease levels of trace, 5%, or 10% and reapplied at 7 or 10 day intervals or according to
Tom-Cast at 15, 20, or 25 disease severity values (DSV) used to control foliar blights
caused by A. dauci and C. carotae in 2002.

 

 

Treatment Yield (kg)z
Initiation timing
0% 5.50 ay
Trace 5.75 a
5% 5.10 b
10% 4.77 b

Application interval

y.

7 day 5.38 a
10 day 5.32 a
Tom-Cast 15 DSV 5.19 a
Tom-Cast 20 DSV 5.39 a
Tom-Cast 25 DSV 5.14 a
Source F value P value
Treatment 2.70 0.0016
Initiation timing 10.47 <.0001
Application interval 0.58 0.6803
Timing*interval interaction 1.44 0.1738
Untreated vs. treated 3.00 0.0886

 

z Carrots from the center 3.05 m of the middle row of each plot were hand-harvested, the
foliage was removed at the crown, and roots were weighed to determine yield on 8
October 2002.

y Initiation timing means followed by the same letter are not significantly different
according to Waller-Duncan Bayesian k-ratio t-test (k-ratio = 100).

" Application interval means followed by the same letter are not signiﬁcantly different
according to Waller-Duncan Bayesian k-ratio t-test (k-ratio = 100).

53

Table 7. Effect of the fungicides chlorothalonil (1.29 kg a.i./ha) alternated with
azoxystrobin (0.11 kg a.i./ha) when initiated prior to disease development (0%) or at
disease levels of trace, 5%, or 10% and reapplied at lO-day intervals or according to
Tom-Cast at 15, 20, or 25 disease severity values (DSV) on yield of ‘Early Gold’ carrots
infected with A. dauci and C. carotae in 2001 and 2002.

 

 

Treatment Yield (kg)z
Untreated 1 3 .34
10 day
0% 14.62
Trace 15.51
5% 14.90
10% 13.39
Tom-Cast 15 DSV
0% 14.57
Trace 14.26
5% 13.90
10% 13.40
Tom-Cast 20 DSV
0% 15.04
Trace 15.86
5% 13.91
10% 13.56
Tom-Cast 25 DSV
0% 14.82
Trace 14.29
5% 13.78
10% 13.81
Source P value P value
Treatment 1 .33 0. 1968

 

Z Carrots from the center 3.05 m of the middle row of each plot were hand-harvested, the
foliage was removed at the crown, and roots were weighed to determine yield on 2
October 2001 and 3 October 2002.

54

Table 8. Effect of the fungicides chlorothalonil (1.29 kg a.i./ha) alternated with
azoxystrobin (0.11 kg a.i.lha) when initiated prior to disease development (0%) or at
disease levels of trace, 5%, or 10% and reapplied at 10-day intervals or according to
Tom-Cast at 15, 20, or 25 disease severity values (DSV) on yield of ‘Cellobunch’ carrots
infected with A. dauci and C. carotae in 2001 and 2002.

 

 

Treatment Yield (kgf
Untreated 9.70
10 day
0% 12.03
Trace 11.17
5% 12.06
10% 11.23
Tom-Cast 15 DSV
0% 11.83
Trace 11.63
5% 10.90
10% 10.59
Tom-Cast 20 DSV
0% 12.39
Trace 11.15
5% 11.45
10% 12.15
Tom-Cast 25 DSV
0% 12.23
Trace 10.81
5% 10.25
10% 10.28
Source F value P value
Treatment 1 .62 0.0775

 

z Carrots from the center 3.05 m of the middle row of each plot were hand-harvested, the

foliage was removed at the crown, and roots were weighed to determine yield on 2
October 2001 and 3 October 2002.

55

Table 9. Effect of the fungicides chlorothalonil (1.29 kg a.i./ha) alternated with
azoxystrobin (0.11 kg a.i./ha) when initiated prior to disease development (0%) or at
disease levels of trace, 5%, or 10% and reapplied at lO-day intervals or according to
Tom-Cast at 15, 20, or 25 disease severity values (DSV) on yield of ‘Prime Cut’ carrots
infected with A. dauci and C. carotae in 2001.

 

 

Treatment Yield (kg)z
Untreated 7.53
10 day
0% 9.03
Trace 9.67
5% . 9.77
10% 10.23
Tom-Cast 15 DSV
0% 9.07
Trace 9.46
5% 9.76
10% 9.05
Tom-Cast 20 DSV
0% 9.22
Trace 9.67
5% 9.27
10% 9.99
Tom-Cast 25 DSV
0% 9.33
Trace 8.58
5% 8.61
10% 8.32
Source F value P value
Treatment 1.38 0.1897

 

z Carrots from the center 3.05 m of the middle row of each plot were hand-harvested, the
foliage was removed at the crown, and roots were weighed to determine yield on 27
September 2001.

56

The interaction of initial spray timings and application intervals was signiﬁcant (P
< 0.05) for the following variables: ‘Early Gold’ AUDPC data for petiole blight incidence
in 2001; ‘Early Gold’ data for the ﬁnal petiole health rating in 2001; ‘Prime Cut’ AUDPC
data for petiole blight incidence in 2001; ‘Prime Cut’ AUDPC data for petiole blight
incidence in 2002; ‘Prime Cut’ AUDPC data for leaf blight in 2002. The signiﬁcant
interaction of initiation timing and application interval indicated that the disease control
provided by the application intervals was dependent on the time of the initial fungicide
application.

Assessment of initial spray timing on ‘Early Gold’ carrots. The time of initial
disease occurrence and progression of disease in untreated plots were different for the
two years this study was conducted. Leaf blight was detected on 30 July and 18 July in
2001 and 2002, respectively (Figure 2). Levels of disease increased, where 100% petiole
infection was observed on 4 September 2002 (Figure 3). In 2001, untreated plots reached
89.5% petiole infection on 28 September. A summary of ﬁnal disease assessments is
listed in Appendix A (Table 31).

The AUDPC data suggest that spray programs that were initiated prior to blight
occurrence or when the ﬁrst sign of disease was detected signiﬁcantly reduced the
incidence of petiole blight in 2002 and percentage of leaf blight throughout the growing
season compared with spray programs that were initiated when 5 or 10% disease
symptoms occurred (Table 10). Petiole blight severity was signiﬁcantly higher for spray
programs that were initiated when 5 or 10% leaf blight was detected compared with spray
programs that were initiated prior to disease or at the ﬁrst disease detection. Petiole

blight severities of spray programs initiated when a trace amount of leaf blight developed

57

2001 2002

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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Figure 2. Disease progress curves for leaf blight caused by A. dauci and C. carotae on
‘Early Gold’ carrots leﬁ untreated or treated with chlorothalonil (1 .29 kg a.i.fha)
alternated with azoxystrobin (0.11 kg a.i./ha) applied prior to blight symptom
development (0%) or at disease levels of trace, 5%, 10% and reapplied every 10 days or
according to Tom-Cast using intervals of 15, 20, or 25 DSVs in 2001 and 2002.

58

2001 2002

 

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60

 

&
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Petiole Infection (%)
N
G

 

Figure 3. Disease progress curves for petiole blight caused by A. dauci and C. carotae
on ‘Early Gold’ carrots left untreated or treated with chlorothalonil (1.29 kg a.i./ha)
alternated with azoxystrobin (0.11 kg a.i./ha) applied prior to blight symptom
development (0%) or at disease levels of trace, 5%, 10% and reapplied every 10 days or
according to Tom-Cast using intervals of 15, 20, or 25 DSVs in 2001 and 2002.

59

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61

were not signiﬁcantly different than the spray programs initiated prior to disease
development (Table 10).

The analysis of AUDPC means for petiole blight incidence in 2001 indicated
signiﬁcant spray initiation timing by application interval interaction (Table 10). This
suggested that the differences observed in the application intervals were dependent on the
spray initiation timing. Initiation timings for the 10-day application interval were all
signiﬁcantly different from one another with spray programs initiated at lower blight
incidence thresholds providing better disease control (Table 11). For the Tom-Cast 15
and 25 DSV application intervals, respectively, initiation timings were similar for sprays
initiated prior to blight occurrence and when a trace of disease symptoms were detected,
and were signiﬁcantly lower than initiation timings of 5 and 10%, which also differed
from one another. Tom-Cast 20 DSV spray programs that started when 5 or 10% blight
occurred were similar but the AUDPC of these were signiﬁcantly higher compared with
programs that were initiated when blight was ﬁrst detected. Also, the AUDPC of the
Tom-Cast 20 DSV interval that started when disease was detected was higher than the
AUDPC of the program that was initially sprayed prior to blight symptom development
(Table 11).

The AUDPC data suggest that petiole health was signiﬁcantly improved by
applying the initial fungicide prior to disease detection or when disease symptoms were
ﬁrst detected compared with spray programs initiated at 5 or 10% leaf blight threshold
(Table 12). In 2002, ﬁnal petiole health ratings indicated no signiﬁcant difference
between spray programs initiated prior to blight symptom development and spray

programs initiated when the ﬁrst sign of disease symptoms were detected. Both the 5 and

62

Table 11. Effect of spray initiation timings for'each application interval on the area
under the disease progress curve of petiole blight caused by A. dauci and C. carotae on
‘Early Gold’ carrots treated with the fungicides chlorothalonil (1.29 kg a.i./ha) alternated
with azoxystrobin (0.11 kg a.i./ha) when initiated prior to disease development (0%) or at
disease levels of trace, 5%, or 10% and reapplied at 10-day intervals or according to
Tom-Cast at 15, 20, or 25 disease severity values (DSV) in 2001.

 

 

Application interval Petiole blight incidence
Initiation timing AUDPC (disease*day)z
10 day
0% 23.54 ay
Trace 531.59 b
5% 1309.56 c
10% 1980.04 d
Tom-Cast 15 DSV
0% 113.28 a
Trace 165.99 a
5% 811.42 b
10% 2550.12 c
Tom-Cast 20 DSV
0% 134.76 a
Trace 513.11 b
5% 2354.31 c
10% 1985.52 c
Tom-Cast 25 DSV
0% 725.61 a
Trace 966.79 a
5% 1523.94 b
10% 2418.96 c

 

Z AUDPC = area under the disease progress curve. Data were transformed using square
root (AUDPC) to stabilize the variance. The table shows back-transformed data.

y Means within each application interval followed by the same letter are not signiﬁcantly
different according to the Waller-Duncan Bayesian k-ratio t-test (k-ratio = 100).

63

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65

10% spray initiation thresholds differed from one another and differed from the lower
spray initiation thresholds (Table 12).

The analysis of the ﬁnal petiole health rating in 2001 indicated a signiﬁcant
interaction between spray initiation and application interval (Table 12). This suggested
that the differences observed in the application intervals were dependent on the spray
initiation timing. Initiation timings of 5 or 10% leaf blight were similar for the 10-day
spray schedule and Tom-Cast 20 DSV treatment, but were signiﬁcantly higher than the
program started at a trace of blight symptoms (Table 13). Programs that were initiated
prior to disease development signiﬁcantly improved petiole health compared with the
program started at a trace amount of blight. Tom-Cast 15 DSV programs initiated prior
to disease development or when a trace amount developed were similar and were
signiﬁcantly lower than programs initiated when 5% blight appeared. The program
started when 10% blight developed was signiﬁcantly higher compared with the program
initiated when 5% blight developed. All of the spray initiation timings for the Tom-Cast
25 DSV treatment were different, and ﬁnal ratings increased as the spray initiation
threshold increased (Table 13).

Assessment of initial spray timing on ‘Cellobunch’ carrots. Disease
progressed rapidly on ‘Cellobunch’ carrots during both years of this study. The incidence
of petiole blight was observed on 5% and 36% of the untreated plants on 8 August 2001
and 7 August 2002, respectively (Figure 4). Untreated plots reached 84% and 100%
petiole blight incidence on 28 September 2001 and 4 September 2002, respectively. Leaf
blight in 2001 progressed from 4% on 17 August to 50% on 22 September (Figure 5). In

2002, severe leaf blight developed during the period of 11 September through 2 October

66

Table 13. Effect of spray initiation timings for each application interval on the ﬁnal
petiole health evaluation assessing disease caused by A. dauci and C. carotae on ‘Early
Gold’ carrots treated with the fungicides chlorothalonil (1.29 kg a.i.lha) alternated with
azoxystrobin (0.11 kg a.i.lha) when initiated prior to disease development (0%) or at
disease levels of trace, 5%, or 10% and reapplied at 10-day intervals or according to
Tom-Cast at 15, 20, or 25 disease severity values (DSV) in 2001.

 

 

Application interval Final
Initiation timing Petiole health2
10 day
0% 1.50 ay
Trace 3.00 b
5% 6.00 c
10% 6.00 c
Tom-Cast 15 DSV
0% 2.25 a
Trace 1.50 a
5% 4.25 b
10% 7.50 c
Tom-Cast 20 DSV
0% 2.00 a
Trace 4.50 b
5% 6.75 c
10% 7.50 c
Tom-Cast 25 DSV
0% 4.25 a
Trace 5.25 b
5% 6.75 c
10% 7.75 d

 

z Petiole health was evaluated using the following scale; where 1 = petioles healthy and
vigorous to 10 = petioles unhealthy, weak, or dead.

y Means within each application interval followed by the same letter are not signiﬁcantly
different according to the Waller-Duncan Bayesian k-ratio t-test (k-ratio = 100).

67

2001

 

 

 

 

   
 

 

 

 

 

 

 

 

 

 

 

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Figure 4. Disease progress curves for petiole blight caused by A. dauci and C. carotae
on ‘Cellobunch’ carrots left untreated or treated with chlorothalonil (1 .29 kg a.i.lha)
alternated with azoxystrobin (0.11 kg a.i.lha) applied prior to blight symptom
development (0%) or at disease levels of trace, 5%, 10% and reapplied every 10 days or
according to Tom-Cast using intervals of 15, 20, or 25 DSVs in 2001 and 2002

68

 

Leaf Blight (%) Leaf Blight (%)

Leaf Blight (%)

Leaf Blight (%)

2001

2002

 

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Figure 5. Disease progress curves for leaf blight caused by A. dauci and C. carotae on
‘Cellobunch’ carrots left untreated or treated with chlorothalonil (1 .29 kg a.i.lha)
alternated with azoxystrobin (0.11 kg a.i./ha) applied prior to blight symptom
development (0%) or at disease levels of trace, 5%, 10% and reapplied every 10 days or
according to Tom-Cast using intervals of 15, 20, or 25 DSVs in 2001 and 2002.

69

when disease in untreated plots progressed from 25 to 70% (Figure 5). A summary of
ﬁnal disease assessments is listed in Appendix A (Table 32).

According to the AUDPC data, spray programs initiated prior to blight occurrence
were most effective in limiting the incidence of petiole blight and leaf blight, and no
similarities were observed among the initiation timings (Table 14). Spray programs
initiated when a trace amount of blight developed were effective in limiting petiole blight
severity and did not differ from programs that started prior to blight occurrence. The 5%
initiation timing was signiﬁcantly different from both the lower thresholds, and it
provided signiﬁcantly better disease control compared with the 10% initiation timing
(Table 14).

The petiole health data suggest that programs initiated prior to disease or when a
trace amount of blight developed signiﬁcantly lowered the AUDPC compared to the 5
and 10% initiation timings, which did not differ (Table 15). In 2001, spray programs
initiated prior to blight occurrence improved ﬁnal petiole health compared with later
initiation timings, which differed ﬁ'om one another. In 2002, spray programs initiated
when a trace amount of blight developed were effective in improving petiole health and
did not differ from programs that started prior to blight occurrence. The 5% initiation
timing was signiﬁcantly different from both the lower thresholds. The 5% initiation
timing provided signiﬁcantly better disease control than the 10% initiation timing (Table
15).

Assessment of initial spray timing on ‘Prime Cut’ carrots. Disease was
detected on 20 July 2001 and 22 July 2002 in the commercial ﬁeld studies. Leaf blight in

untreated plots reached 60% by 27 September in 2001 and 45% by 30 September in 2002

70

 

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74

(Figure 6). In 2001, the incidence of petiole blight in untreated plots was 19% on 2
August and increased to 92% by 28 August (Figure 7). In 2002, the incidence of petiole
blight in untreated plots was 24% on 5 August and progressed to 85% by 19 August
(Figure 7). A 7-day application schedule was included in the 2002 commercial ﬁeld trial,
and it controlled disease effectively when applications were initiated at low disease
thresholds (Figure 8). A summary of ﬁnal disease assessments is listed in Appendix A
(Table 33).

The 2001 leaf blight AUDPC data indicated that programs started when 5% blight
symptoms developed provided signiﬁcantly less disease control than programs initiated at
earlier disease thresholds but signiﬁcantly limited leaf blight compared with programs
started when 10% blight developed (Table 16). Spray programs initiated when 10%
blight developed resulted in a higher AUDPC for petiole blight severity compared with
the programs that started at lower disease incidence thresholds. Treatments that were
initiated prior to disease, when a trace amount of disease was detected, or when 5% leaf
blight occurred did not differ in limiting petiole blight severity (Table 16).

The analysis of AUDPC for petiole blight incidence in 2001 indicated a
signiﬁcant interaction between the initiation timing and the application intervals,
suggesting that the differences observed in the application intervals was dependent on the
spray initiation timing (Table 16). The initiation timing for the lO-day and Tom-Cast 2O
DSV intervals, respectively, differed from one another, with the programs started prior to
blight occurrence providing the highest disease control (Table 17). When following the
Tom-Cast 15 DSV application interval, the spray thresholds of 5 and 10% blight did not

differ and provided signiﬁcantly less disease control compared with the program that was

75

2001 2002

 

 

{lo-day ilO-day l
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Leaf Blight (%)
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Leaf Blight (%)
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101

 

 

 

Figure 6. Disease progress curves for leaf blight caused by A. dauci and C. carotae on
‘Prime Cut’ carrots left untreated or treated with chlorothalonil (1.29 kg a.i.lha)
alternated with azoxystrobin (0.11 kg a.i.lha) applied prior to blight symptom
development (0%) or at disease levels of trace, 5%, 10% and reapplied every 10 days or
according to Tom-Cast using intervals of 15, 20, or 25 DSVs in 2001 and 2002.

76

2001 2002

 

       
 

 

 

 

 

 

  

 
 

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v %
W N‘ A? g? N N N W N
Figure 7. Disease progress curves for petiole blight caused by A. dauci and C. carotae
on ‘Prime Cut’ carrots left untreated or treated with chlorothalonil (1.29 kg a.i./ha)
alternated with azoxystrobin (0.11 kg a.i./ha) applied prior to blight symptom
development (0%) or at disease levels of trace, 5%, 10% and reapplied every 10 days or
according to Tom-Cast using intervals of 15, 20, or 25 DSVs in 2001 and 2002.

77

2002

 

 

 

 

 

 

 

 

 

 

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Figure 8. Disease progress curves for petiole and leaf blight caused by A. dauci and C.
carotae on ‘Prime Cut’ carrots left untreated or treated with chlorothalonil (1.29 kg
a.i.lha) alternated with azoxystrobin (0.11 kg a.i.lha) applied prior to blight symptom
development (0%) or at disease levels of trace, 5%, 10% and reapplied every 7 days in
2002.

78

 

 

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80

Table 17. Effect of spray initiation timings for each application interval on the area
under the disease progress curve of petiole blight caused by A. dauci and C. carotae on
‘Prime Cut’ carrots treated with the fungicides chlorothalonil (1 .29 kg a.i.lha) alternated
with azoxystrobin (0.1 1 kg a.i./ha) when initiated prior to disease development (0%) or at
disease levels of trace, 5%, or 10% and reapplied at lO-day intervals or according to
Tom-Cast at 15, 20, or 25 disease severity values (DSV) in 2001.

 

 

Application interval Petiole blight incidence
Initiation timing AUDPC (disease*day)z
10 day
0% 1049.92 ay
Trace 1434.77 b
5% 2276.12 c
10% 3281.62 d
Tom-Cast 15 DSV
0% 742.57 a
Trace 1090.16 b
5% 2294.45 0
10% 2563.06 c
Tom-Cast 20 DSV
0% 1377.56 a
Trace 1 810.05 b
5% 2175.69 c
10% 2924.79 (1
Tom-Cast 25 DSV
0% 2723.46 b
Trace 1823.63 a
5% 2876.55 bc
10% 3060.42 c

 

z AUDPC = area under the disease progress curve.
y Means within each application interval followed by the same letter are not significantly
different according to the Waller-Duncan Bayesian k-ratio t-test (k-ratio = 100).

81

initiated when a trace amount of disease was detected. The highest disease control was
achieved when the initial application was made prior to blight symptom development and
subsequent applications were made according to Tom-Cast 15 DSV. This program
differed from the initiation timing where sprays were applied following blight detection.
Tom-Cast 25 DSV spray programs initiated when a trace amount of disease was detected
resulted in higher disease control compared to programs that were initiated prior to blight
symptom development or when disease was evident on 5% of the foliage. Programs that
started when 5% of the foliage was blighted did not differ from programs that were
initiated when 10% leaf blight occurred (Table 17).

The 2002 AUDPC data suggest that spray programs initiated when a trace amount
of blight symptoms were detected effectively limited petiole blight severity and were
comparable with the programs that were initiated prior to blight development (Table 18).
Programs that started when 5 or 10% blight developed did not differ and were
signiﬁcantly different than programs that were initiated at lower blight incidence
thresholds (Table 18).

There was a signiﬁcant interaction between initiation timing and application
interval for AUDPC of petiole blight incidence in 2002, indicating that the differences
observed in the application intervals depended on the spray initiation timing (Table 18).
Seven-day spray programs that were initiated prior to disease occurrence did not differ
from programs that were started aﬁer a trace amount of disease was present (Table 19).
Programs that were started when 5% blight developed were signiﬁcantly less effective in
controlling blight compared with programs that were initiated at lower initiation timings,

and the 5% blight initiation timing provided signiﬁcantly better disease control compared

82

 

 

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83

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84

Table 19. Effect of spray initiation timings for each application interval on the area
under the disease progress curve of petiole blight caused by A. dauci and C. carotae on
‘Prime Cut’ carrots treated with the fungicides chlorothalonil (1 .29 kg a.i./ha) alternated
with azoxystrobin (0.11 kg a.i./ha) when initiated prior to disease development (0%) or at
disease levels of trace, 5%, or 10% and reapplied at 7 or 10 day intervals or according to
Tom-Cast at 15, 20, or 25 disease severity values (DSV) in 2002.

 

 

Application interval Petiole blight incidence
Initiation timinL AUDPC (disease*day)z
7 day
0% 167.20 ay
Trace 403.85 a
5% 2953.36 b
10% 3506.80 c
10 day
0% 3207.73 b
Trace 935.06 a
5% 3414.87 bc
10% 3745.63 c
Tom-Cast 15 DSV
0% 203.17 a
Trace 1206.41 b
5% 3573.35 c
10% 4064.68 (1
Tom-Cast 20 DSV
0% 616.44 a
Trace 1933.81 b
5% 3670.45 c
10% 3281.77 c
Tom-Cast 2S DSV
0% 2156.40 a
Trace 2015.16 3
5% 4131.24 c
10% 3427.55 b

 

Z AUDPC = area under the disease progress curve.
3’ Means within each application interval followed by the same letter are not significantly
different according to the Waller-Duncan Bayesian k-ratio t-test (k-ratio = 100).

85

with programs that were initiated when 10% blight developed. Ten-day spray programs
that were initiated when a trace amount of disease developed provided better disease
control than programs initiated prior to blight development or when sprays were applied
at the 5% blight threshold. Programs that were initiated when 10% blight developed
provided the least disease control, and were not signiﬁcantly different from the programs
that were started when 5% blight symptoms developed. Tom-Cast 15 DSV spray
programs that were initiated prior to disease symptom development were most effective
in controlling disease. Programs that were initiated when a trace amount of disease was
detected, when 5% blight developed, and when 10% blight developed were all
signiﬁcantly different from one another, with the least control provided by the 10%
initiation timing. Tom-Cast 20 DSV program signiﬁcantly reduced petiole blight
incidence more effectively compared with programs that were initiated at a trace amount
of disease. Programs that were initiated when 5 or 10% blight developed did not differ,
however, both programs did not control disease as effectively as the programs that were
initiated when a trace amount of disease was detected. Tom-Cast 25 DSV programs that
were initiated prior to blight symptoms or when disease was evident on a trace amount of
the foliage did not differ, and provided better disease control than programs started when
10% blight developed. Programs that were started when 5% blight symptoms occurred
were the least effective in controlling disease, and were signiﬁcantly different than
programs initiated when 10% blight developed (Table 19).

There was a signiﬁcant interaction between the initiation timing and the
application interval for AUDPC of leaf blight in 2002, indicating that the differences

observed in the application intervals depended on the spray initiation timing (Table 18).

86

Using the 7-day and Tom—Cast 20 DSV applications intervals, respectively, programs that
were initiated prior to disease development were most effective in controlled leaf blight
(Table 20). Programs that were started when a trace amount of blight developed, when
5% blight developed, or when 10% blight developed were all signiﬁcantly different ﬁom
one another, with the 10% initiation timing resulting in the highest AUDPC. Ten-day
spray programs that were started after a trace amount of blight developed were the most
effective in controlling leaf blight compared with programs that were initiated prior to
disease development or when 5% blight occurred. Programs that were started when 10%
blight developed provided the least disease control and were signiﬁcantly different from
programs that were initiated prior to blight symptoms or when disease was evident on 5%
of the foliage. Tom-Cast 15 DSV spray programs that were initiated when disease was
apparent on a trace amount of the foliage were not signiﬁcantly different than programs
that were initiated prior to disease development. Programs that were started when 5%
leaf blight occurred controlled leaf blight less effectively than programs started at earlier
thresholds but were more effective than the programs that started when 10% blight
occurred. Tom-Cast 25 DSV spray programs that were initiated prior to disease
development provided the best disease control and were signiﬁcantly different from
programs that were started when a trace amount of the foliage was blighted. Programs
that were initiated when 5 or 10% blight developed did not differ from one another, but
were signiﬁcantly less effective in controlling leaf blight than the programs initiated
when a trace amount of blight developed (Table 20).

The 2002 AUDPC data of petiole health and ﬁnal petiole health indicated that

programs that were initiated prior to disease development were similar to programs that

87

Table 20. Effect of spray initiation timings for each application interval on the area
under the disease progress curve of leaf blight caused by A. dauci and C. carotae on
‘Prime Cut’ carrots treated with the fungicides chlorothalonil (1 .29 kg a.i./ha) alternated
with azoxystrobin (0.11 kg a.i./ha) when initiated prior to disease development (0%) or at
disease levels of trace, 5%, or 10% and reapplied at 7 or 10 day intervals or according to
Tom-Cast at 15, 20, or 25 disease severity values (DSV) in 2002.

 

 

Application interval Leaf blight
Initiation timing AUDPC (disease"‘day)z
7 day
0% 159.00 ay
Trace 213.00 b
5% 310.38 c
10% 350.88 (1
10 day
0% 274.63 b
Trace 176.00 a
5% 284.50 b
10% 527.88 c
Tom-Cast 15 DSV
0% 138.50 a
Trace 174.88 a
5% 286.50 b
10% 413.63 c
Tom-Cast 20 DSV
0% 173.50 a
Trace 229.13 b
5% 388.50 c
10% 432.25 (1
Tom-Cast 25 DSV
0% 199.88 a
Trace 238.00 b
5% 404.88 c
10% 395.50 c

 

z AUDPC = area under the disease progress curve.
y Means within each application interval followed by the same letter are not signiﬁcantly
different according to the Waller-Duncan Bayesian k-ratio t-test (k-ratio = 100).

88

.l.“

 

were sprayed when a trace amount of disease developed (Table 21). Sprays that were
applied when 5% blight occurred provided less disease control compared with spray
programs that were initiated at lower disease thresholds, but were more effective in
improving petiole health than programs that were initiated when 10% blight symptoms
occurred (Table 21).

Assessment of fungicide application interval on ‘Early Gold’ carrots. The
main effect of application interval had a signiﬁcant effect on all ‘Early Gold’ disease
assessments except the AUDPC data for petiole blight incidence in 2002 (Table 10).
Disease progressed rapidly in 2002 (Figure 3), and an increase in petiole blight resulted
compared with the 2001 growing season. The AUDPC data suggest that the lO-day
interval and Tom—Cast 15 DSV intervals limited petiole blight severity when compared
with the Tom-Cast 25 DSV interval (Table 10). The Tom-Cast 20 DSV interval was
similar to both the 10-day, Tom-Cast 15 DSV, and Tom-Cast 25 DSV intervals. Leaf
blight AUDPC was controlled most effectively by the 10-day or Tom-Cast 15 DSV
application intervals when compared to the Tom-Cast 20 or 25 DSV intervals, which
differed from one another (Table 10).

The AUDPC and 2002 ﬁnal rating data for petiole health suggest that the lO-day
and Tom-Cast 15 DSV application intervals improved petiole health compared with the
Tom-Cast 20 and 25 DSV intervals (Table 12). Spray programs using the Tom-Cast 20
or 25 DSV intervals did not differ and were the least effective in controlling disease
(Table 12).

Assessment of fungicide application interval on ‘Cellobunch’ carrots.

Application interval had no effect on the AUDPC values of petiole blight incidence or

89

Table 21. Effects of spray initiation timings and application intervals on the area under
the disease progress curve and ﬁnal ratings for petiole health of ‘Prime Cut’ carrots
treated with the fungicides chlorothalonil (1.29 kg a.i./ha) alternated with azoxystrobin
(0.11 kg a.i./ha) when initiated prior to disease development (0%) or at disease levels of
trace, 5%, or 10% and reapplied at 7 or 10 day intervals or according to Tom-Cast at 15,
20, or 25 disease severity values (DSV) for control of A. dauci and C. carotae in 2002.

 

 

 

Petiole health
Treatment AUDPC (disease*day)’ Final ratingy
Initiation timing
0% 125.93 a" 4.90 a
Trace 131.08 a 5.00 a
5% 157.30 b 6.10 b
10% 190.80 c 6.75 c
Application interval
7 day 137.69 abW 4.94 a
10 day 163.88 cd 6.06 bc
Tom-Cast 15 DSV 131.34 a 5.38 ab
Tom-Cast 20 DSV 151.69 be 5.81 bc
Tom-Cast 25 DSV 171.78 (1 6.25 c
Source F value P value F value P value
Treatment 5.90 <.0001 4.05 <.0001
Initiation timing 22.90 <.0001 14.26 <.0001
Application interval 6.02 0.0004 4.05 0.0056
Timing*interval interaction 0.70 0.7480 0.63 0.8068
Untreated vs. treated 16.89 <.0001 14.50 0.0003

 

z AUDPC = area under the disease progress curve.

y Petiole health was evaluated using the following scale; where l = petioles healthy and
vigorous to 10 = petioles unhealthy, weak, or dead.
" Initiation timing means within a column followed by the same letter are not
signiﬁcantly different according to Waller-Duncan Bayesian k-ratio t-test (k-ratio =

100).

w Application interval means within a column followed by the same letter are not
signiﬁcantly different according to Waller-Duncan Bayesian k-ratio t-test (k—ratio =

100).

90

severity (Table 14). According to the AUDPC data, the lO-day and Tom-Cast 15 DSV
intervals were similar and most effective in controlling leaf blight when compared with
the Tom-Cast 20 and 25 DSV intervals, which did not differ (Table 14). Similarly, the
10-day and Tom-Cast 15 DSV intervals reduced the petiole health AUDPC compared
with the Tom-Cast 20 and 25 DSV intervals, but the later intervals did not differ from the
Tom-Cast 15 DSV schedule (Table 15). The ﬁnal petiole health data suggest that the 10-
day and Tom-Cast 15 DSV intervals were similar in 2001 and 2002 and improved petiole
health compared with the Tom-Cast 20 and 25 DSV intervals, which did not differ (Table
15).

Assessment of fungicide application interval on ‘Prime Cut’ carrots.
According to the 2001 AUDPC data, the 10-day, Tom-Cast 15 DSV, and Tom-Cast 20
DSV were equally effective in controlling petiole blight severity and leaf blight (Table
16). Tom-Cast 25 DSV was the least effective in controlling petiole blight severity and it
did not differ from the Tom-Cast 20 DSV application interval. Additionally, Tom-Cast
25 DSV was not effective in controlling leaf blight and was signiﬁcantly different from
all other application intervals (Table 16).

In 2002, application interval did not have a signiﬁcant effect on the AUDPC of
petiole blight severity in 2002 (Table 18). The AUDPC of petiole health and ﬁnal petiole
health data suggest that Tom-Cast 15 DSV and the 7-day interval were most effective in
limiting disease (Table 21). The AUDPC data indicate that the 7-day schedule was not
signiﬁcantly different than Tom-Cast 20 DSV. The 10-day schedule was similar to Tom-
Cast 20 DSV and Tom-Cast 25 DSV, which provided the least disease control. The 2002

ﬁnal petiole data suggest that the Tom-Cast 15 DSV interval did not differ from the 10-

91

day schedule or Tom-Cast 20 DSV. Tom-Cast 25 DSV, which was the least effective in
improving petiole health, was not signiﬁcantly different than the 10-day schedule or
Tom-Cast 20 DSV application interval (Table 21).

DISCUSSION

Carrot growers in Michigan have been concerned about applying fungicides when
environmental conditions do not favor blight development. The cost of these unneeded
sprays became paramount as production costs continued to increase. It was desirable to
develop methods for reducing fungicide input to economically produce carrots. The
goals of this study were to investigate disease incidence thresholds determined by ﬁeld
scouting for timing the initial fungicide application and to examine the use of the Tom-
Cast disease forecasting system for timing fungicide application intervals to control
Alternaria and Cercospora blights on carrots.

The economic beneﬁts of using ﬁeld scouting to time initial sprays and using
Tom-Cast to time subsequent sprays are exempliﬁed in situations where growers do not
use either disease management strategy. In 2001, standard lO-day fungicide schedules
required nine sprays for each cultivar tested, whereas the number of applications was
reduced to eight (‘Early Gold’ and ‘Cellobunch’) or remained the same with nine sprays
(‘Prime Cut’) by using Tom-Cast 15 DSV to determine spray intervals. Applying the
initial application when blight was ﬁrst detected in the ﬁeld saved an additional three
sprays (‘Early Gold’ and ‘Cellobunch’) and one spray (‘Prime Cut’). In 2002, standard
fungicide schedules required nine sprays (‘Early Gold’ and ‘Cellobunch’) and eight
sprays (‘Prime Cut’), whereas the number of fungicide applications was reduced to six

applications (‘Early Gold’ and ‘Cellobunch’) and or remained the same with nine

92

applications (‘Prime Cut’) by using Tom-Cast 15 DSV to determine spray intervals.
Applying the initial fungicide application when blight was ﬁrst detected in the ﬁeld saved
one additional spray (‘Early Gold’, ‘Cellobunch’, and ‘Prime Cut’). In seasons with
conditions highly favorable for blight development, the Tom-Cast program may not
reduce the number of sprays or production costs, but it may be beneﬁcial in improving
the timing of fungicide applications to prevent severe blight epidemics and crop loss.

Ben-Noon et a1. (2001) examined timings of spray initiation for controlling A.
dauci on carrots and attempted to create a disease threshold model describing when the
ﬁrst spray should be applied. Initial sprays were delayed until 14 and 28 days aﬁer the
common management practice in the growing area. Higher fungicide efﬁcacy was
observed with spray schedules that were initiated earlier in the season relative to the ﬁrst
occurrence of disease. The model was not validated, and recommendations were made to
apply fungicides in a prophylactic manner to achieve leaf blight control. The scouting
studies conducted in this research indicated different results. The time of spray initiation
was successfully delayed until the ﬁrst detection of disease symptoms, without
compromising disease control. In many cases, the scouting and Tom-Cast program
resulted in disease control that was similar to standard fungicide schedules that were
initiated prior to disease occurrence while eliminating up to four sprays per season.

The results of the present study agree with the ﬁndings of Gillespie and Sutton
(1979). One to three sprays were omitted by delaying the initial fungicide application
until blight symptoms developed on 1 to 2% of the foliage. Conversely, delaying the
initial fungicide application is not recommended for other cr0ps. Keinath et a1. (1996)

tested a scouting-based spray program for scheduling ﬁmgicide applications for

93

controlling early blight of tomato. According to this spray program, ﬁelds were scouted
twice per week until disease symptoms appeared on 3 to 6% of the foliage when a weekly
fungicide program commenced. The scouting program delayed the initial fungicide
application for 42 days and saved six sprays compared with the standard 7-day fungicide
program. The scouting program resulted in lower yields of extra-large fruit and increased
disease severity compared with the standard 7-day program. The negative impact of the
scouting program presented a risk to growers, and it was recommended that growers
continue to apply fungicides prior to disease occurrence (Keinath et al., 1996).

Tom-Cast has been used to successfully time fungicide applications for managing
other pathogens of vegetable crops. Tom-Cast was evaluated as a disease management
tool for timing fungicide applications to control purple spot (Stemphylium vesicarium
(Wallr.) E. Simmons) on asparagus (Meyer et al., 2000). The Tom-Cast spray program
prompted an equal or fewer number of sprays and provided better disease control than the
l4-day standard program. Additionally, some newly established asparagus plots
managed according to Tom-Cast resulted in increased fern stands (Meyer et al., 2000).

The yield measurements recorded in these studies do not reﬂect yields that may
be recorded in a commercial production situation where carrots are mechanically
harvested. All carrots in 3.05 m of the center row were hand harvested, whereas yield
losses may have become evident if plots were harvested mechanically. Therefore,
differences in yield (where applicable) are attributed to leaf blight’s ability to reduce the
photosynthetic capacity of plants. A large-scale ﬁeld trial, where carrots are
mechanically harvested, is needed to determine the effect of treatments on yield reduction

attributed to the condition of foliage.

94

Tom-Cast should be used in conjunction with other effective IPM methods.
Cultural controls, such as crop rotation and the plowing under of carrot residue following
harvest, should continue to prevent A. dauci and/or C. carotae inoculum from
accumulating in infected carrot foliar residues (Pryor et al., 2002). New fungicides, as
they become available, should be tested for disease control efﬁcacy when used in a
scouting and Tom-Cast spray program. The results of these studies may prompt others to
examine the effect of using systemic fungicides for the initial application when disease
symptoms are present in the ﬁeld. In addition, the use of disease scouting and Tom-Cast

may be explored for use in other cropping systems.

95

 

‘ m'cuxhr‘.
A . n i
I.

 

LITERATURE CITED

Arcelin, R., and Kushalappa, AC. 1991. A survey of carrot diseases on muck soils in
the southwestern part of Quebec. Canadian Plant Disease Survey 71 :147-153.

Ben-Noon, E., Shtienberg, D., Shlevin, E., Vintal, H., and Dinoor, A. 2001.
Optimization of chemical suppression of Alternaria dauci, the causal agent of
Alternaria leaf blight in carrots. Plant Disease 85: 1 149-1 156.

Bird, G., Bishop, B., Graﬁus, E.J., Hausbeck, M.K., Jess, L.J., Kirk, W., and Pett, W.
2002. Insect, disease and nematode control for commercial vegetables. East
Lansing, Michigan State University. E - 312, 44-47.

F ancelli, M.I., and Kimati, H. 1991. Occurence of iprodione resistant strains of
Alternaria dauci. Summa Phytopathologica 17:135-146.

Gillespie, T.J., and Sutton, J .C. 1979. A predictive scheme for timing fungicide
applications to control Alternaria leaf blight in carrots. Canadian Journal of Plant
Pathology 1:95-99.

Hausbeck, M.K., and Harlan, BR. 2003. Chemical control of Alternaria and Cercospora
leaf blights in carrot, 2002. Fungicide and Nematicide Tests 58 (in press).

Hausbeck, M.K., Cortright, B.D., and Lindennan, SD. 2000. Chemical control of
Alternaria blight in carrot, 1999. Fungicide and Nematicide Tests 55:153-154.

James, R.V., and Stevenson, W.R. 1999. Evaluation of selected fungicides to control
carrot foliar blights, 1998. Fungicide and Nematicide Tests 54:131.

James, R.V., Stevenson, W.R., and Rand, RE. 1999. Evaluation of carrot cultivars and
breeding selections to identify resistance to foliar blights, 1999. Madison,
University of Wisconsin. Wisconsin vegetable disease control trials, 1999, 73-75.

Keinath, A.P., DuBose, V.B., and Rathwell, PI. 1996. Efﬁcacy and economics of three
fungicide application schedules for early blight control and yield of fresh-market
tomato. Plant Disease 80: 1277-1282.

Madden, L., Pennypacker, SP, and MacNab, A.A. 1978. FAST, a forecast system for
Alternaria solam' on tomato. Phytopathology 68:1354-1358.

Meyer, M.P., Hausbeck, M.K., and Podolsky, R. 2000. Optimal fungicide management
of purple spot of asparagus and impact on yield. Plant Disease 84:525-530.

Pitblado, RE 1992. Development and implementation of Tom-Cast. Ontario Ministry
of Agriculture and Food.

96

Pryor, B.M., Strandberg, J .O., Davis, R.M., Nunez, J .J ., and Gilbertson, R.L. 2002.
Survival and persistence of Alternaria dauci in carrot cropping systems. Plant
Disease 86:1115-1122.

Shaner, G., and Finney, RE. 1977. The effect of nitrogen fertilization on the expression
of slow-mildewing resistance in Knox wheat. Phytopathology 67:1051-1056.

Steel, R.G.D., Torrie, J.H., and Dickey, D. 1997. Principals and procedures of statistics:
a biometrical approach. 3rd ed. McGraw-Hill, New York, pp.197-198.

Strandberg, J .O. 1988. Establishment of Alternaria leaf blight on carrots in controlled
environments. Plant Disease 72:522-526.

Strandberg, J .O., Bassett, M.J., Peterson, CE, and Berger, RD. 1972. Sources of
resistance to Alternaria dauci. (Abstr.) Hort Science 7:345.

Wamcke, D.D., Christenson, D.R., Jacobs, L.W., Vitosh, M.L., and Zandstra, EH. 1992.
Fertilizer recommendations for vegetable crops in Michigan. East Lansing, MI,
Michigan State University. E - 5503, 20-24.

Zandstra, EH. 2002. Weed control guide for vegetable crops. East Lansing, MI,
Michigan State University. E - 433, 16.

Zandstra, B.H., Graﬁus, E.J., Wamcke, DD, and Lacy, ML. 1986. Commercial

vegetable recommendations: carrots. East Lansing, MI, Michigan State
University, Cooperative Extension Services. E-1437, 1-6.

97

APPENDIX A

SUMMARY OF WEATHER DATA, FUNGICIDE APPLICATION DATES, AND
FINAL DISEASE ASSESSMENTS FOR FIELD STUDIES IN 2001 AND 2002

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112

APPENDIX B

PESTICIDE APPLICATION EQUIPMENT STUDIES IN 2001

113

CARROT (Daucus carota ‘Goliath’) R.S. Bounds and MK. Hausbeck

Alternaria Blight; Alternaria dauci Michigan State University
Cercospora Blight; Cercospora carotae Department of Plant Pathology
East Lansing, MI 48824

Evaluation of spray application equipment to manage foliar blights of carrot, 2001.

This study was conducted at a cooperator’s farm in Oceana County, M1 on a Freesoil
sand ﬁeld previously planted to corn. Carrot ‘Goliath’ seeds were planted on 27 Apr at a
spacing of 1.75 in. to rows spaced 18 in. apart on three-row beds centered 64 in. apart.
Treatment plots were seven beds wide and 40 ft long with 10 it of unsprayed buffer
between plots and one bed of unsprayed carrots on either side of the plot. The center bed
of the plot was used as an untreated drive row, and the three beds to the left and right
were sprayed with different spray nozzle systems. Fungicides were applied with a trailer
spray rig equipped with two independent spray nozzle systems pulled by a 40 hp high
clearance tractor traveling at 3 mph. The three treated beds on the left of the drive row
were sprayed with a conventional boom elevated 12 to 16 in. above the crop canopy and
equipped with eleven XR11003VS ﬂat fan nozzles spaced 20 in. apart. Spray solutions
were mixed in S-gal tanks pressurized by C02 and calibrated to deliver 20 gal/A at a
nozzle pressure of 20 psi. The three treated beds on the right of the drive row were
sprayed with a boom located 4 to 5 ﬁ above the crop canopy that was mounted with three
air-assisted nozzles spaced 64 in. apart. The motor used to generate power for the air-
assisted system was operated at a hydraulic pressure of 1600 psi that propelled the fans to
spin at 5000 rpm. Spray solutions were mixed in a 30-gal tank and the boom was
pressurized by a hydraulic roller pump calibrated to deliver 10 gal/A. Weeds, insects,
fertilization, and irrigation were managed according to standard production practices.
Five treatments were randomly assigned within each of four blocks. Nozzle type was not
randomized for treatment plots because the tractor only traveled in one direction down
the drive rows. As a result, nozzle type is confounded with direction of travel and
consequently the side of the plot. The analyses assume that no systematic differences
exist between the two sides of the plot. As a whole, the experiment represents a split plot
design in which fungicide treatments are the whole plots and nozzle types are the sub-
plots. Seven applications were made at lO-day intervals on 12 and 23 Jul; 2, 14, and 23
Aug; and 4 and 13 Sep. Disease assessments were recorded on 8 and 28 Aug; 15 and 29
Sep; and 9 Oct from the middle bed in the center 10 ft of the middle row from both sides
of each plot. Carrots in the center 10 ii of the middle row of each plot were hand-

harvested, the foliage removed at the crown, and roots weighed to determine yield on 9
Oct.

Disease pressure was light until 1 Sep, when a severe epidemic of Alternaria and
Cercospora blights developed. The interaction between nozzle type and treatment was
not signiﬁcant for any of the data analyzed, so the best treatment did not depend on
nozzle type. The air-assisted nozzles signiﬁcantly reduced the percentage of plants with
petiole lesions throughout the season based on the AUDPC and at the time of ﬁnal
evaluation compared with the conventional nozzles (Table 34). Disease severity on the
petioles did not differ between nozzle types nor did petiole health. All fungicide regimes

114

signiﬁcantly reduced disease severity on petioles and improved petiole health compared
with the untreated but did not signiﬁcantly affect yield (Table 35). Quadris alternated
with Bravo, regardless of nozzle type, signiﬁcantly reduced the percentage of plants with
petiole lesions compared with Quadris alternated with Kocide and the untreated control.

115

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117

CARROT (Daucus carota ‘Goliath’) R.S. Bounds and MK. Hausbeck

Alternaria Blight; Alternaria dauci Michigan State University
Cercospora Blight; Cercospora carotae Department of Plant Pathology
East Lansing, MI 48824

Evaluation of spray application equipment and reduced fungicide rates to manage
foliar blights of carrot, 2001.

This study was conducted at a cooperator’s farm in Oceana County, M1 on a Freesoil
sand ﬁeld previously planted to corn. Carrot ‘Goliath’ seeds were planted on 27 Apr at a
spacing of 1.75 in. to rows spaced 18 in. apart on three-row beds centered 64 in. apart.
Treatment plots were seven beds wide and 40 it long with 10 ft of unsprayed buffer
between plots and one bed of unsprayed carrots on either side of the plot. The center bed
of the plot was used as an untreated drive row, and the three beds to the left and right
were sprayed with different spray nozzle systems. Fungicides were applied with a trailer
spray rig equipped with two independent spray nozzle systems pulled by a 40 hp high
clearance tractor traveling at 3.0 mph. The three treated beds on the left of the drive row
were sprayed with a conventional boom elevated 12 to 16 in. above the crop canopy and
equipped with eleven XR11003VS ﬂat fan nozzles spaced 20 in. apart. Spray solutions
were mixed in S-gal tanks pressurized by C02 and calibrated to deliver 20 gal/A at a
nozzle pressure of 20 psi. The three treated beds on the right of the drive row were
sprayed with a boom located 4 to 5 it above the crop canopy that was mounted with three
air-assisted nozzles spaced 64 in. apart. The motor used to generate power for the air-
assisted system was operated at a hydraulic pressure of 1600 psi that propelled the fans to
spin at 5000 rpm. Spray solutions were mixed in a 30-ga1 tank and the boom was
pressurized by a hydraulic roller pump calibrated to deliver 10 gal/A. Weeds, insects,
fertilization, and irrigation were managed according to standard production practices.
Fungicides were applied at 75% of the labeled rate and at the labeled rate. Seven
treatments were included in this study: an untreated control, Bravo 82.5WDG applied at
1.1 and 1.4 lb/A; Kocide 53.8DF applied at 1.1 and 1.5 lb/A; and Quadris 2.08F applied
at 4.7 and 6.2 ﬂ oz/A. Treatments were randomly assigned within each of four blocks.
Nozzle type was not randomized for treatment plots because the tractor only traveled in
one direction down the drive rows. As a result, nozzle type is confounded with direction
of travel and consequently the side of the plot. The analyses assume that no systematic
differences exist between the two sides of the plot. As a whole, the experiment
represents a split plot design in which fungicide treatments are the whole plots and nozzle
types are the sub-plots. Treatments were compared by decomposing the treatment SS
into four components to address the following questions: (1) Do the untreated plots differ
from the average treated plot; (2) Is there a fungicide main effect that explains overall
differences in the three fungicides used; (3) Is there a rate main effect that explains
overall differences in the two rates used; and (4) Is there an interaction between fungicide
and rate to explain if differences in the fungicides depend on rate at which the fungicide
was applied? Seven applications were made at lO-day intervals on 12 and 23 Jul; 2, 14,
and 23 Aug; and 4 and 13 Sep. Disease assessments were recorded on 8 and 28 Aug; 15
and 29 Sep; and 9 Oct from the middle bed in the center 10 ft of the middle row from
both sides of each plot. Carrots in the center 10 ft of the middle row of each plot were

118

hand-harvested, the foliage removed at the crown, and roots weighed to determine yield
on 9 Oct.

Disease pressure was light until 1 Sep, when a severe epidemic of Alternaria and
Cercospora blights developed. The interaction between nozzle type and treatment was
not signiﬁcant for any of the data analyzed, so the best treatment did not depend on
nozzle type. Furthermore, since the previous interaction was not signiﬁcant, the best
nozzle type did not depend on the fungicide used or the rate applied. The air-assisted
nozzles, irrespective of the fungicide or rate used, signiﬁcantly reduced petiole blight and
improved petiole health compared with the conventional nozzles (Table 36). The
interaction between fungicide and rate was not signiﬁcant for either petiole blight or
petiole health, so the best fungicide did not depend on the rate applied. In addition, the
main effect of rate was not signiﬁcant. The signiﬁcant main effect of fungicide indicates
that Bravo and Quadris signiﬁcantly reduced the AUDPC, petiole blight incidence,
petiole blight severity, and improved petiole health compared with Kocide (Table 37).
Bravo signiﬁcantly reduced petiole blight incidence compared with Quadris. Yield was
not signiﬁcantly affected by nozzle type or treatment, but yield losses may have become
evident if plots were harvested mechanically. Foliar blight control was improved by
applying fungicides with the air-assisted nozzles, and the control provided by the
fungicides was not affected by the rate applied.

119

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