Janna Beckerman, Associate Professor of Plant Pathology, Purdue University
Powdery mildew usually develops as an obvious white powdery growth on the upper surface of the leaf. The white powdery substance that occurs on infected leaves consists of powdery mildew spores and mycelium. Early, severe infection of young leaf tissue can cause twisting and distortion , with powdery mildew occuring on both the upper and lower leaf surfaces. Although powdery mildew rarely kills ornamental landscape plants, it will make them so unattractive that you take matters into your own hands!
Moderate, autumn temperatures (cool to warm, depending on the particular powdery mildew) and high relative humidity favors powdery mildew development. Poor air circulation also favors powdery mildew development due to the accumulation of pockets of high humidity around the leaf canopy. Strange as it may seem, wet leaves actually deter powdery mildew infection.
A variety of powdery mildew-resistant shrubs, annuals and perennials are available, and provide a first line of defense against this group of fungi. Powdery mildews can usually be effectively managed by the timely use of fungicides coupled with the use of disease-resistant varieties. For more information on resistant varieties, and fungicides for the management of the powdery mildews, see Powdery Mildew of Landscape Plants, available on line at: http://www.extension.purdue.edu/extmedia/BP/BP-5-W.pdf.
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Fig 1. Some phlox are more resistant to powdery mildew than others.
Fig 2. Severe powdery mildew of ninebark (Physocarpus).
Fig 3. Powdery mildew resistant cultivars look better with less work and fewer pesticides.
All photos by Janna Beckerman