How to Manage Pests
UC Pest Management Guidelines
Pathogen: Podosphaera macularis
(Reviewed 12/09, updated 6/12)
In this Guideline:
Symptoms and Signs
Caneberry leaves infected with powdery mildew initially may develop light green (chlorotic) patches on leaves. Leaves and shoots later are covered with white to gray mycelial growth. Affected leaves may be curved, twisted, or otherwise distorted. Severely infected shoots become long and spindly with dwarf leaves that curl upward. Severely diseased plants may be stunted.
Comments on the Disease
In California, the powdery mildew pathogen overwinters as mycelium in buds or on the surface of first-year canes (primocanes). Later, conidia are produced abundantly on the surface of infected tissue and serve as secondary inoculum for repeated cycles of infection throughout the growing season. The spores are airborne and the development of disease is favored by warm, dry weather.
Blackberries and varieties of red, black, and purple raspberries vary in their susceptibility to powdery mildew. Where powdery mildew is a serious problem, avoid planting highly susceptible cultivars. The blackberries from the Arkansas breeding program such as Navaho, Apache, and Arapaho tend to have very low susceptibility to powdery mildew as well as other diseases.
Practices that allow good air circulation and direct light penetration have been reported helpful in limiting the spread of the disease. These include proper plant spacing, cane thinning, and maintaining narrow rows. Removing late-forming suckers with powdery mildew symptoms and cutting back of floricanes (second-year canes) to a horticulturally desired height may reduce primary sources of inoculum.
Incidence and severity of powdery mildew tends to be higher in macrotunnels because the warm, dry conditions found inside the tunnels favor disease development. Free water is not necessary for the disease to develop, but higher humidity favors the disease.
Organically Acceptable Methods
The use of resistant varieties, maintaining good air circulation in the planting, and sprays of sulfur or potassium bicarbonate are acceptable for use in organically certified crops.
Treat when disease is first evident.
UC IPM Pest Management Guidelines:
Acknowledgment for contributions to Diseases: