Why Do My Grapevine Leaves Have Downy Mildew?

Downy mildew (Plasmopara viticola) is a destructive oomycete pathogen specific to grapevines and one of the most significant diseases facing home growers in cool, wet climates. Unlike powdery mildew which thrives in dry warmth, downy mildew spreads explosively in warm, wet conditions — making British and northern European summers almost ideal for it. A severe infection can defoliate a vine and destroy the entire crop within a matter of weeks if left untreated.

Identifying downy mildew on leaves

The first symptom is an irregularly shaped, pale or oily-looking yellow-green patch on the upper surface of the leaf, bounded by leaf veins giving it an angular appearance. Turn the leaf over: the corresponding area on the underside develops a white, downy or cottony growth — the fungal sporangiophores — which is diagnostic for this disease. As infection progresses, affected areas turn brown and the leaf may curl, die, and fall early. Young leaves and shoot tips are the most susceptible.

What downy mildew does to fruit

Infection of the flower cluster causes entire bunches to turn white and covered in the downy growth (this is called white rot or grey rot of the cluster). Infected berries after fruit set shrivel, dry up, and turn brown or grey without the typical mould associated with botrytis — a phenomenon called leather rot. A heavily infected cluster is completely lost. Protecting the bunches at flowering and just after fruit set is the most critical window in the disease management calendar.

Copper-based fungicides

Copper hydroxide or Bordeaux mixture (copper sulphate and slaked lime) are the traditional treatments for downy mildew and are approved for organic use. Apply preventatively from when new shoots are 10–15 cm long and repeat every 7–14 days during wet weather. Copper accumulates in soil over many years of use, so apply at the minimum effective rate. Modern synthetic fungicides based on dimethomorph or mandipropamid can also be used by conventional growers.

Cultural prevention

Remove badly infected leaves and bunches to reduce the inoculum level in the canopy. Keep the canopy open through regular summer pruning to improve air circulation and help leaves dry quickly after rain. Avoid overhead irrigation, which wets the foliage — drip irrigation or surface watering keeps leaves dry and inhospitable to the pathogen. Clear fallen leaves at the end of the season as the pathogen overwinters in leaf litter on the soil.

Resistant varieties

Several disease-resistant grape varieties offer significant resistance to downy mildew. PIWI varieties (pilzwiderstandsfähig — fungus-resistant) such as Solaris, Johanniter, Prior, and Aromera carry multiple resistance genes and rarely suffer severe outbreaks. For growers who want to minimise spraying, choosing a resistant variety is the most effective long-term solution.