Cooperative Extension MG Manual Home Plant Pathology

PLANT PATHOLOGY: DIAGNOSTIC KEY   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 ] KEY PROBLEMS ON VEGETABLES SYMPTOMS POSSIBLE CAUSES CONTROL & COMMENTS 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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