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 plant parts  Aphids  Pollen

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

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