Disease Management

Terry Kirkpatrick, Plant PathologyM. Elena Garcia, Horticulture

University of Arkansas

Plant Disease

A malfunction of one or more plant systems due to continuous irritation that results in some symptom – Horsfall &Cowling, 1977

At least 90% of plant disease control is accomplished through proper cultural methods and sanitation.

Unfortunately, modern agriculture sometimes ignores the fundamental concepts for convenience.

Types of Diseases

Biotic: result of infection of plant by a pathogen; able to spread to healthy plants

Types of pathogens: fungi, bacteria, viruses, nematodes, parasitic plants

Abiotic: result of environmental or cultural problem; cannot be transmitted

Types: light, temperature, hail, lightning, fire, wire girdling, pot-bound roots, air pollution, chemical injury, herbicide injury, nutrient deficiencies, chemical toxicity

Biotic Diseases

Fungi ------ ~80%

Bacteria ----- ~15%

Nematodes ----- <5%

Viruses ------ <5%

The Disease Triangle

Disease development depends on:1. Presence of pathogen

(source of inoculum)2. Host- Plant

susceptibility3. Environmental

conditions must be favorable (light, moisture, temperature, soil fertility)

ENVIRONMENT

Total of conditions favoring disease

PATH

OGEN

Tota

l of v

irulen

ce, a

bund

ance

, etc.

HOSTTotal of conditions favoring

susceptibility

Amount of disease

HOST

Immune-no association other than coincidentalcausal entity (pathogen) has no effect.

Susceptible – unable to suppress a pathogen; adverse response of plant to causal organismResistant-ability to suppress the activity of

a pathogen; may or may not have adverse effect on plant.

Susceptible Resistant

Pathogen – an organism that is capable of causing disease

Virulent Avirulent

Virulence – relative ability of the pathogen to cause disease

Avirulence – inability of a pathogen to cause disease

In order for disease to occur, both a susceptible host and a virulent pathogen must be present

Environmental Manipulation

Env

iron

men

t Host

Pathogen

You can’t control the weather……

But you can control the plant’s environment, at least to some degree

Environmental management for disease control

• Irrigation: availability; quality; type• Site selection• Pruning; thinning; trellising; plant spacing; row

spacing• Beds vs. flat planting• Annual vs. perennial (strawberry)• Mulching• Fertility; pH; overall plant nutrition balanced• Timely harvest

How do diseases Spread?

Soil-borne diseasesWind-borne diseasesSeed-borne diseasesVector-transmitted diseases

Soil-Borne Diseases

Introduced into soilInoculation

WindInfected seedsInfected transplantsOther infected plantsContaminated toolsWater

Crop residues provide an environment for pathogen Can affect roots or any part of the plant

Smut, bacterial wilt, fusarium wilt, dampening-off

Wind-Borne Diseases

Spore transportationMay be great distances

Transport directly to aerial potions of plantsInoculate soilRusts, powdery mildew, late and early blight of tomato, apple scab

Seed-Borne Diseases

Recommended to purchase certified disease-free seedsSeed sterilizationBacterial blights and anthracnose of peasPotato ‘seeds’Viral diseases may spread by seeds

Vector-Transmitted Diseases

InsectsCarried from plant to plant on the mouth parts of insects

PeoplePruning NematodesFungi

Epidemiology1. Single cyclic disease – most soilborne pathogens.

0

10

20

30

40

50

60

70

Disease

Time

Epidemiology1. Multicyclic disease – Foliar pathogens

0102030405060708090100

Disease

Time

lag

exponential

stationary

Moisture on foliage is a primary factor.

Symptoms caused by fungi:

May be local or generalNecrotic symptoms

Leaf spotsBlightCankerDiebackRoot rotDamping-offBasal stem rotSoft or dry rotAnthracnoseScabDecline

Excessive growth symptoms

ClubrootGallsWartsWitches’ broomsLeaf curls

Additional symptomsWiltRustSmutMildew

Fungal Symptoms

Anthracnose- Pumpkin Anthracnose- Strawberry

Sherrie Smith, University of Arkansas, Cooperative Extension

Sherrie Smith, University of Arkansas, Cooperative

Symptoms caused by bacteria:Galls: crown, twig, or caneHairy rootSoft rotsCankers Wilt Scab Blight Leaf spotsRoot nodules of legumes

Fire blight

Sherrie Smith, University of Arkansas Cooperative Extension

Bacterial Symptoms

Watermelon Bacterial Fruit Blotch

Sherrie Smith, University of Arkansas, Cooperative Extension

Cabbage Black Rot

Symptoms caused by viruses:May occur on any part of plantStuntingPremature deathMosaics

MottlingStreakRing or line patternVein clearingVein bandingChlorotic spots

Ring spots

APLPV - American Plum Line Pattern Virus - Plums

Sherrie Smith - University of Arkansas, Cooperative Extension

Pictures of Viral SymptomsBean- Yellow Mosaic Virus Tomato-Tobacco Mosaic

http://www.avrdc.org/LC/tomato/tomv/htmlSherrie Smith, University of Arkansas, Cooperative Extension

Some Symptoms caused abiotic diseases:

Low temperatureLate frost tip or bloom necrosisBark splitInternal branch necrosisWinter drying of evergreensCold water ringsLeaf margin necrosisFrost injury to fruits

High temperatureSunscald on fruitWater coreBlossom end rot (citrus)

Light Etiolation (low light)Scald (high light)

Low moistureLeaf scorchWilting

High moistureFlood damageOedemaRot

Low oxygenBlack heart of tubers

Some Symptoms caused abiotic diseases:

Herbicide injuryLeaf or fruit deformitiesChlorosisNecrosisStunting Death

Air pollutionChlorosisBleached leavesStuntingLeaf margin necrosisPremature defoliation

Sherrie Smith, University of Arkansas, Cooperative Extension Photo courtesy NARA, photographer Gene Daniels/U.S. EPA

Ozone-damaged plant (left) and normal plant (right)Grazon Damage

Managing Plant Diseases

Manage cropping system in a way that prevents disease from occurringIdentify disease

Biotic or abioticFungi, bacteria, virus, or nematodeFoliar or soil-borne

Take disease triangle into consideration:Improve quality and ability of host to withstand diseaseReduce or eliminate populations of pathogensEncourage environmental conditions favorable for hosts and unfavorable for pathogens

Why is Disease ID Important?

Altering irrigation system to reduce leaf wetness will help against foliar disease, but not soil-borne diseaseCrop rotations affect soil-borne disease, but not wind- or insect-dispersed diseasesFungicides control fungal disease, but not viruses

Sanitation: A Key Strategy to Disease Prevention

Plant in sites free of diseaseClean equipment to prevent spread of disease through fields

Clean pruning tools in 10% bleach solution between cuts

Remove crop residue to reduce inoculum buildupRemove weeds that may serve as pathogen host

Prune diseased portion from plantsRemove severely infected plants from field

Prevent further spread of viruses

Properly dispose of infected debris

Place infected debris in center of hot compost heapBurn debrisPlace in sealed container in trash

Cultural Practices to Improve Host Health

Choose resistant cultivarsAvoid excess or deficient crop nutritionUse transplants rather than seeds to avoid exposure to soil-borne seedling diseasesPlant seeds shallowly to limit pathogen exposureCorrectly prune woody plants to improve air circulation and light infiltration

Scab resistant apple cultivar ‘Liberty’ (left) with susceptible cultivar ‘McIntosh’ (right)

Cultural Practices that may reduce pathogen presence

Adjust soil pHLiming negatively affects some fungal pathogens (e.g. club root )Increasing pH inhibits survival of some bacteria (e.g. scab, cotton root rot)

Prune out diseased portion of plants to avoid further spread

Crop rotations break cycle of certain pathogens

Spatially: move location of species within fieldTemporally: grow different species from year to yearKeep in mind many pathogens attack numerous species within a plant family

Cultural Practices that affect environmental conditions

Use appropriate types of irrigation

Overhead irrigation encourages spread of foliar disease

Avoid overwateringImprove soil drainageImprove air circulation through wider row spacing, decreased planting density, and pruning

Time tasks appropriatelyCool soils slow germination of many seeds (e.g. corn, beans), increasing susceptibility to soil-borne diseaseLate-season pruning or fertilization discourages hardening-off, increasing occurrence of abiotic disease (e.g frost injury)

Post-harvest cold storage to delay growth of soft rots

Physical Control

Soil solarizationIncreased temperatures due to solar heat kill many soil-borne pathogens near soil surfaceCover soil with clear plasticSoil temperatures may reach 52°C

Biological Control

Limited options for disease controlIncrease diversity of soil microbes through proper soil management and addition of organic matterCross-protection

Inoculation of plant with mild strain of disease to provide protection against more virulent strainsExample: Galltrol-A®; nonpathogenic strain of Agrobacterium; protects against crown gall

Coniothyrium minitansFungal pathogen of Sclerotinia sclerotiorum and S. minor (cause of white mold on cole crops, carrots, and beansOMRI-Listed product: Contans® WG

Other control

Chemical Control

Potassium-, Sodium-, or Aluminum-Bicarbonates

Formulations differ in efficacy against certain diseases (e.g. Potassium bicarbonate more effective in control of powdery mildew)Studies have found a variety of concentrations most effective, ranging from 0.5-2%

Some benefit seen from addition of surfactantsFoliar burning resulted when concentration too high

OMRI-Listed products: Kaligreen® and MilStop® broad spectrum foliar fungicide

Chemical Control

Sulfur : prevents fungal spores from germinating; phytotoxic

Also has some insecticidal and acaricidal propertiesToxic to beneficial insects, fish, mammalsShould not be used if oil has been sprayed in the previous month

Chemical Control

Lime-sulfur: kills recently germinated fungal sporesphytotoxicSpraying during high temperatures (>85F) increases phytotoxicityToxic to mammals

Copper: inhibit growth and germination of sporesphytotoxic Labeled for use on over 100 crops to control fungal and bacterial diseasesShould be used with care due to toxicity to several non-target species OMRI-Listed products: Champion WP ®, Crop Clean COCS ®, Britz Copper Sulfur 15-25 Dust ®, Nordox 75WG ®, Concern® Copper soap Fungicide, Cueva Fungicide Concentrate ®, Cueva Fungicide Ready to Use ®, Lilly Miller Ready to Use Cueva Copper Soap Fungicide®

Resourceshttp://www.longislandhort.cornell.edu/vegpath/photos/index.htmhttp://ipmnet.org/plant-disease/glossary.cfmhttp://plantpathology.uark.edu/2539.htmhttp://plantclinic.cornell.edu/http://www.ag.auburn.edu/hort/landscape/exam_pics.html#diseases

http://www.nysaes.cornell.edu/pp/resourceguide/

Acknowledgements

This presentation address general organic production practices. It is to be to use in planning and conducting organic horticulture trainings. The presentation is part of project funded by a Southern SARE PDP titled “Building Organic Agriculture Extension Training Capacity in the Southeast”Project Collaborators• Elena Garcia, University of Arkansas CES

Heather Friedrich, University of ArkansasObadiah Njue, University of Arkansas at Pine BluffJeanine Davis, North Carolina State UniversityGeoff Zehnder, Clemson UniversityCharles Mitchell, Auburn UniversityRufina Ward, Alabama A&M UniversityKen Ward, Alabama A&M UniversityKaren Wynne, Alabama Sustainable Agriculture Network