Symptoms
Yellow or angular (vein-bounded) spots on the upper leaf surface, matched underneath by a grey-violet or brown coating of spores. It differs from powdery mildew, whose white coating sits on the upper surface. It develops in humidity and cool nights and is dangerous to cucumbers, onions and grapevines.
Treatment
Remove infected leaves, improve air movement and reduce humidity, and if needed apply fungicides (different from those used against powdery mildew). Acting quickly is important.
Prevention
Airy planting spacing, watering at the base in the morning, avoiding wetting the leaves in the evening, resistant varieties, and crop rotation.
Sources
- Atlas of vegetable diseases
