Worldwide distribution. On crops, ornamentals, weeds in many families: cotton, cucumber, onion, legume, potato, and more. Common and potentially serious fungal disease.
Lower leaves infected first with whitish powdery patches on undersides, yellow irregular spots above. Unlike other powdery mildew: little fungal growth over the leaf surface.
Spread by spores on the wind.
Cultural control: nursery: avoid overcrowding; weed; check plants before planting. Field: remove volunteers; do not overapply N; avoid moisture stress; choose areas with good air movement; resistant varieties.
Chemical control: commercial plantings: i) wettable sulphur, ii) chlorothalonil, or iii) triazoles. Organic regime: horticultural oil, potassium bicarbonate or wettable sulphur.(oils are eradicants). Apply wettable sulphur early morning or evening. Never within 2 weeks of the last application of an oil spray. READ LABEL. If few plants: use normal strength milk diluted 1 part in 10 parts of water.