Generally, we think of powdery mildew appearing in the hot, dry dog days of August. That is the case with most plants; and the disease is usually most obvious in August. Powdery mildew on dogwood is a bit earlier than on most other hosts in the landscape. Look for it now and be ready to treat it as soon as symptoms appear. Conditions have varied in Illinois. Some parts of the state had enough rain that mildew won't be showing for a while. In Champaign, everyone was smiling when we finally got a half-inch of rain last week. Powdery mildew has appeared on campus on a few hosts already this season.

The fungus grows superficially on the surface of the host and forms 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.

Powdery mildew of dogwood is most severe on crowded plants, in a shaded location, or where air circulation is poor. Dogwoods in the open, as specimen trees, are less likely to be infected. 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°F.

Try pruning 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, choose a product recommended under the appropriate host in the 2001 Illinois Commercial Landscape and Turfgrass Pest Management Handbook or the Home, Yard, and Garden Pest Guide (formerly, Homeowners' Guide to Pest Management). These manuals list chemicals in a table at the end of each disease chapter. Look at these tables to get information on trade and common name and on mobility. Further information on powdery mildews is available in Report on Plant Disease (RPD) no. 611 and no. 617. These reports are available in Extension offices or on the Web at http://www.ag.uiuc.edu/~vista/horticul.htm. (Nancy Pataky)