AZ Master Gardener Manual: Key to Specific Vegetables (Corn and Cucurbits)

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Plant Pathology

PLANT PATHOLOGY: DIAGNOSTIC KEY [continued]

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Ch. 4, pp. 7 - 9

[ 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 ]

KEYS TO PROBLEMS ON SPECIFIC VEGETABLES

CORN

SYMPTOMS	CAUSES	CONTROLS
Ears not completely filled with kernels	• Poor pollination	• Plant in blocks of at least 3-4 short rows instead of 1 long row; hand pollinate
Ears not completely filled with kernels	• Birds	• Put paper bag over ear after pollination
White smooth or black powdery galls on leaves, stalk, ears, or tassels	• Smut (fungal disease)	• Cut off galls before they turn black; remove old plant debris; tolerant varieties
Plants stunted with yellow and green stripe or mosaic pattern; pale yellow	• Maize dwarf mosaic (virus disease)	• Weed control, especially Johnson grass; aphid control; destroy affected plants; do not handle healthy plants after affected ones
Leaves reddish on margins	• Phosphorus deficiency	• Amend soil as needed
Leaves reddish on margins	• Virus disease	• Weed control before corn emerges; aphid control; remove affected plants
Distorted leaf or stalk: leaves may fail to unfurl or stalk may be bent	• Herbicide injury
Caterpillar feeding on the tip of the ear	• Corn earworm	• Apply registered insecticide during silking to prevent infestation; inject 1/2 medicine dropper-ful of mineral oil into silk channel as silks start to dry
Young plants chewed off at ground level	• Cutworms	• Place cardboard collars around seedlings; use registered insecticide

CUCURBITS

SYMPTOMS	CAUSES	CONTROLS
No fruit produced	• Poor pollination	• Be patient, male and female flowers are not produced at the same time at first; bee activity may be low due to cool weather or use of insecticides; spray insecticides in late afternoon
Misshapen or bitter fruit produced	• Poor pollination	• See above
	• Dry soil	• Supply water
	• Poor soil fertility	• Amend soil as needed
Watersoaked, sunken, brown or usually black spot at blossom end of fruit only	•Calcium deficiency caused by uneven soil moisture and poor supply of calcium to fruit during early development	• Water during dry periods; calcium foliar spray
Watersoaked,sunken brown or black spots on fruit not restricted to blossom end; root rot may also be present	• Fungal fruit rot	• Submit sample for diagnosis
Wilted plants	• Dry soil	• Supply water
	• Bacterial wilt	• Control cucumber beetles
	• Root rot (fungal disease)	• Improve drainage; rotate; remove old plant debris
	• Fusarium wilt (fungal disease)	• Tolerant varieties if available; rotate
	• Root knot nematode	• Soil pasteurization; rotate
Circular or irregular brown spots on leaves and/or fruit	• Fungal or bacterial disease	• Submit sample for diagnosis
White, powdery growth on leaves; may be on both leaf surfaces	• Powdery mildew (fungal disease)	• Resistant varieties; use registered fungicide; remove old plant debris
Yellow spots on upper leaf surfaces; grayish fuzzy growth on underside of spots	• Downy mildew (fungal disease)	• Resistant varieties; use registered fungicide; remove old plant debris
Yellow and green mottled pattern on leaves; strapped appearance, i.e. abnormally narrow with leaf veins stretched out at leaf margins so leaves appear feathery	• Virus disease	• Weed control before plants emerge; aphid control; remove affected plants
	• Herbicide injury	• Do not spray lawn herbicides on hot days; spray after wind has died down
Holes chewed in leaves and stalks; yellow-green beetles with black stripes or spots	• Cucumber beetles	• Hand pick; heavy mulching; use registered insecticide
Squash and pumpkin leaves wilt, eventually become black and crisp; plant bugs present	• Squash and plant bugs	• Use registered insecticide; hand pick

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