Walk around your garden, neighborhood, or landscape and you are likely to see powdery mildew diseases, which are caused by a fungal pathogen that thrives during conditions of warm to hot days with cool nights, and when dews form on leaves. The fungus grows superficially on the surface of the host, using special structures that penetrate into the host tissue. Most of the fungus appears on the surface as a grayish mildew.

Powdery mildew diseases affect woody and herbaceous ornamentals as well as vegetable, cereal, and fruit crops. Clinic samples with powdery mildew include rose, crabapple, lilac, dogwood, zinnia, and sycamore, but many other hosts are possible. These fungal diseases are easy to identify because of the characteristic white to light grayish powdery growth, primarily on leaves. Also look for stunting, curling of leaves, chlorosis, premature leaf drop, and deformation of flower buds. Apples and crabapples are commonly infected, but scab or some other accompanying disease often gets blamed for the stunting, chlorosis, and leaf curling. Even the easy-to-diagnose white powdery growth can be masked on plant species with heavy pubescence.

Disease will be most severe on crowded plants, in a shaded location, or where air circulation is poor. Unlike most fungal diseases, powdery mildew is not as destructive when rains are frequent. High relative humidity (but not rain) is needed for spores to germinate, and mildew develops rapidly in extended periods of warm, dry weather when morning dews are heavy. Ideal disease conditions are 90 to 99 percent relative humidity at temperatures of 66 to 72 degrees F.

When planning next year’s garden, look in seed catalogs and garden centers for cultivars resistant to mildew. Pruning out diseased wood (especially on rose and crabapple) during the normal pruning period will greatly reduce overwintering inoculum. Try to prune plants to allow better air circulation within the plant as well as within the planting. Never handle the infected plants when they are wet. As usual, plants should be maintained in high vigor to withstand disease attack.

Fungicides are available to control the mildews, and if sprays are begun at the first sign of mildew, control can be attained. Often, however, damage is only aesthetic and the actual vitality of the plant is not affected. If you decide to use a fungicide, use one of the products recommended under the appropriate host in the Illinois Commercial Landscape and Turfgrass Pest Management Handbook, 1998–1999. Further information on powdery mildews is available in Report on Plant Diseases Nos. 611 and 617.