plant pathogens and biocontrol agents. plant pests pathogens predators weeds
TRANSCRIPT
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Plant Pathogens and Biocontrol Agents
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Plant Pests
Pathogens Predators Weeds
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Symptoms of Microbial Diseases in Plants
Necrosis - death of plant cells; may appear as spots in localized areas
Canker - localized necrosis resulting in lesion, usually on stem Wilt - droopiness due to loss of turgor Blight - Loss of foliage Chlorosis - loss of photosynthetic capability due to bleaching
of chlorophyll Hypoplasia - stunted growth Hyperplasia - excessive growth Gall - tumor
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Pathogens : Viruses
Transmission of viruses Insect vectors - especially aphids, whiteflies,
leafhoppers, mealybugs, ants Nematodes Seeds from infected parent plants
Airborne transmission Infected plant parts Aphids Pollen
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Pathogens: Bacteria Generally Gram-negative bacilli: species of
Erwinia, Pseudomonas, Xanthomonas, Agrobacterium, and Corynebacterium
Dispersal from plant generally passive by water, wind-blown water, animals, agricultural workers
In warm, humid climates, where dew and rain are common, dispersal of bacteria by rain-splash is the major means of disease spread
Airborne spread on rafts of plant material
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Pathogens: Fungi Over 70% of all major crop diseases are
caused by fungi Thousands of fungal species recognized as
plant pathogens Fungal diseases cost more than $3.5 billion to
US farmers alone In general spores of most fungal pathogens
are adapted for airborne transport
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Plant Pests and Their Control by Fungi and Bacteria
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Plant Pathogenic Nematodes
Obligate parasites Feed on roots of plants Some never live in soil, they survive in host
and are spread by insect vectors Reduces crop yield and increases risk of
infection through wounds
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Insects
Damage or destroy crops before and after harvest
Larval stage often most destructive Injury plants directly by using plant for food
or shelter and indirectly by spreading pathogens
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Basic feeding patterns
Chewing Insects Either larvae or adults Tear or bite portions of the plants May eat their way through the plant causing holes and
tunnels Others eat whole plant
Sucking Insects Pierce the plant and sucks up the sap Results in curling, stunting, deformed parts
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Weeds
“Unloved plant” Injurious to agricultural crops Loss is a direct result of competition for
light, water, nutrients
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Control Measures
Chemicals widely used to control plant pests and diseases
Dangers of pesticide use apparent
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Integrated Pest Management (IPM)
Multifaceted approach to disease control Sanitation Crop rotation Genetic resistance Biological Control
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Biological Control Use of living organisms to reduce disease due to
competition or antagonism i.e.. ladybugs to control aphids
The aim is to reduce dependence on chemicals Today emphasis on microorganisms
Bacillus thuringiensis for insect control Several Pseudomonas species for control of bacterial
and fungal pathogens Numerous fungi for insects, nematodes, fungal
pathogens
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Bacillus thuringiensis
Common soil bacterium well known for its ability to produce crystalline proteins with insecticidal properties
Since 1960s Bt available as a safe naturally occurring biopesticide Use as a dried inoculum containing endospores and
crystals of insecticidal proteins used as sprays or dusts for a wide variety of insects -
especially Lepidopteran
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Bt Toxins
Toxins activated by enzymes in insect gut Kill insects by binding to membranes in
digestive system and creating pores in membrane~contents leak into body cavity
Harmless to humans, natural enemies of arthropods, and non-target organisms
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Bacillus thuringiensis
B.t. subspecies kurstaki is widely used in caterpillar control in agriculture and forestry
B.t. subspecies israelensis is active against mosquitoes and black flies
B.t. subspecies tenebrionis is active again beetle larvae
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Bt Uses
Spray ApplicationsBt toxins degrade within a few daysEndospores can survive for several years
after spray applications Genetic Engineering with Bt genes
Transfer into crop plantsTransfer other bacteria
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Pseudomonas species
Pseudomonas fluorescens for control of fire blight (also may control apple blue mold)
Fire blight – bacterial disease of apples and pears caused by Erwinia amylovora
Pseudomonas out competes Erwinia Reduces use of streptomycin which has been
helpful since many Erwinia strains resistant