Gail Ruhl, Sr. Plant Disease Diagnostician, Purdue University
Powdery mildew, a fungal disease, appears as a white, powdery fungal growth of mycelium and spores on the upper leaf surface. Leaves and flowers may become infected, wither and die.
Conditions including high humidity, warm days, and cool nights favor powdery mildew development. The disease is common in crowded plantings and in areas with restricted air movement.
There is no cure for infected plants however powdery mildew can usually be managed with timely use of preventative fungicides coupled with the use of disease resistant varieties. Preventative fungicide sprays help protect healthy plant tissue from infection. Products containing active ingredients such as triforine, chlorothalonil and sulfur are labeled for control of powdery mildew. Be sure to follow label directions. Spraying won’t cure already-infected leaves, but it will prevent the disease from spreading to the rest of the plant.
For more information on resistant varieties and fungicides for management of powdery mildews, see BP-5-W: Powdery Mildew of Plants (pdf file).
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Powdery mildew on peonies
White, powdery fungal
growth typical of powdery
mildew on the leaves of garden phlox.
Severe powdery mildew
infection progressing from lower to
upper leaves on this phlox plant.