🦠 Downy mildew

Downy mildew is a vine disease caused by an organism called Plasmopara viticola. It looks like a fungus but belongs to a different group. The disease likes damp, warm weather and is especially dangerous in rainy years and in maritime climates.

The signs are easy to recognise. Yellow oily spots appear on the upper side of leaves and a white downy growth on the underside. Flower clusters and berries are affected too. If not controlled, the leaf dries and falls, and the crop can be lost.

Downy mildew was brought to Europe from America in the 19th century. Bordeaux mixture, a blend of copper sulphate and lime, was devised against it. Copper is still used, including in organic viticulture. Growers also breed varieties resistant to the disease. These are hybrid grapes and PIWI varieties, which need fewer treatments.

Canopy management, leaf thinning and good airflow around the vine are also used to reduce the risk. Downy mildew, along with esca and black rot, is one of the main vine diseases.