control of plant diseases as plant pathologists, we don't study morphology, life cycles, and...
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CONTROL OF PLANT DISEASESCONTROL OF PLANT DISEASES
As 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)
** Images and lecture material were not entirely created by J. Bond. ** Images and lecture material were not entirely created by J. Bond. Some of this material was created by others.** Some of this material was created by others.**
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 effect on the development of resistance to the control measure by the pathogen:measure 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 control
There are four basic types of control There are four basic types of control measures:measures:
a. Biological controla. Biological control
b. Cultural controlb. Cultural 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 control
2. 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 — inhibition of pathogen through antibiotics produced by the antagonist - produced by the antagonist - examples: examples: streptomycin streptomycin (antibacterial, from actinomycete), penicillin (antibacterial, (antibacterial, from actinomycete), penicillin (antibacterial, from fungus)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 — antagonist makes the environment unsuitable for the pathogen (modifies pH, temperature, moisture)for 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 control
Antagonism 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
Note: Slide is the property of B.M. Pryor, U. Arizona
Cultural controlCultural controlCultural (physical) control — manipulation of the environmentCultural (physical) control — manipulation of the environment
There 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 (time) plant cotton late to control damping-off caused by late 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 between-row spacing to open the canopy and reduce diseases that spread in the humid, protected canopy diseases that spread in the humid, protected canopy environmentenvironment
Cultural controlCultural controlCultural (physical) control — manipulation of the environmentCultural (physical) control — manipulation of the environment
4.4. 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 (1912), which resulted from epidemics of chestnut blight and Dutch elm from epidemics of chestnut blight and Dutch elm diseasedisease
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)
Note: Slide is the property of B.M. Pryor, U. Arizona
ChemicalChemicalApplication of pesticidesApplication of pesticides
PesticidePesticide — chemical that kills a pest (fungicide, bactericide, — chemical that kills a pest (fungicide, bactericide, nematicide, etc.); fungicides as examples, since fungi are nematicide, etc.); fungicides as examples, since fungi are the largest group of plant pathogensthe largest 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 fungicide
b.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. OrganicOrganic
a.a. Protective fungicidesProtective fungicides (preventative, contact)-- protect infection (preventative, contact)-- protect infection courtcourt
1.1. thiram (Thiram, Tersan) — seed and bulb treatment of vegetablesthiram (Thiram, Tersan) — seed and bulb treatment of vegetables
2.2. dichloran (Botran) ~ used against dichloran (Botran) ~ used against Botrytis Botrytis on vegetables and flowerson vegetables and flowers
3.3. Azoxystrobin (Quadris), Azoxystrobin (Quadris), Pyraclostrobin (Headline/Cabrio/Insignia) -- -- used against leaf spots and blights, fruit rotsused against leaf spots and blights, fruit rots
Protective/Contact FungicidesProtective/Contact Fungicides
Are sprayed onto foliage to form a Are sprayed onto foliage to form a protective barrier protective barrier
Do not protect emerging plant shootsDo not protect emerging plant shoots
Must be applied frequentlyMust be applied frequently• Usually on a schedule, also based upon Usually on a schedule, also based upon
forecasting systems forecasting systems
Note: Slide is the property of B.M. Pryor, U. Arizona
ChemicalChemicalTypes of fungicides and selected examples: Types of fungicides and selected examples:
2.2. OrganicOrganicb.b. Systemic fungicidesSystemic fungicides (curative) — are (curative) — are
absorbed through foliage or roots and are absorbed through foliage or roots and are translocated upward through the xylem; translocated upward through the xylem; control already established pathogens and control already established pathogens and protect against new infectionsprotect against new infections
i.i. metalaxyl (Ridomil, Apron) -- controls oomycetesmetalaxyl (Ridomil, Apron) -- controls oomycetes
ii.ii. benomyl (Benlate) — broad-spectrum fungicidebenomyl (Benlate) — broad-spectrum fungicide
iii.iii. propiconazole (Tilt)/ tebuconazole (Folicur) — propiconazole (Tilt)/ tebuconazole (Folicur) — broad-spectrum fungicidebroad-spectrum fungicide
iv.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. InorganicInorganic
2.2. OrganicOrganic
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
Fungicide ResistanceFungicide Resistance
Fungicide Resistance – the Fungicide Resistance – the inheritable modification of a inheritable modification of a fungus to a fungicide. Fungi fungus to a fungicide. Fungi become less sensitive to a become less sensitive to a fungicide due to a mutation.fungicide due to a mutation.
Note: Slide is the property of B.M. Pryor, U. Arizona
Actions That Lead To Actions That Lead To ResistanceResistance
Continuous use of a single fungicide or Continuous use of a single fungicide or fungicides with the same mode of fungicides with the same mode of actionaction
Not using tank mixes of multiple Not using tank mixes of multiple fungicidesfungicides
Not alternating fungicides with different Not alternating fungicides with different modes of actionmodes of action
Note: Slide is the property of B.M. Pryor, U. Arizona
Examples of Fungi Examples of Fungi ResistanceResistance
FungicideFungicide PathogenPathogen DiseaseDisease
Tersan 1991Tersan 1991 SclerotiniaSclerotinia Dollar Dollar SpotSpot
SubdueSubdue PythiumPythium Pythium BlightPythium Blight
Chipco 26019 Chipco 26019 MicrodochiumMicrodochium Pink Snow Pink Snow MoldMold
BayletonBayleton SclerotiniaSclerotinia Dollar Dollar SpotSpot
Note: Slide is the property of B.M. Pryor, U. Arizona
Fungicide Resistance Fungicide Resistance Management StrategiesManagement Strategies
Alternate or tank mix fungicides with Alternate or tank mix fungicides with different modes of actiondifferent modes of action
Minimize applications of fungicides that Minimize applications of fungicides that are susceptible to resistance developmentare susceptible to resistance development
Use label ratesUse label rates Integrate other control methods with Integrate other control methods with
fungicide usagefungicide usage Use fungicides to prevent disease Use fungicides to prevent disease
development instead of as a curative development instead of as a curative actionaction
Note: Slide is the property of B.M. Pryor, U. Arizona