Flowering dogwood trees near Townsend Hall on the University of Delaware campus were observed with powdery mildew symptoms on May 13, which is very early. Now is the time to apply a fungicide to valuable susceptible trees. Look for the white talcum-like growth on the leaves in the center of the tree or on water sprouts. When powdery mildew shows up early on dogwood I recommend that infected trees be sprayed to control this potentially debilitating disease. Homeowner labelled fungicides include, Immunox, Infuse, Bayer Advanced Tree, Shrub and Flower Disease Control, and others. I have had very good success with applications of 1% solutions of Sunspray horticultural oil, neem oil (Triact), and potassium carbonate, (Kaligreen, Remedy). Commercial applicators can use Eagle, Cleary's 3336, Spectro, Banner MAXX, Heritage, Compass and others. Apply when first signs appear and reapply every 2-3 weeks until early August. If you use horticultural oil, neem oil or potassium bicarbonate time those sprays to cover new flushes of growth during the season for the best protection.
Powdery mildew on dogwood in a nursery. Photo from the University of Kentucky, http://www.ca.uky.edu/HLA/Dunwell/KHC/NurseryUpdate13_2003.htmlInformation from Bob Mulrooney, Extension Plant Pathologist, UD.
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