Crop Pathogens and Disease (Chapter 10 p.310-321, 331-332)

1. Crop Pathogens and Disease (Chapter 10 p.310-321, 331-332) Diseased Plant: Physiology negatively altered. Pathogen: “Living” organism causing disease. Pathogenicity: Capacity of pathogen to cause disease. Host Range: Crop species pathogen can invade and grow on. Northern Corn Leaf Blight Exserohilum turcicum “Pull those weeds”

2. Identifying Crop Pathogens Disease: Caused by microorganisms or environmental conditions. How do we separate the two? Observe Diseased Plant: Cells malfunction Plant physiology altered Symptoms Appear: physiological effects external changes Infectious Disease: Caused by living organism. Noninfectious Disease: Result from conditions weather, soil, chemical injury Phomopsis Helianthi of Sunflower

3. Crop Disease Proof Damping off in Soybeans Koch’s Postulates (1876): Identifies causal agent (pathogen). Four criteria: • Organism observed with disease. • Organism isolated from infected plant. • Organism produces same disease when inoculated on plant. • Organism re-isolated from diseased plant. Wheat Crown And Root Rot

4. Crop Disease and Control Bipolaris maydis 1971 Southern Corn Leaf Blight Race T Annual disease loss of 10-20%. Corn = 11% Wheat and Soybeans = 14% Potato = 20-25% Many possible causes of crop disease. Living: Fungi Bacteria Nematodes Non-Living: Viruses Most plants are immune to most pathogens. 1845 Late Blight Of Potato

5. Types of Crop Losses Wheat Streak Mosaic Virus Direct Yield Loss: Compete for water and nutrients. Block vascular system. Increases respiration. Harvest loss: Lodging Ear dropping or shattering Hazards: Toxic Aflotoxin in corn and peanuts Allergic Asian Soybean Rust

6. Fungal Diseases Northern Corn Leaf Blight Greatest number and diversity of diseases. Greatest economic loss. Attack all plant parts. Parasitic – attack live tissue Saprophytic – attack dead tissue Adaptable: Survive in diverse environments Overcome genetic resistance Infect by Conidia (spores). Conidia  germinate  epidermis Rusts, smuts, stalk rot, downy mildews, leaf blights Systemic chemical control effective. Conidia Mycelium

7. Bacterial Diseases Flea Beetle Stewart’s Bacterial Wilt of Corn Parasitic or saprophytic. Soft rot, leaf spot, galls, blights, vascular disease. Prefer warm moist areas Enter through stomata or wounds (hail). Spread by wind, irrigation, seed, vegetative propagation Genetic resistance best. Examples: Goss’ bacterial wilt of corn Stewart’s bacterial wilt of corn Common scab of potato Bacterial blight of alfalfa Bacterial soft rots Potato Scab

8. Viral Diseases Barley Yellow Dwarf Genetic resistance best. Non-Living: Protein coat + RNA/DNA inside Enter through wounds Pollen grain, insects, fungi, seed, nematodes Systemic: Occur in phloem fluids Alter leaf chlorophyll Mosaic, mottled, striped Alter growth Curling, edge crinkling, excessive branching, stunting Examples: Beet curly top Potato witches broom and yellow dwarf Maize dwarf mosaic, Barley yellow dwarf. Maize Dwarf Mosaic

9. Nematode Diseases Roundworms: in soil and water. Microscopic eel-like worms in soil & water. Root knot and cyst nematodes: Most crops susceptible to root knot Cyst common on potato, tomato, egg plant, soybean, cereals, clover Attach to roots, stems, leaves, flowers Cause root galls, root lesions, stunted roots, injured root tips. Light colored foliage, stunted growth, wilting Examples: Soybean cyst nematode Sugar beet cyst nematode Corn cyst nematode.

10. Requirements for Disease Disease Triangle Epidemic Disease: Severe over wide area Only occasionally Catastrophic damage Example: 1971 Southern Corn Leaf Blight Race T (14%) Endemic Disease: Occurs year after year Moderate to severe damage Examples: Corn stalk rot Corn root rot Susceptible Crop Favorable Environment Pathogen Disease

11. Disease Development Stages: • Inoculation: Host pathogen contact • Penetration: Pathogen enters host • Infection: Pathogen invades cells for nutrients. Hypersensitive Reaction:self-induced tissue death around invasion site. • Dissemination: Pathogen spreads • Overwinter / Oversummer: Pathogen dormant or alternate on host

12. Stages of Disease Development Overwintering Stage Pathogen survives on alternate host, residue, soil or seed Dissemination Secondary Inoculum Inoculation of New Plants Primary Inoculum Infection Incubation Infection Symptoms appear: pathogen multiplies rapidly in host plant. Inoculation/ Penetration Pathogen enters the host plant stomata or wounds Incubation Pathogen becomes established in host plant

13. Genetic Disease Control Resistance: Plant gene resists infection. Best for bacteria and viruses. Tolerance: Plant withstands inoculation. Mechanical or chemical exclusion of pathogen. Avoidance: Morphological structures (stomata, wax, thick cuticle, pubescence) discourages inoculation. Biochemical: plant toxins Reach maturity before disease severe. Single gene resistance to fungus disease in tomato

14. Genetic Disease Control Corn Ear Rot New Pathogen Races: • sexual reproduction, mutation, introduction • development of resistant varieties a continual process • race against time. Types of Resistance: Vertical (single gene): One or a few nuclear genes Not durable  wheat rust Horizontal (polygenic): Numerous nuclear genes Durable Cytoplasmic: Genes in cytoplasm Southern corn leaf blight Yellow leaf blight Wheat Rust

15. Cultural Disease Control Field operations that alter host/pathogen life cycle to avoid inoculation. Proper Crop Management. Tillage and cultivation  reduce inoculum Weed control  reduces alternate host Drainage  reduces soil pathogens Disease free seed  off to good start Crop Rotation  reduces inoculum Examples: Late planted winter wheat  reduces fall growth and crown or root rot. Early planting spring oats  avoids high rust levels. Tradeoff: $$$$$$$$$$$$$$$

16. Chemical Disease Control: (preventative) Peanuts Not common (11% of pesticides) Most effective for fungal diseases when systemic. Expensive Limitation for hay and grazing Apply before inoculation and re-apply. Once inoculation chemical control impossible. Costs: Not economical against rust on grain crops Economical on high value crops. Chemical seed treatment routine. Controls damping-off. No control for seed transmitted diseases No control for viral or bacterial diseases. Poor Bermudagrass

17. How Pathogens Affect Plants Photosynthesis: Reduce photosynthetic area Degrade chlorophyll Respiration: Increased respiration and metabolism Translocation: Vascular system blocked Root uptake reduced, tissue killed. Growth: Reduced biomass Plant Death: Entire plant or plant parts die Economic Product: Reduce crop yield. Reduced marketable plant part Reduced quality Sudden Death in Soybeans Alfalfa Root Rot

18. Non-Infectious Diseases Salt Damage Result of weather, soils and chemicals. Temperature Moisture Wind, hail, lightning Soil pH Nutrients Pesticide application rate Pesticide carry-over Pollutants Jet fuel Degree of Air Filtration Good Poor Hail Air Pollution Damage