Why Do My Grapevines Have White Powdery Mildew?

Powdery mildew (Erysiphe necator, also known as Uncinula necator) is one of the most prevalent fungal diseases of grapevines worldwide, and home growers in warm, dry summers will almost inevitably encounter it at some point. Unlike many fungal diseases that spread in wet conditions, grape powdery mildew actually thrives in warm, dry weather with moderate humidity — making it particularly problematic in greenhouse-grown vines and in sheltered wall positions. Early identification and prompt treatment prevent it from ruining the crop.

Recognising powdery mildew

The fungus produces a characteristic white or pale grey powdery coating on the surfaces of young leaves, shoots, and developing berries. On leaves it typically starts as small powdery patches that spread to cover most of the surface. Infected young shoots may be stunted and distorted. On berries, the powder appears at or just after fruit set and causes the skin to crack and develop a brown corky web of russeting beneath the white coating — severely affected berries become unmarketable. An unpleasant musty or mushroom-like odour is sometimes noticeable when the infection is heavy.

Treatment with sulphur

Wettable sulphur is the traditional and most effective treatment for grape powdery mildew, and it is approved for organic use. Apply as a spray to all surfaces of the vine as soon as you see the first signs of infection, and repeat every 10–14 days while conditions remain warm and humid. Do not apply sulphur when temperatures exceed 30°C (85°F) as it can cause leaf scorch, and do not apply within four weeks of a neem or horticultural oil treatment.

Potassium bicarbonate as an alternative

A spray of potassium bicarbonate raises the pH on the leaf surface to a level that the fungus cannot tolerate. It is less persistent than sulphur but effective as a knockdown treatment on light to moderate infections and is safe to apply in warm weather. Some growers alternate bicarbonate and sulphur treatments to reduce resistance risk.

Prevention through canopy management

Powdery mildew overwinters in dormant buds and bark, and spreads most rapidly in a dense canopy where air movement is limited. Summer pruning — removing crowded laterals and opening the canopy — reduces the warm, stagnant microclimate the fungus prefers. For greenhouse vines, ventilate on warm days and avoid the combination of high temperature and high humidity.

Resistant varieties

Some modern grape varieties carry partial resistance to powdery mildew — particularly PIWI varieties such as Regent, Solaris, and Villaris. Growing a resistant variety does not eliminate the need for monitoring, but it dramatically reduces the frequency and severity of outbreaks and is a good long-term solution for growers who prefer to minimise spraying.