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. Late Blight is managed first with cultural controls suited to permaculture systems: sanitation, spacing, irrigation style, and resistant planting stock. Causal agent: Phytophthora infestans. In regenerative gardens, prioritize sanitation, airflow, and unbroken soil cover over calendar spraying, and intervene early when symptoms appear.
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. Compare several plants and note whether symptoms spread after rain, insect feeding, or wet foliage. Photograph undersides of leaves and cut stems when wilt is present to refine the diagnosis. Rule out simple drought, fertilizer burn, and herbicide drift before assuming a pathogen - abiotic injury often mimics disease.
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. Mulch to reduce splash, clear season-end debris, and avoid working plants when wet. Design beds for airflow and diverse plantings that support beneficial insects without creating permanent humid pockets around susceptible crops.
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. Prefer drip irrigation, morning watering, and open spacing so leaves dry quickly. Disinfect pruners between cuts on woody plants. Use copper, sulfur, or biological products only as labeled protectants when pressure is high - they rarely reverse advanced infections alone. Destroy (do not casually compost) highly infectious debris.