Powdery Mildews-NOT ALL THE SAME

Gail Ruhl, Senior Plant Disease Diagnostician, Purdue Plant and Pest Diagnostic Lab

Powdery mildew (PM) is the name given to a group of diseases with a grayish-white, powdery coating of spores and fungal mycelia visible on the surface of leaves, stems, flower petals and fruit. Powdery mildew is caused by several closely related fungi, each having a limited host range. In other words, observing powdery mildew on lilac leaves should not cause concern about spread to nearby zinnias.

All PM fungi are obligate parasites, meaning that they must grow and reproduce on living tissue. PM survives from one season to the next as spherical, thick-walled, fruiting bodies, called chasmothecia (previously called cleistothecia), on the bark of branches and stems of woody, perennial hosts and also on fallen, infected leaf debris beneath plantings. In spring or early summer, airborne spores from overwintering chasmothecia infect susceptible leaves to once again begin the infection cycle.

The reason we often see powdery mildew infections in the absence of rain events is that all PM species can germinate and infect susceptible tissues in the absence of free water. In fact, water on plant surfaces for extended periods of time will actually inhibit germination and can kill the spores of most PM fungi! We do not recommend this as a control measure because moisture on leaves promotes the growth of most other diseases.

The best method of control for PM is prevention. Cultural practices that will decrease the severity of PM in the landscape include avoiding planting those cultivars which are highly susceptible and alleviating high humidity by spacing plants far enough apart to allow good air movement to quickly dry the foliage. Although chemical control is seldom warranted in the home landscape for powdery mildew, preventative fungicide sprays are available for use on prized ornamentals and vegetables that require protection.

See these publications for more information:

https://www.extension.purdue.edu/extmedia/bp/bp-5-w.pdf

https://www.extension.purdue.edu/extmedia/bp/id-414-w.pdf