Garden Club Powder Mildew Powdery mildews are common fungal diseases on many ornamental plants in Minnesota.
Common hosts are rose, phlox, alpine current, zinnia, begonia, dogwood, and lilac.
In most cases, powdery mildew generally does not seriously harm the plant. Both relative humidity and temperature are factors in the development of the disease. It is most prevalent in cooler temperatures and high humidity. Crowded plants and lack of air circulation are factors in the development of the disease. In Minnesota, increased levels of powdery mildew occur in the Spring and again in the Fall.