Downy Mildew
Peronospora spp., Pseudoperonospora spp., Bremia lactucae, Plasmopara viticola
Overview
A group of oomycete diseases causing angular, yellow to brown lesions on upper leaf surfaces with corresponding fuzzy, grayish-purple sporulation on the undersides. Unlike powdery mildew, downy mildew thrives in cool, wet conditions. Downy Mildew is managed first with cultural controls suited to permaculture systems: sanitation, spacing, irrigation style, and resistant planting stock. Causal agent: Peronospora spp., Pseudoperonospora spp., Bremia lactucae, Plasmopara viticola. In regenerative gardens, prioritize sanitation, airflow, and unbroken soil cover over calendar spraying, and intervene early when symptoms appear.
Symptoms
Angular yellow to brown lesions on upper leaf surfaces, bounded by veins. Grayish-purple fuzzy growth on leaf undersides. Leaves may curl, wilt, and die. Systemic infections cause stunting. 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
Humid
Prevention
Use resistant varieties. Improve air circulation. Avoid overhead watering. Water in the morning so foliage dries quickly. Remove crop debris. Rotate crops on a 2-3 year cycle. 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. Remove infected leaves immediately. Phosphorous acid products provide systemic protection. Destroy crop residues after harvest. 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.