James A. Chatfield, Ohio State University Extension, Northeast District/Horticulture and Crop Science; Nancy A. Taylor, Plant Pathology, C. Wayne Ellett Plant and Pest Diagnostic Clinic, Ohio State University; Erik A. Draper, Ohio State University Extension, Geauga County; and Joseph F. Boggs, Ohio State University Extension, Hamilton County/Southwest District.
Many plants are affected by powdery mildew diseases. The most familiar way to recognize powdery mildew diseases is the actual sign of the powdery mildew fungus growing on the upper leaf surface. We are all familiar with these tell-tale powdery white fungal mycelial threads, and if you are Clark Kent, you can see the microscopic chains of spores growing from this mycelia. Less familiar are the symptoms of reddening and/or bronzing of leaf tissue on dogwood, magnolia, and rose and distortion of new growth on planetrees, dogwood, and rose that accompany these diseases.
Control involves improving air movement, to the extent possible, with judicious pruning and plant siting and use of powdery-mildew-resistant varieties when available. Also recognize that in many cases the disease causes little harm to plant health, and fungicides may be used when necessary.
Common powdery mildew fungicides include triforine (e.g., Funginex), thiophanate-methyl (e.g., Cleary's 3336), and propiconazole (e.g., Banner). Read and follow the fungicide label carefully when applying to a particular plant.