Latin name: Sphaerotheca macularis
Reason for Concern:
1) Disease on foliage, new canes, and fruit of red raspberry in the Pacific Northwest.
2) Most prevalent in years when weather is warm and humid.
3) It can affect winter hardiness.
Raspberry leaves infected with powdery mildew develop spots on the upper surface, often with powdery white fungal growth on the lower leaf surface. Upper surfaces of leaves have a mottled appearance similar to that caused by mosaic virus infection. Infected shoot tips and fruit may also become covered with white, fungal growth. Severely infected shoots become long and spindly with dwarf leaves that curl upward. Severely diseased plants may be dwarfed or stunted with distorted plant parts. Infected fruit may become covered with a white, mealy mat of fungus. Severely infected berries fail to size properly and wither and die.
The fungus overwinters in dormant buds of stunted cane tips or on primocanes. Optimum conditions for spore germination and infection are 65 to 80°F with relative humidity of 97 to 99%. In May, leaves develop lesions that produce fungal spores that serve as a secondary inoculum for repeated cycles of infection throughout the growing season. The spores are airborne and the development of disease is favored by moderate, dry weather. If the disease continues to develop, small, secondary-infection lesions appear on vegetative tissue and developing fruit.