Last week, we discussed dogwood anthracnose, a potentially devastating disease of dogwoods. Now that we have you looking closely at your dogwoods, we want to alert you to the fact that you will soon be seeing powdery mildew on that host. Central Kentucky reported the disease a couple of weeks ago and, based on past experiences, that means we can expect to see the disease any time now.

The disease will be most severe on crowded plants or 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% relative humidity at temperatures of 66 to 72°F.

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 white powdery growth that looks like a grayish mildew as it ages. The powdery mildew disease on dogwood is caused by Microsphaera species and/or by Phyllactinia species. Although most other powdery mildews in our landscape cause symptoms in mid- to late July, the powdery mildew fungi on dogwood are active all summer. We see symptoms starting much earlier on this species. Because the fungi are favored by high humidity and short-term predictions are favorable for high humidity, watch for the disease now.

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. On many landscape plants, damage from powdery mildew is only aesthetic, and the actual vitality of the plant is not affected. The mildew diseases of dogwood have the potential to cause more long-term damage to the tree. If you have a specimen tree that has been infected in the past, you may need to use a protective fungicide now before symptoms appear. 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 2000 or in the Illinois Homeowners’ Guide to Pest Management. A newer product not yet listed in either manual (but will be added with the next revision) is Immunox. It will work to protect your dogwood from powdery mildew, too. Further information on powdery mildews is available in Report on Plant Diseases Nos. 611 and 617.