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For most crops, the goal is to save most of the plant population, not ... kills a pest (fungicide, bactericide, nematicide, etc.); fungicides as examples, ... – PowerPoint PPT presentation

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  • 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)

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
  • 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

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

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

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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

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
  • 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

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
  • 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
  • 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

Cultural control
  • Cultural (physical) control manipulation of the
  • Irrigation
  • Pathogens can be spread in irrigation water or
    favored by wet soils-example late blight
  • 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

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)

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)

  • 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
  • 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

  • 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.

  • 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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