Late Blight
Phytophthora infestans
Overview
A devastating oomycete disease of tomatoes and potatoes that can destroy entire crops within days under favorable conditions. The same pathogen caused the Irish Potato Famine of the 1840s.
Symptoms
Large, irregular, water-soaked, grayish-green lesions on leaves that quickly turn brown and papery. White fuzzy sporulation on leaf undersides in humid conditions. Stems develop dark brown lesions. Tubers show firm, brown, granular rot.
Environmental Conditions
Cool
Prevention
Plant resistant varieties. Use certified disease-free seed potatoes. Destroy volunteer potatoes. Ensure good air circulation. Monitor weather forecasts for blight-favorable conditions.
Management
Apply copper-based fungicides preventively during cool, wet weather. Remove and destroy infected plants immediately. Do not compost infected material. Harvest potatoes in dry weather and cure before storage.