A group of disease pathogens prefers hot, humid weather! These are the powdery mildews. Recent outbreaks on zinnia, phlox, lilac, sycamore, and oak confirm this preference. Powdery mildew fungi in Illinois all prefer hot, humid days. The spores germinate on foliage when relative humidity is 23 to 99% but not in free moisture (rain). Look for this disease now on many perennials, as well as annuals, shrubs, and even trees and turf. Common hosts in Illinois are lilac, zinnia, phlox, and rose, but other species are affected.
Although powdery mildew is not thought to be a major threat to plant growth, it is undesirable in production systems and on focal plants in the landscape. It may spread quickly, especially in humid weather.
Refer to issue 10 of this newsletter for preventive practices. If you see powdery mildew now, you can slow the spread by using a fungicide. Scout for the disease and then treat the plants according to label directions. Often damage is minor and treatment unnecessary; but watch plants with a history of problems. Consult the Commercial Landscape & Turfgrass Pest Management Handbook or the Home, Yard, & Garden Pest Guide for a list of registered fungicides by host and by disease. These manuals are available in Extension offices. Report on Plant Disease, no. 617, "Powdery Mildews of Ornamentals," is available at http://www.ag.uiuc.edu/~vista/horticul.htm or in Extension offices and provides details.