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.
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.
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.
Management
Apply copper-based fungicides preventively. Remove infected leaves immediately. Phosphorous acid products provide systemic protection. Destroy crop residues after harvest.