Especially at risk are open-land and hot house cucumbers. Other cucurbit may also become affected. Without treatment, infection of the entire crop may occur within 14 days.
Downy mildew may occur throughout the entire growing season, however, primarily starting in July. The disease spreads particularly fast on cold nights with dew formation.
Yellow spots, frequently limited angularly by the leaf veins, appear on the upper side of the leaf. (Picture 1) The brownish spore lawn which is typical for mildew appears on the underside of the leaf (Picture 2). Later on, the plants look burnt (Picture 3).