Late Blight
Risk near you, next 48h
See whether your local forecast favours this disease right now.
When it strikes
About
Late blight (Phytophthora infestans) is the most destructive disease of tomato and potato, capable of wiping out a crop in days during cool, wet weather. It caused the Irish potato famine and remains a serious threat for home and commercial growers alike.
Symptoms
Dark, water-soaked greasy patches on leaves that rapidly turn brown; white fuzzy sporulation on leaf undersides in humid mornings; firm brown lesions on stems and fruit. Whole plants can collapse within days.
Organic Treatment
Remove and bag infected foliage immediately. Apply copper-based fungicide preventively before symptoms in wet spells. Improve airflow and avoid overhead watering. Severely infected plants should be destroyed, not composted.
Chemical Treatment
Protectant fungicides (chlorothalonil, mancozeb) on a 5–7 day schedule, or systemic products (cymoxanil, mandipropamid) when pressure is high. Rotate modes of action to avoid resistance.
Prevention
Plant resistant varieties, space widely for airflow, water at the base in the morning, stake plants off the ground, and never save tubers or fruit from infected plants.