Downy mildew is a destructive grapevine disease caused by the oomycete Plasmopara viticola. It is not a true fungus, although it is commonly described as a fungal disease. The pathogen thrives in warm, wet conditions and can infect leaves, shoots, inflorescences and young grape berries. Severe infections reduce photosynthesis, damage the crop and weaken the vine.
Early symptoms on leaves may appear as pale yellow or oily-looking spots on the upper surface, often called oil spots. Under humid conditions, white downy growth may appear on the underside of affected leaves. Infected flower clusters and young berries can turn brown, shrivel or become covered with downy growth. Symptoms vary with the stage of infection and weather.
Downy mildew spreads through spores and requires suitable moisture and temperature conditions for infection. Rainfall, prolonged leaf wetness and dense canopies can increase disease pressure. The pathogen can survive between seasons in vineyard debris and produce primary infections when conditions become favourable.
Management combines canopy ventilation, monitoring, removal of infected material where appropriate and permitted plant-protection treatments. Copper-based products are used in many regions, including organic viticulture, but application rates and environmental restrictions are regulated. Integrated disease management and local forecasting are important because repeated treatments can affect soil and ecosystems.
Example: a wet spring in Bordeaux, Champagne or British Columbia can create conditions favourable to downy mildew, requiring close vineyard monitoring.
Downy mildew is caused by an oomycete and is strongly associated with wet conditions and downy growth on leaf undersides. Powdery mildew is a true fungal disease that can develop without free water on the plant surface and appears as powdery white growth.
Chef’s note: downy mildew is a vineyard disease, not a wine fault. Its impact is on vine health, grape yield and fruit quality before harvest.
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