Downy mildew
A water-mould disease that causes yellow patches on top of leaves and grey-purple fuzz underneath, spreading fast in cool, wet weather.
Also known as: Peronospora, Basil downy mildew
Symptoms and identification
Angular yellow patches on the upper side of leaves, limited by the veins. Underneath, a grey, purple or brown fuzzy growth appears, especially after humid nights. Leaves later brown and die. Common on basil, cucumber, gourds and lettuce.
Causes
Caused by water moulds (oomycetes) that need leaf wetness to infect. Cool nights, heavy dew, the monsoon and crowded plants favour it.
Organic treatment
Remove infected leaves at once. Spray a copper-based fungicide approved for organic use every 7–10 days in wet weather, covering leaf undersides.
Prevention
Space plants for airflow, water at the base in the morning, and avoid wetting leaves. Grow resistant varieties and remove infected plants early.
Chemical treatment (use with care)
Use a fungicide labelled for downy mildew on that crop (for example mancozeb or metalaxyl-based products), following the label and rotating products.