Wine Arcana

Concept

Powdery mildew

Powdery mildew is a grapevine disease caused by the fungus Erysiphe necator. Its pale, powder-like growth can affect leaves, shoots, and berries. Infection can reduce the crop and change the fruit entering the winery, making it a concern for wine quality as well as vineyard productivity.

How an epidemic develops

The fungus can survive winter in dormant buds or in small fruiting bodies on the vine. These provide routes for the first infections of the season. Subsequent cycles spread spores through the vineyard when conditions favour growth. Temperature is particularly important; hot extremes and favourable moderate warmth have different effects on the organism.

University of California guidance uses weather-based risk assessment to help judge disease pressure. Canopy management also matters: opening dense growth improves access for protective treatments and changes the shaded environment around susceptible tissue. Early infections deserve attention because they can supply spores for later cycles.

Why infected grapes change wine

The Australian Wine Research Institute describes effects extending beyond the visible coating. Infected fruit can have altered composition, greater susceptibility to other microbes, and different fermentation behaviour. Reported wine effects include earthy or mushroom-like aromas, increased bitterness, and changes in texture; their extent depends on the fruit and severity of infection.

This creates practical consequences for sorting and maceration. Removing affected fruit limits what enters the fermenter, while shorter contact with skins may reduce extraction of unwanted components. The appropriate response depends on whether the intended wine is white or red and on the condition of the lot.

Powdery mildew and downy mildew are separate diseases with different causal organisms and infection biology. A grape's resistance to one therefore needs to be evaluated separately from its resistance to the other.

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