Late blight
A fast-spreading disease of tomatoes and potatoes that causes dark, greasy patches on leaves and fruit and can kill plants within days.
Also known as: Phytophthora blight, Potato blight
Symptoms and identification
Large, irregular, greyish-green to dark brown water-soaked patches appear on leaves, often with a white fuzzy growth underneath in damp weather. Stems develop dark lesions, and fruit gets firm, greasy brown patches.
Causes
The water mould Phytophthora infestans spreads rapidly through cool, wet and humid weather, carried by wind and rain from nearby infected plants. It survives in infected potato tubers and plant debris.
Organic treatment
Remove and bin affected plants or parts immediately; do not compost them. Copper-based sprays applied before infection can protect healthy plants during risky weather.
Prevention
Grow resistant varieties, space plants well, water at the base and avoid planting near potatoes. Watch closely during cool, cloudy, wet spells.
Chemical treatment (use with care)
Use a fungicide labelled for late blight as a preventive spray during cool, wet weather, following the label and harvest interval.