Plant Pathogens and Biocontrol Agents Plant Pests n

Plant Pathogens and Biocontrol Agents

Plant Pests n Pathogens n Predators n Weeds

Symptoms of Microbial Diseases in Plants n Necrosis - death of plant cells; may appear as spots in n n n 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 n Transmission of viruses ¨ Insect vectors - especially aphids, whiteflies, leafhoppers, mealybugs, ants ¨ Nematodes ¨ Seeds from infected parent plants n Airborne transmission ¨ Infected ¨ Aphids ¨ Pollen plant parts

Pathogens: Bacteria n Generally Gram-negative bacilli: species of Erwinia, Pseudomonas, Xanthomonas, Agrobacterium, and Corynebacterium n Dispersal from plant generally passive by water, wind-blown water, animals, agricultural workers n In warm, humid climates, where dew and rain are common, dispersal of bacteria by rain-splash is the major means of disease spread n Airborne spread on rafts of plant material

Pathogens: Fungi n Over 70% of all major crop diseases are caused by fungi n Thousands of fungal species recognized as plant pathogens n Fungal diseases cost more than $3. 5 billion to US farmers alone n In general spores of most fungal pathogens are adapted for airborne transport

Plant Pests and Their Control by Fungi and Bacteria

Plant Pathogenic Nematodes n Obligate parasites n Feed on roots of plants n Some never live in soil, they survive in host and are spread by insect vectors n Reduces crop yield and increases risk of infection through wounds

Insects n Damage or destroy crops before and after harvest n Larval stage often most destructive n Injury plants directly by using plant for food or shelter and indirectly by spreading pathogens

Basic feeding patterns n 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 n Sucking Insects ¨ Pierce the plant and sucks up the sap ¨ Results in curling, stunting, deformed parts

Weeds n “Unloved plant” n Injurious to agricultural crops n Loss is a direct result of competition for light, water, nutrients

Control Measures n Chemicals widely used to control plant pests and diseases n Dangers of pesticide use apparent

Integrated Pest Management (IPM) n Multifaceted approach to disease control n Sanitation n Crop rotation n Genetic resistance n Biological Control

Biological Control n Use of living organisms to reduce disease due to competition or antagonism ¨ i. e. . ladybugs to control aphids n The aim is to reduce dependence on chemicals n 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 n n Common soil bacterium well known for its ability to produce crystalline proteins with insecticidal properties Since 1960 s 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 activated by enzymes in insect gut n Kill insects by binding to membranes in digestive system and creating pores in membrane~contents leak into body cavity n Harmless to humans, natural enemies of arthropods, and non-target organisms n

Bacillus thuringiensis B. t. subspecies kurstaki is widely used in caterpillar control in agriculture and forestry n B. t. subspecies israelensis is active against mosquitoes and black flies n B. t. subspecies tenebrionis is active again beetle larvae n

Bt Uses n Spray Applications ¨ Bt toxins degrade within a few days ¨ Endospores can survive for several years after spray applications n Genetic Engineering with Bt genes ¨ Transfer into crop plants ¨ Transfer other bacteria

Pseudomonas species n n 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