Fungicide spray programs need to be started as soon as new leaves appear in the spring. Mulching also helps keep water from splashing up onto the leaves. Powdery Mildew Powdery mildew is a fungus disease that affects young leaves, causing them to curl and twist and develop a purple coloration. As the disease progresses, leaves become covered with white powdery fuzz. Whereas blackspot is usually most severe on the lower part of the plant, mildew affects the top part of the plant.
Mature leaves are less likely to be affected.
Mildew is spread by wind and develops rapidly during periods of warm, humid days followed by cool nights. Mildew is actually discouraged by the presence of water on the leaves. Infection can be reduced through sanitation and fungicide spray programs. Prune out all dead and diseased canes to reduce initial fungus infection. Because new growth is especially susceptible, thorough coverage of new growth with fungicide is important.