Powdery mildew is a common disease of summer. It begins when we have cool nighttime temperatures coupled with warm, humid days. Symptoms continue even when days are warmer.
Powdery mildew is a “parasitic” fungus. It rests on the leaf surface and absorbs sap from plant cells. For this reason infected leaves are often twisted and gnarled.
Euonymus, rose, phlox, and crapemyrtle are common hosts. Powdery mildews are host specific — they cannot survive without the proper host plant. The species which causes powdery mildew on roses does not attack lilac. The mildew that attacks dogwoods does not attack phlox. Powdery mildews are caused by over l,600 species of fungi.
Control options include:
Mancozeb (click for sources), Bonide Infuse (click for sources), chlorothalonil (click for sources), propiconazole (click for sources) and myclobutanil (click for sources) are commonly available from local garden centers.
powdery mildew on crapemyrtle