Blackberries are seldom severely infected by powdery mildew. It is occasionally a serious problem on susceptible varieties of red and black raspberries, especially the Black Hawk and Latham varieties. Infected plants may be stunted and less productive.
Symptoms and Disease Cycle
The disease is caused by the fungus Sphaerotheca macularis, which overwinters in infected cane tips and dormant buds. When temperatures reach 50 to 60°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 plantings with poor air circulation.
Unlike with most fungi, free water will reduce the incidence of this disease. Plant disease-resistant varieties when possible. Removing suckers that are infected with powdery mildew and pruning canes in the spring to a desirable height can reduce sources of the disease. Practices that allow good air circulation, such as cane thinning, proper plant spacing, and maintaining narrow rows, can be helpful in control. Apply fungicide sprays (see Table 7.5) when symptoms first appear, usually from midsummer through fall. If powdery mildew was severe in black raspberries the previous season, begin fungicide sprays in mid-June. Three to four applications might be required.