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Plant Pathology


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  MG Manual Reference
Ch. 4, pp. 15 - 17
[ 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 ]



Dark brown irregular spots with target rings and yellow haloes develop on leaves, stems, and fruit; spots on fruit are often at stem end and are sunken • Early blight (fungal disease) • Resistant varieties; use registered fungicide; remove old plant debris; rotate tomatoes with beans or sweet corn
Phoma rot (fungal disease) Phoma rot is not as common as early blight and you will be safe recommending early blight controls
Uniformly small (1/8") chocolate brown spots or dark spots with tan centers develop on leaves from bottom of plant to top; spots sometimes form on stems but never on fruits; leaves shrivel Septoria leaf spot (fungal disease) • Use registered fungicide; remove old plant debris
• Bacterial spot • Not as common as Septoria leaf spot; control for bacterial spot is to use chlorox treated seed and Kocide spray if available
Flowers do not set fruit and drop from plant • Night temperatures are below 55 degrees • Wait until temperatures become more favorable; it may help to shake the flowers by gently tapping the stems to effect self-pollination
• or day temperatures exceed 90 degrees F
Light tan spots on upper leaf surfaces; dense olive green moldy growth on undersurface of spots • Gray leaf mold (fungal disease) • Mainly a greenhouse problem: provide adquate ventilation to avoid high humidity; fungicides used to control other diseases will control this disease in the garden
Small (1/8") chocolate brown spots on leaves and fruit; spots on fruit are raised • Bacterial spot • Use chlorox treated seed; avoid overhead watering; use fixed copper bactericide (e.g. Kocide) if available locally; remove old plant debris; rotate
Large, sunken, tan watersoaked spot develops on blossom end of fruit; spot turns black and mold may grow • Blossom end rot, caused by calcium deficiency to developing fruits • Calcium deficiency is a problem when developing fruits receive uneven moisture: supply water during dry periods; mulch
General browning of tomato skin; brown speckling of walls between seed cavities apparent when fruit is cut open • Internal browning (virus disease) • Resistant varieties (to tobacco mosaic virus); weed control; do not handle healthy plants after diseased ones; remove affected plants
Extreme malformation and scarring of fruit • Catfacing, caused by cool weather or herbicide injury during fruit formation  
Tiny white winged insects on undersides of leaves • Whiteflies • Yellow sticky boards (smeared with grease) will attract and trap adults or use registered insecticide
Yellow-orange blotches that do not ripen at stem end of fruit or white, papery spot on side of fruit facing sun • Sunscald • Prevent foliar diseases that cause leaf drop and expose fruits to sun
Leaves distorted with "strapped" or feathery look (leaves narrower than normal, tips stretched out into thin projection, veins very close together) • Herbicide injury • Do not spray lawn herbicides during hot weather; spray after wind has died down in late afternoon
• Cucumber mosaic (virus disease) • It is impossible to distinguish these two problems based on symptoms alone; however, if sample comes in during spring when lawn herbicides are being sprayed, strongly suspect herbicide injury; virus is controlled by removing affected plants,weed control and aphid control
Leaves roll upward, feel leathery, but remain green; plants are not stunted • Excess water • Common physiological disorder after wet periods; varieties Big Boy, Floramerica, and Beefsteak are especially susceptible
Plants wilted; bottom leaves may turn yellow; brown discoloration inside stem Verticillium wilt • Rotate tomatoes with cereals, grasses, or legumes; avoid rotation with potatoes, peppers, or eggplant; resistant varieties
• Walnut wilt, caused by toxin from walnut or butternut trees • Do not plant tomatoes near trees; sever roots bordering garden and place barrier between tree and garden
• Waterlogged soil • Improve drainage
Plants stunted, wilted, and yellow; nodules on roots • Root knot nematode • Rotate; remove old plant debris; soil pasteurization if necessary
Young plants cut off at ground level • Cutworms • Use cutworm collars or registered insecticide
Young plants with many tiny holes in leaves • Flea beetles • Tomatoes will tolerate a lot of flea beetle damage if they are healthy; when necessary, use registered insecticide
Large holes in leaves; caterpillars present • Tomato hornworm • Handpick, spray with registered insecticide or B. thuringiensis spores

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