CONTROL OF PLANT DISEASES

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CONTROL OF PLANT DISEASES

• As plant pathologists, we don't study morphology,
  life cycles, and spread of pathogens because it's
  so interesting instead, the main purpose behind
  understanding pathogens and the diseases they
  cause is so diseases can be controlled.
• For most crops, the goal is to save most of the
  plant population, not selected individuals.
  exception tree crops (citrus, pecan, timber)

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Basic principles of disease control

• Control strategies can be divided into two groups
  based on their effect on the development of
  resistance to the control measure by the
  pathogen
• Eradicative control measures designed to
  eliminate the entire pathogen population -
  examples pesticides, vertical or complete
  resistance - These tend to select for resistant
  variants of the pathogen. Why? All individuals
  are affected, so the pathogen must adapt or die.
• Management control measures designed to reduce
  the pathogen population by destroying a portion
  of the population - examples horizontal or
  partial resistance, antagonism, cultural
  practices, quarantine - These do not apply heavy
  selection pressure to the pathogen. Why? Portions
  of the pathogen population remain unaffected, no
  pressure to adapt.
• Of the two, we prefer to use management
  strategies.

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Disease control

• There are four basic types of control measures
• a. Biological control
• b. Cultural control (includes physical control)
  
• c. Legislative and regulatory control
• d. Chemical control

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Biological control

• Manipulation of biotic entities host and
  antagonistic microorganisms
• 1. Host resistance - control based on the genes
  and the resistance mechanisms they control
• Van der Plank described two types of resistance
  (1960s these are the "classics)
• Vertical resistance resistance that is
  effective against some, but not all, races of a
  pathogen decreases the effective amount of
  incoming inoculum (avirulent races can't infect),
  but does not reduce the rate of disease
  development (virulent races are not affected)
• Horizontal resistance resistance that is
  effective against all races of the pathogen
  decreases the rate of disease development for all
  races

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Biological control

• b. Resistance has been defined in many other ways
  since Van der Plank, including systems based on
  epidemiologic effects, number of genes involved,
  how long the resistance lasts under field
  conditions additional terms you should be
  familiar with are
• tolerance plants are diseased, but they do not
  yield less than healthy plants
• induced resistance a normally susceptible plant
  treated with an avirulent strain of a pathogen
  gives a resistant reaction when challenged later
  with a strain that is virulent

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Biological control

• 2. Antagonists control using microorganisms
  that inhibit the growth, development, or
  reproduction of pathogens
• Four types of activity
• Antibiosis inhibition of pathogen through
  antibiotics produced by the antagonist -
  examples streptomycin (antibacterial, from
  actinomycete), penicillin (antibacterial, from
  fungus)
• Competition two organisms attempt to utilize
  the same limiting factors (nutrients, oxygen)
  supply not large enough to support both
  antagonist and pathogen
• Amensalism antagonist makes the environment
  unsuitable for the pathogen (modifies pH,
  temperature, moisture)
• Parasitism predation antagonist directly
  attacks the pathogen example nematode-trapping
  fungi

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Biological control

• Antagonism frequently operates under natural
  conditions difficult to manipulate due to the
  modifying effects of the environment may be
  important in suppressive soils soils in which
  the pathogen cannot establish, develop, or
  survive
• example Queensland avocado grove has been
  productive for 34 years even though researchers
  routinely collect a virulent isolate of
  Phytophthora from the soil root rot is common in
  nearby groves, but very rare in the grove with
  suppressive soil

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Cultural control

• Cultural (physical) control manipulation of the
  environment
• There are many types of cultural control. Here
  are few selected examples
• Crop rotation rotate crops and varieties over
  seasons to reduce pathogen inoculum levels This
  is probably the most widely employed control
  measure in agriculture! example rotate soybean
  with corn to control soybean cyst nematode
• Selection of planting date or planting location
  choose a time/place favorable for the host,
  rather than the pathogen avoid pathogen or its
  vector example (time) plant cotton late to
  control damping-off caused by Pythium (warm soil)
• Seeding rate and canopy density adjust
  within-row and between-row spacing to open the
  canopy and reduce diseases that spread in the
  humid, protected canopy environment

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Cultural control

• Cultural (physical) control manipulation of the
  environment
• Irrigation
• Pathogens can be spread in irrigation water or
  favored by wet soils-example late blight
  (Phytophthora)
• Pathogens can be controlled by flooding -
  example Fusarium wilt on banana
• Control insects and weeds insects vector
  viruses and other pathogens weeds serve as
  alternate hosts for pathogens or vectors and
  increase canopy density
• Sanitation keep area free of diseased plant
  material by pruning diseased branches
  (fireblight), plowing under or burning debris,
  washing and sterilizing harvesting and processing
  equipment (Rhizopus soft rot) poor sanitation
  contributed to the late blight outbreak that
  caused the Irish famine
• Heat or refrigeration -- hot air, hot water, or
  steam treatments are used to kill pathogens in
  seed or propagation materials harvested fruits
  and vegetables are kept refrigerated

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Legislative and regulatory control

• Quarantine detention and associated practices
  for preventing the entry of diseased materials or
  pathogens into an area relatively inexpensive
  can be at federal or state level (CA citrus)
• APHIS (Animal and Plant Health Inspection
  Service) agency within USDA that runs
• PPQ (Plant Protection and Quarantine) agency
  responsible for federal quarantines -established
  by the Plant Quarantine Act (1912), which
  resulted from epidemics of chestnut blight and
  Dutch elm disease
• Pest and Disease Survey national database all
  pests on major crops in each state
• Action programs -- eradicate or contain pests
  that get past quarantine worked for citrus
  canker (FL) didn't work for potato golden
  nematode on Long Island, NY (birds) sugarcane
  smut, FL (hurricanes)

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Legislative and regulatory control

• Quarantine
• Inspection and certification programs state
  level plants/seeds grown under conditions
  unfavorable for pathogens and are inspected to be
  sure that pests are not transported along with
  packing material
• Pesticide labeling and applicator certification
  these activities are under the control of the EPA
  (Environmental Protection Agency)

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Chemical

• Application of pesticides
• Pesticide chemical that kills a pest
  (fungicide, bactericide, nematicide, etc.)
  fungicides as examples, since fungi are the
  largest group of plant pathogens
• Types of fungicides and selected examples
• Inorganic
• Sulfur -- oldest known fungicide
• Copper oldest formulated fungicide is the
  Bordeaux mixture (downy mildew of grape) still
  the most widely used copper fungicide in the
  world
• Organic
• Protective fungicides -- protect infection court
• thiram (Thiram, Tersan) seed and bulb treatment
  of vegetables
• dichloran (Botran) used against Botrytis on
  vegetables and flowers
• azoxystrobin (Quadris) -- used against leaf spots
  and blights, fruit rots

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Chemical

• Types of fungicides and selected examples
• Organic
• Systemic fungicides are absorbed through
  foliage or roots and are translocated upward
  through the xylem control already established
  pathogens and protect against new infections
• metalaxyl (Ridomil, Apron) -- controls oomycetes
• benomyl (Benlate) broad-spectrum fungicide
• propiconazole (Tilt) broad-spectrum fungicide
• aldicarb (Temik) broad spectrum bacteria,
  nematodes, etc.

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Chemical

• Types of fungicides and selected examples
• Inorganic
• Organic
• Fumigant highly volatile, small molecular
  weight compounds with activity against a wide
  variety of pathogens (not limited to fungi)
  dangerous to humans
• example methyl bromide currently being pulled
  from market due to danger to nontarget organisms,
  including humans

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