CONTROL OF PLANT DISEASESCONTROL OF PLANT DISEASESAs plant pathologists, we don't study morphology, life As plant pathologists, we don't study morphology, life

cycles, and spread of pathogens because it's so cycles, and spread of pathogens because it's so interesting; instead, the interesting; instead, the main purposemain purpose behind behind understanding pathogens and the diseases they understanding pathogens and the diseases they cause is so diseases can be cause is so diseases can be controlled.controlled.

For most crops, the For most crops, the goal is to save most of the plant goal is to save most of the plant population,population, not selected individuals. not selected individuals. exception: exception: tree tree crops (citrus, pecan, timber)crops (citrus, pecan, timber)

Basic principles of disease control:Basic principles of disease control:Control strategiesControl strategies can be divided into can be divided into two groupstwo groups based on their based on their

effect on the development of resistance to the control measure effect on the development of resistance to the control measure by the pathogen:by the pathogen:

1.1. EradicativeEradicative control measures — designed to eliminate the control measures — designed to eliminate the entire pathogen population - entire pathogen population - examples: examples: pesticides, vertical or pesticides, vertical or complete resistance - These tend to select for resistant complete resistance - These tend to select for resistant variants of the pathogen. Why? All individuals are affected, so variants of the pathogen. Why? All individuals are affected, so the pathogen must adapt or die.the pathogen must adapt or die.

2.2. ManagementManagement control measures — designed to reduce the control measures — designed to reduce the pathogen population by destroying a portion of the population pathogen population by destroying a portion of the population - - examples: examples: horizontal or partial resistance, antagonism, horizontal or partial resistance, antagonism, cultural practices, quarantine - These do not apply heavy cultural practices, quarantine - These do not apply heavy selection pressure to the pathogen. Why? Portions of the selection pressure to the pathogen. Why? Portions of the pathogen population remain unaffected, no pressure to adapt. pathogen population remain unaffected, no pressure to adapt.

* Of the two, we * Of the two, we preferprefer to use to use managementmanagement strategies. strategies.

Disease controlDisease controlThere are four basic types of control There are four basic types of control

measures:measures:a. Biological controla. Biological controlb. Cultural control (includes physical b. Cultural control (includes physical

control) control) c. Legislative and regulatory control c. Legislative and regulatory control d. Chemical controld. Chemical control

Biological controlBiological controlManipulation of biotic entities; host and Manipulation of biotic entities; host and

antagonistic microorganismsantagonistic microorganisms1. 1. Host resistanceHost resistance - control based on the - control based on the genesgenes

and the resistance mechanisms they controland the resistance mechanisms they controla.a. Van der Plank described two types of Van der Plank described two types of

resistance (1960s; these are the "classics‘’):resistance (1960s; these are the "classics‘’):i.i. Vertical resistance — resistance that is effective Vertical resistance — resistance that is effective

against some, but not all, races of a pathogen; against some, but not all, races of a pathogen; decreases the effective amount of incoming decreases the effective amount of incoming inoculum (avirulent races can't infect), but does inoculum (avirulent races can't infect), but does not reduce the rate of disease development not reduce the rate of disease development (virulent races are not affected)(virulent races are not affected)

ii.ii. Horizontal resistance — resistance that is effective Horizontal resistance — resistance that is effective against all races of the pathogen; decreases the against all races of the pathogen; decreases the rate of disease development for all racesrate of disease development for all races

Biological controlBiological controlb. Resistance has been defined in many other ways since b. Resistance has been defined in many other ways since

Van der Plank, including systems based on: Van der Plank, including systems based on: epidemiologic effects, number of genes involved, how epidemiologic effects, number of genes involved, how long the resistance lasts under field conditions; long the resistance lasts under field conditions; additional terms you should be familiar with are: additional terms you should be familiar with are:

i.i. tolerancetolerance — plants are diseased, but they do not — plants are diseased, but they do not yield less than healthy plantsyield less than healthy plants

ii.ii. induced resistanceinduced resistance — a normally susceptible plant — a normally susceptible plant treated with an avirulent strain of a pathogen gives treated with an avirulent strain of a pathogen gives a resistant reaction when challenged later with a a resistant reaction when challenged later with a strain that is virulent strain that is virulent

Biological controlBiological control2. 2. AntagonistsAntagonists — control using microorganisms that inhibit the — control using microorganisms that inhibit the

growth, development, or reproduction of pathogens growth, development, or reproduction of pathogens

Four types of activity:Four types of activity:1.1. AntibiosisAntibiosis — inhibition of pathogen through antibiotics produced — inhibition of pathogen through antibiotics produced

by the antagonist - by the antagonist - examples: examples: streptomycin (antibacterial, from streptomycin (antibacterial, from actinomycete), penicillin (antibacterial, from fungus)actinomycete), penicillin (antibacterial, from fungus)

2.2. CompetitionCompetition — two organisms attempt to utilize the same — two organisms attempt to utilize the same limiting factors (nutrients, oxygen); supply not large enough to limiting factors (nutrients, oxygen); supply not large enough to support both antagonist and pathogensupport both antagonist and pathogen

3.3. AmensalismAmensalism — antagonist makes the environment unsuitable for — antagonist makes the environment unsuitable for the pathogen (modifies pH, temperature, moisture)the pathogen (modifies pH, temperature, moisture)

4.4. Parasitism & predationParasitism & predation — antagonist directly attacks the — antagonist directly attacks the pathogen pathogen example: example: nematode-trapping funginematode-trapping fungi

Biological controlBiological controlAntagonism frequently operates under natural Antagonism frequently operates under natural

conditions; difficult to manipulate due to the modifying conditions; difficult to manipulate due to the modifying effects of the environment; may be important in effects of the environment; may be important in suppressive soilssuppressive soils — soils in which the pathogen cannot — soils in which the pathogen cannot establish, develop, or surviveestablish, develop, or survive

example: example: Queensland avocado grove has been Queensland avocado grove has been productive for 34 years even though researchers productive for 34 years even though researchers routinely collect a virulent isolate of routinely collect a virulent isolate of PhytophthoraPhytophthora from the soil; root rot is common in nearby groves, but from the soil; root rot is common in nearby groves, but very rare in the grove with suppressive soilvery rare in the grove with suppressive soil

Cultural controlCultural controlCultural (physical) control — manipulation of the environmentCultural (physical) control — manipulation of the environmentThere are many types of cultural control. Here are few selected There are many types of cultural control. Here are few selected

examples:examples:1.1. Crop rotationCrop rotation — rotate crops and varieties over seasons to — rotate crops and varieties over seasons to

reduce pathogen inoculum levels * This is probably the most reduce pathogen inoculum levels * This is probably the most widely employed control measure in agriculture! widely employed control measure in agriculture! example: example: rotate soybean with corn to control soybean cyst nematoderotate soybean with corn to control soybean cyst nematode

2.2. Selection of planting date or planting locationSelection of planting date or planting location — choose a — choose a time/place favorable for the host, rather than the pathogen: time/place favorable for the host, rather than the pathogen: avoid pathogen or its vector avoid pathogen or its vector example: example: (time) plant cotton late (time) plant cotton late to control damping-off caused by to control damping-off caused by Pythium Pythium (warm soil)(warm soil)

3.3. Seeding rate and canopy densitySeeding rate and canopy density — adjust within-row and — adjust within-row and between-row spacing to open the canopy and reduce diseases between-row spacing to open the canopy and reduce diseases that spread in the humid, protected canopy environmentthat spread in the humid, protected canopy environment

Cultural controlCultural controlCultural (physical) control — manipulation of the environmentCultural (physical) control — manipulation of the environment4.4. IrrigationIrrigation

a.a. Pathogens can be spread in irrigation water or favored by wet soils-Pathogens can be spread in irrigation water or favored by wet soils-eexample: xample: late blight late blight (Phytophthora) (Phytophthora)

b.b. Pathogens can be controlled by flooding - Pathogens can be controlled by flooding - example: example: Fusarium wilt on Fusarium wilt on bananabanana

5.5. Control insects and weedsControl insects and weeds — insects vector viruses and other — insects vector viruses and other pathogens; weeds serve as alternate hosts for pathogens or pathogens; weeds serve as alternate hosts for pathogens or vectors and increase canopy densityvectors and increase canopy density

6.6. SanitationSanitation ~ keep area free of diseased plant material by pruning ~ keep area free of diseased plant material by pruning diseased branches (fireblight), plowing under or burning debris, diseased branches (fireblight), plowing under or burning debris, washing and sterilizing harvesting and processing equipment washing and sterilizing harvesting and processing equipment (Rhizopus soft rot); poor sanitation contributed to the late blight (Rhizopus soft rot); poor sanitation contributed to the late blight outbreak that caused the Irish famineoutbreak that caused the Irish famine

7.7. Heat or refrigerationHeat or refrigeration -- hot air, hot water, or steam treatments are -- hot air, hot water, or steam treatments are used to kill pathogens in seed or propagation materials; harvested used to kill pathogens in seed or propagation materials; harvested fruits and vegetables are kept refrigeratedfruits and vegetables are kept refrigerated

Legislative and regulatory controlLegislative and regulatory control1.1. QuarantineQuarantine — detention and associated practices for — detention and associated practices for

preventing the entry of diseased materials or pathogens preventing the entry of diseased materials or pathogens into an area; relatively inexpensive; can be at federal or into an area; relatively inexpensive; can be at federal or state level (CA citrus)state level (CA citrus)a.a. APHIS (Animal and Plant Health Inspection Service) — APHIS (Animal and Plant Health Inspection Service) —

agency within USDA that runs:agency within USDA that runs:i.i. PPQPPQ (Plant Protection and Quarantine) — agency (Plant Protection and Quarantine) — agency

responsible for federal quarantines -established by responsible for federal quarantines -established by the the Plant Quarantine ActPlant Quarantine Act (1912), which resulted from (1912), which resulted from epidemics of chestnut blight and Dutch elm diseaseepidemics of chestnut blight and Dutch elm disease

b.b. Pest and Disease SurveyPest and Disease Survey — national database; all pests on — national database; all pests on major crops in each statemajor crops in each state

c.c. Action programsAction programs -- eradicate or contain pests that get past -- eradicate or contain pests that get past quarantine quarantine worked for: worked for: citrus canker (FL); citrus canker (FL); didn't work for; didn't work for; potato golden nematode on Long Island, NY (birds); potato golden nematode on Long Island, NY (birds); sugarcane smut, FL (hurricanes)sugarcane smut, FL (hurricanes)

Legislative and regulatory controlLegislative and regulatory control

1.1. QuarantineQuarantine — —

2.2. Inspection and certification programsInspection and certification programs — state — state level; plants/seeds grown under conditions level; plants/seeds grown under conditions unfavorable for pathogens and are inspected unfavorable for pathogens and are inspected to be sure that pests are not transported along to be sure that pests are not transported along with packing materialwith packing material

3.3. Pesticide labeling and applicator certificationPesticide labeling and applicator certification — these activities are under the control of the — these activities are under the control of the EPA (Environmental Protection Agency)EPA (Environmental Protection Agency)

ChemicalChemicalApplication of pesticidesApplication of pesticidesPesticidePesticide — chemical that kills a pest (fungicide, bactericide, — chemical that kills a pest (fungicide, bactericide,

nematicide, etc.); fungicides as examples, since fungi are the nematicide, etc.); fungicides as examples, since fungi are the largest group of plant pathogenslargest group of plant pathogens

Types of fungicides and selected examples: Types of fungicides and selected examples: 1.1. InorganicInorganic

a.a. SulfurSulfur -- oldest known fungicide -- oldest known fungicideb.b. CopperCopper — oldest formulated fungicide is the Bordeaux mixture — oldest formulated fungicide is the Bordeaux mixture

(downy mildew of grape); still the most widely used copper (downy mildew of grape); still the most widely used copper fungicide in the world fungicide in the world

2.2. OrganicOrganica.a. Protective fungicidesProtective fungicides -- protect infection court -- protect infection court

i.i. thiram (Thiram, Tersan) — seed and bulb treatment of vegetablesthiram (Thiram, Tersan) — seed and bulb treatment of vegetablesii.ii. dichloran (Botran) ~ used against dichloran (Botran) ~ used against Botrytis Botrytis on vegetables and flowerson vegetables and flowersiii.iii. azoxystrobin (Quadris) -- used against leaf spots and blights, fruit rotsazoxystrobin (Quadris) -- used against leaf spots and blights, fruit rots

ChemicalChemicalTypes of fungicides and selected examples: Types of fungicides and selected examples: 2.2. OrganicOrganic

b.b. Systemic fungicidesSystemic fungicides — are absorbed through — are absorbed through foliage or roots and are translocated upward foliage or roots and are translocated upward through the xylem; control already established through the xylem; control already established pathogens and protect against new infectionspathogens and protect against new infections

i.i. metalaxyl (Ridomil, Apron) -- controls oomycetesmetalaxyl (Ridomil, Apron) -- controls oomycetesii.ii. benomyl (Benlate) — broad-spectrum fungicidebenomyl (Benlate) — broad-spectrum fungicideiii.iii. propiconazole (Tilt) — broad-spectrum fungicidepropiconazole (Tilt) — broad-spectrum fungicideiv.iv. aldicarb (Temik) – broad spectrum – bacteria, aldicarb (Temik) – broad spectrum – bacteria,

nematodes, etc.nematodes, etc.

ChemicalChemicalTypes of fungicides and selected examples: Types of fungicides and selected examples: 1.1. InorganicInorganic2.2. OrganicOrganic

3.3. Fumigant — highly volatile, small molecular weight Fumigant — highly volatile, small molecular weight compounds with activity against a wide variety of compounds with activity against a wide variety of pathogens (not limited to fungi); dangerous to pathogens (not limited to fungi); dangerous to humanshumans

example: example: methyl bromide; currently being pulled from methyl bromide; currently being pulled from market due to danger to nontarget organisms, market due to danger to nontarget organisms, including humansincluding humans