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Diagnosis and Prevention of VegetableDiseases in Organic systems

Daniel S. EgelDepartment of Botany and Plant Pathology

Purdue University

4369 N. Purdue Rd.

Vincennes, IN 47591

(812) 886-0198 egel@purdue.edu

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Learn about what diseasesare important for your

crop/location

VerticilliumTMVTimber rot

SpeckSeptoriaRoot knot

Leaf moldLate blightGray mold

Fusarium wiltFusarium crown rotEarly blight

CankerBacterial spotAnthracnose

Tomato Diseases

There may be many possible diseases listed for your crop.

However, not all of them will be important for your location

and situation. It makes sense to learn which diseases are

most important for your area and to learn the

characteristics of each disease.

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Unusual in IndianaYesNoLate blight

CommentsResistance

Maybe

Seed

borneDisease

Primarily greenhouse

diseasesNoNo

Leaf mold/

gray mold

NoNoEarly blight/

Septoria

NoYesCanker

SpeckYesBacterialspot/speck

Stake & mulchNoYesAnthracnose

Residue borne tomato diseases-rotations of

2-4 years and fall tillage critical

Diseases which are residue borne will survive in the soil

only a long as the residue (leaves, stems, fruit) for that crop

survives. In general, the length of the rotation for residue

borne diseases are shorter than the rotations required to

manage soil borne diseases.

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Wide host range.YesNoVerticillium

wilt

Includes buckeye rot and

root rotNoNo

Phytopthora

diseases

CommentsResistance

Maybe

seed

borneDisease

NoNoWhite mold(timber rot)

Wide host range. Causal

agent survives manyyears in soil.

YesNoRoot knot

3 races existYesYesFusariumwilt

NoNoFusarium

crown rot

Soil borne tomato diseases-rotations of

more than 4 years and fall tillage critical

The organisms that cause soil borne diseases may survive

long periods of time in the soil without any host. In most

vegetable systems, long rotations will keep soil pathogens

from rapid increases in populations. However, rotations

alone will not successfully manage soil borne diseases.

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Virus diseasesPowderymildew

Plectosporiumblight

Phytophthora blightFusariumfruit rot

Downy mildewBacterial fruit spotBlack rot

Pumpkin Diseases

Pumpkin diseases have very different characteristics and

thus need to be managed differently.

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Also affects squashNoYesBacterial

fruit spot

CommentsResistance

Maybe

seed

borneDisease

Readily windbornePartialNoPowdery

mildew

NoNo

Plecto-

sporium

blight

aka, gummy stemblight

NoYesBlack rot

Residue borne pumpkin diseases-rotations of

2-4 years and fall tillage critical

Rotations of 2 to 4 years will help to manage these

diseases of pumpkin.

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CommentsResistance

Maybe

Seed

BorneDisease

Water management

critical. Avoid

rotations withsolanaceous crops.

NoNoPhytophthorablight

NoYesFusarium

fruit rot

Soil borne pumpkin diseases-rotations of

more than 4 years and fall tillage critical

Long rotations are needed to avoid building up populations

of these soil borne pathogens of pumpkins.

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CommentsResistance

Maybe

seed

borneDisease

Overwinters in SE USor on weed hosts

NoNoVirusdiseases

Overwinters in SE USNoNoDowny

mildew

Pumpkin diseases that are neither

soil or residue borne

The organisms which cause these diseases do not survive

in the soil or residue.

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Post Planting Disease ManagementPost Planting Disease Management

••Fungicides/bactericidesFungicides/bactericides

••Apply materials preventativelyApply materials preventatively

••Importance of the latent periodImportance of the latent period

Latent period-Latent period-

the time between infection and symptom productionthe time between infection and symptom production. .

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Septoria Septoria leaf spot of tomato-latent period about 6 days.leaf spot of tomato-latent period about 6 days.

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Downy mildew of pumpkin-latent period 4-12 days.Downy mildew of pumpkin-latent period 4-12 days.

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1. The latent period means that there are normally1. The latent period means that there are normally

more infections present than the eye can see. more infections present than the eye can see.

2. Even the best fungicide/bactericide2. Even the best fungicide/bactericide

merely merely slows the production of slows the production of symptoms. symptoms.

3. Therefore, it is best to apply a pesticide in a 3. Therefore, it is best to apply a pesticide in a

preventative manner. preventative manner.

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High Disease

Low Disease

Hypothetical Fungicide Trial

Most fungicides will not completely control a disease.

Historically, those conventional fungicides which have had

success result in from 1/10 to 1/3 s much disease as

untreated crops.

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Biofungicides on snap beans 1

A=allowed, R=restricted, x=not listed

170

32R, ATrilogy + Kaligreen

77

140R, R, A, xOxidate + Trilogy + Kaligreen + Cohere

60

102AContans

43A, R, xBallad + Champion + Biotune

80

91A, xBallad + Biotune

% controlOMRI*Product

White Mold

*Organic Materials Review Institute

This information is based on published reports from 2006. The two

numbers for each organic treatment represent two different trails.

Note that sometime the trials differed greatly in the end result. If a

treatment is listed as controlling over 100% compared with the

control, the disease in this treatment was worse than the control.

Many organically labeled compounds require precise knowledge of

the characteristics of the compound.

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Biofungicides on snap beans 2

A=allowed, R=restricted, x=not listed

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106R, ATrilogy + Kaligreen

69

62R, R, A, xOxidate + Trilogy + Kaligreen + Cohere

160

49AContans

78A, R, xBallad + Champion + Biotune

0

65A, xBallad + Biotune

% controlOMRIProduct

Gray Mold

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Why scout?

Identify disease problems during the season

•To change practices this year

•For next year

Scout for diseases

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How to scout:How to scout:

Scout your crops on a regular basis (calendar). Scouting

supplies include a hand lens (10X or higher), paper for

notes, self sealing bags for samples, a marker pen, field

guide and possibly a digital camera.

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A=Healthy leaf; B=infectious pattern;

C=non-infectious pattern (nutrition?);

D=non-infectious pattern (stress?)

These diagrams illustrate some generalizations in

determining an infectious disease problem from a

nutritional or stress induced problem. Note that infectious

foliar diseases often produce a random distribution of

lesions on the leaf.

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Old leaf Old leaf chlorosis-muskmelonchlorosis-muskmelon

An example of a non-infectious problem. In this case, no

corrective action is required.

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Interveinal chlorosis-muskmelonInterveinal chlorosis-muskmelon

These muskmelon leaves probably suffer from a nutritional

problem.

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Alternaria Alternaria leaf blightleaf blight of muskmelonof muskmelon

A lesion caused by an infectious disease often results in a

lesion with an internal pattern such as the ridges that can

be observed here. Note that the ridges in this lesion are

concentric and resemble a ‘bulls-eye’. Such a pattern is

characteristic of diseases caused by the fungus Alternaria.

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Septoria Septoria leaf spot-tomatoleaf spot-tomato

Note the internal pattern present in these lesions caused

by Septoria. The margin of the lesions are a chocolate

brown and the center is gray. Note also the random

pattern of lesions on the leaves.

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Septoria Septoria leaf spot-tomatoleaf spot-tomato

A close up of these same lesions. Note that there are

small dark spots in the center of these lesions. These are

the structures of the fungus that causes Septoria leaf spot

and can bee seen with a hand lens.

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Contact herbicide damage-watermelonContact herbicide damage-watermelon

The random pattern of the lesions on this leaf could be

mistaken for an infectious disease. However, the lesions

observed here have little internal pattern. This is typical of

a lesion caused by a substance that kills the tissue on

contact, but does not cause the lesion to expand over time.

This leaf damage was caused by a contact herbicide.

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Contact herbicide damage-watermelonContact herbicide damage-watermelon

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Downy mildew-pumpkinDowny mildew-pumpkin

These chlorotic (yellow) lesions are limited by the leaf

veins, therefore the lesions are angular in appearance.

Note that some of the lesions are necrotic (brown).

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Downy mildew-pumpkinDowny mildew-pumpkin

This pumpkin leaf has been infected by the fungus that causes

downy mildew. The fungus can be seen growing on the leaf in the

areas circled in red, causing rectangular ‘fuzzy’ looking areas. The

downy mildew fungus can often be observed on a leaf when wet with

dew as in this photo. The observation of the downy mildew fungus

on the leaf is diagnostic, that is, can be considered proof of the

identity of the disease.28

Downy mildew-pumpkinDowny mildew-pumpkin

The margins of these pumpkin leaves are necrotic, causing

the leaf to assume a ‘burnt’ appearance. These symptoms

were caused by downy mildew of pumpkin.

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Powdery mildew of pumpkinPowdery mildew of pumpkin

The fungus that causes powdery mildew assumes a

talcum-like appearance. Although many different plants

are affected by powdery mildew, the fungi are specialized

to a particular family of plants. For example, powdery

mildew of pumpkin can affect other cucurbits, but cannot

affect tomato.30

Pumpkin virusPumpkin virus

This leaf is representative of the mosaic symptoms that can

occur as a result of virus infection. Papaya ringspot,

watermelon mosaic virus and zucchini mosaic virus are all

potyviruses, infect cucurbits and may result in symptoms

such those shown here.

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Pumpkin virusPumpkin virus

Leaf strapping is another symptom sometimes observed in

virus infections. The leaves shown here are infected by

one of the potyviruses discussed in the previous slide.

More than one virus may be present in the infection.

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Pumpkin virusPumpkin virus

Fruit may also have symptoms of virus infection such the

mottling seen above. Aphids are responsible for vectoring

potyviruses in pumpkins.

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Fusarium Fusarium wilt-watermelonwilt-watermelon

The wilting observed here can be distinguished from the

wilt of a plant that lacks water from the one-sided nature of

the symptom. This is typical of a vascular wilt disease.

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Fusarium Fusarium wilt-watermelonwilt-watermelon

Vascular discolorationVascular discoloration

HealthyHealthy rootsroots

In this photo, the reason why the plant in the previous slide

had a one-sided wilt can be seen. The vascular tissue in

half o this stem is discolored and non-functional. The roots

of plants affected by a vascular disease will look healthy.

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DiseasedDiseased rootsroots

Diseased roots will look discolored and rotten. Such roots

may result in a wilted plant, but not in the one-sided wilt

observed previously.

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Using a DiagnosticUsing a Diagnostic LabLab

1.1. CollectCollect fresh, donfresh, don’’t send over weekend.t send over weekend.

2.2. Send several examplesSend several examples

3.3. Crush proof containerCrush proof container

4.4. Provide lots of informationProvide lots of information

5.5. DonDon’’t pullt pull plants-do bag rootsplants-do bag roots

6.6. Press leavesPress leaves

7.7. Contact labContact lab