Download Plant Pathogens and Biocontrol Agents

Plant Pathogens
and Biocontrol
Agents
Plant Pests
 Pathogens
 Predators
 Weeds
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
Pathogens : Viruses

Transmission of viruses
 Insect
vectors - especially aphids, whiteflies,
leafhoppers, mealybugs, ants
 Nematodes
 Seeds from infected parent plants

Airborne transmission
 Infected
 Aphids
 Pollen
plant parts
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
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
Plant Pests and
Their Control by
Fungi and Bacteria
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
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
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
Weeds
 “Unloved plant”
 Injurious to agricultural crops
 Loss is a direct result of competition for
light, water, nutrients
Control Measures
 Chemicals widely used to control plant
pests and diseases
 Dangers of pesticide use apparent
Integrated Pest Management (IPM)
 Multifaceted approach to disease control
 Sanitation
 Crop rotation
 Genetic resistance
 Biological Control
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
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

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

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

Bt Uses

Spray Applications
 Bt
toxins degrade within a few days
 Endospores can survive for several years
after spray applications

Genetic Engineering with Bt genes
 Transfer
into crop plants
 Transfer other bacteria
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