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Plant Pathology

  MG Manual Reference
Ch. 4, pp. 1 - 3

[ Diagnostic Key: vegetables | specific vegetables; asparagus, bean, beet, carrot, cole crops, corn, cucurbits, eggplant, lettuce, onion, pea, pepper, potato, tomato | tree fruits| specific fruits; apple, stone, citrus | ornamentals | specific ornamentals; rose family, rose, palm, pine ]


Poor fruit yield; fruit may be small and have poor taste • Uneven moisture • Supply water during dry periods
• Poor soil fertility • Amend soil as needed
Plants grow slowly; leaves light green • Insufficient light • Thin plants; do not plant in shade
• Cool weather  
• Poor soil • Amend soil as needed
• Improper pH • Amend soil as needed
• Excess water • Do not overwater; improve drainage
Seedlings don't emerge • Dry soil • Supply water
• Seeds washed away • Do not overwater; treat seed with fungicide
• Damping-off (fungal problem)  
• Incorrect planting depth  
• Slow germination due to weather  
• Root maggots • Use registered soil insecticide
Wilted seedlings; seedlings fall over • Dry soil • Supply water
• Damping-off (fungal disease) • Do not overwater; treat seed with registered fungicide
• Root maggots • Use registered soil insecticide
Chewed seedlings • Rodents, rabbits, or birds • Place fence around garden
• Slugs • Use slug bait (beer or commercial slug bait)
• Various insects • Use registered insecticide
Wilted plants; bottom leaves may turn yellow • Dry soil • Water
• Root rot (fungal disease) • Do not overwater; remove old plant debris; rotate
• Vascular wilt (fungal disease) • Resistant varieties; rotate
• Root knot nematode • Rotate; soil pasteurization
• Walnut wilt (mainly affecting tomato) • Do not plant near walnut or butternut trees; sever tree roots bordering garden and place barrier between tree and garden
• Waterlogged soil • Improve drainage
General leaf yellowing; no wilting • Nutrient or mineral deficiency • Amend soil as needed
• Insufficient light • Thin plants
Leaves stippled with tiny white spots • Spider mites • Treat with registered miticide; insecticidal soap spray
• Air pollution (ozone)  
Leaf margins turn brown and shrivel • Dry soil • Supply water
• Fertilizer burn • Soil test for soluble salts level; do overapply fertilizer; flush soil with water
• Potassium deficiency • Amend soil as needed
• Cold injury  
Discrete brown spots on leaves; some spots may coalesce • Fungal or bacterial leaf spot disease • See controls under specific disease; submit sample for diagnosis if necessary
• Chemical injury • Do not apply chemicals that are not registered for use on the plant; apply chemicals at registered rates; some chemical injury occurs from drift
White powdery growth on upper leaf surfaces • Powdery mildew (fungal disease) • Use registered fungicide
Leaves shredded or stripped from plant • Hail damage  
• Rodents • Place fence around garden
• Slugs • Use slug bait (either beer or commercial bait)
Leaves with yellow and green mosaic or mottle pattern; leaves may be puckered and plants stunted • Virus disease • Resistant varieties if available; weed control; remove affected plants; remove old plant debris
Leaves curled, puckered, or distorted • Herbicide injury (common on tomato, cucumber) • If lawn herbicides are used, apply after wind has died down and do not apply in heat of day
• Virus disease • Same controls as above
• Aphids • Treat undersides of leaves with registered insecticide and/or hard stream of soapy water

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