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.

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CONTROL OF PLANT DISEASES

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Control of plant diseases

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)


Basic principles of disease control

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.


Disease control

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

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


Biological control1

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 control2

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


Biological control3

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


Cultural control

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


Cultural control1

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


Legislative and regulatory control

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 control1

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)


Chemical

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


Chemical1

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.


Chemical2

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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