Brambles are seldom severely infected by powdery mildew. Occasionally, it is a serious problem on susceptible cultivars of red and black raspberries, especially the varieties Latham and Black Hawk. Infected plants may be stunted and less productive

Symptoms and Disease Cycle

The disease is caused by the fungus Sphaerotheca macularis. The fungus overwinters in infected cane tips and dormant buds. When temperatures reach 50 to 60 degrees F, the spores are discharged and spread by wind. The characteristic sign of this disease is a white, powdery growth, primarily on the underside of the leaves. Infected leaves are dwarfed and twisted and have a yellow appearance on the upper surface. Powdery mildew is favored by warm weather without rainfall and is most serious in years and in plantings where there is poor air circulation.

Disease Management

Plant disease-resistant cultivars when possible. Avoid the cultivars Viking, Ottawa, Latham, and Glen Clova. Chief, Marcy, and Malling Orion are resistant, and Logan is immune. Removing suckers that are infected with powdery mildew and pruning canes in the spring to a desirable height may reduce sources of the disease. Practices that allow good air circulation such as cane thinning, proper plant spacing, and maintaining narrow rows may be helpful in control. If powdery mildew was severe in black raspberries the previous season, begin fungicide sprays in mid June. Three to four applications may be required.