Downy Mildew Control — How to Identify, Treat & Prevent Downy Mildew in Grapes & Vegetables
Downy mildew is a devastating oomycete disease that spreads explosively in wet and humid conditions, causing yellow lesions and white sporulation that rapidly defoliates grapes, cucurbits, and onion. Preventive fungicide applications before rain events are the cornerstone of effective management.
How to identify downy mildew
Watch for these signs in your field.
Yellow angular patches on upper leaf surface
Pale yellow to bright yellow angular patches appear on the upper surface of leaves, bounded by leaf veins — a distinctive sign of downy mildew infection in grapes and cucurbits.
White-grey downy growth on leaf underside
Corresponding white to grey-purple downy or cottony fungal sporulation appears on the lower leaf surface directly below the yellow patches — visible in early morning or wet conditions.
Rapid defoliation in severe cases
In severe or untreated infections, affected leaves turn completely brown, dry up, and fall prematurely, leading to defoliation that weakens the plant and reduces yield and fruit quality.
How to control downy mildew
Follow this step-by-step protocol for best results.
Apply preventive fungicide before rain
Downy mildew spreads rapidly in wet, humid conditions. Apply Metalaxyl + Mancozeb 72% WP (2.5 g/L) or Cymoxanil + Mancozeb 69% WP (2.5 g/L) before a forecasted rain period, not after. Preventive sprays are far more effective than curative applications.
Ensure full coverage on leaf undersides
Downy mildew sporulates on the lower leaf surface. Use a high-pressure sprayer to ensure full coverage of leaf undersides where spores are produced. Add a sticker-spreader to the spray solution to improve adhesion and rainfastness.
Rotate fungicide groups and improve air circulation
Rotate between contact (Mancozeb) and systemic (Metalaxyl, Cymoxanil) fungicide groups each spray to prevent resistance. Remove infected leaves and improve canopy airflow by pruning. Avoid overhead irrigation which creates the humid leaf wetness that downy mildew requires to sporulate.
Which crops are at risk?
Tap a crop for the full pest management guide.
Downy Mildew — Common Questions
What is the difference between downy mildew and powdery mildew?
Downy mildew shows yellow lesions on the upper leaf surface with white-grey downy sporulation on the underside — it thrives in cool, wet conditions. Powdery mildew shows white powdery growth on the upper leaf surface and thrives in warm, dry conditions with high humidity. They require different fungicide groups for effective control.
What is the best preventive spray schedule for grapes?
In grapes, start preventive sprays with Metalaxyl + Mancozeb 72% WP (2.5 g/L) at bud burst and repeat every 10–14 days during the monsoon season. Always spray before rain, not after. In high-pressure seasons, include a Cymoxanil-based spray every third application to target the systemic phase of infection.
Is there an organic option for downy mildew control?
Copper-based products such as Copper oxychloride 50% WP (3 g/L) or Bordeaux mixture (1%) provide organic-approved downy mildew suppression. They are contact protectants and must be applied preventively before infection. Repeat every 7–10 days during wet periods. Biocontrol agents like Pseudomonas fluorescens can also reduce inoculum load when used as soil and foliar treatments.
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