Downy Mildew (Peronospora/Plasmopara): Water-Dependent, Algae-Related Disease

Downy mildew, caused by oomycetes (Peronospora, Plasmopara, Pseudoperonospora)—water molds related to algae, not true fungi—requires leaf wetness and cool temps (spring). Fuzzy white/gray/brown/purple growth on leaf undersides; yellow/green/brown/purple mottling above; stunting, leaf loss. Fungicides don't work; cultural control is primary.

Key Distinction: Downy vs. Powdery Mildew

Prevention (Best Control)

• Water from below ONLY. Drip, soaker, hand-water at base. Wet leaves = infection highway.
• Airflow: Space, prune, ventilate greenhouses.
• Sanitation: Remove all dead plant material fall/winter—overwinters on debris.
• Resistant varieties: Many cucurbit, lettuce, spinach, basil, grape lines.

If Infected

• No effective curative chemicals for home gardeners.
• Preventive oomycetesides (mandipropamid, cyazofamid, oxathiapiprolin, fosetyl-Al) for commercial/repeated issues.
• Eliminate moisture: stop overhead water, improve airflow, reduce humidity (indoor).
• Mild cases: wait for warm weather—downy mildew dies out >75°F / 24°C.

Downy mildew is a water disease. Dry leaves, clean beds, and resistant genetics stop it. Wet leaves and last year's debris guarantee it.