This time of year we get a lot of questions from customers about how to treat powdery mildew. This year has been especially bad for powdery mildew and other fungal diseases such as black spot and blight because we've had mild, damp weather farther along into the growing season. These conditions create a perfect habitat for diseases to develop and spread. Powdery mildew can be especially difficult to contain because it spreads very quickly.
To combat the spread of powdery mildew first it is best to remove any infected leaves and toss them in the trash (not the composter). Then spray the rest of the plant with a garden fungicide. I recommend using Fertilome's Broad Spectrum Landscape and Garden Fungicide containing Chlorothalonil (which we carry at the store). The good news is, now that the sun is shining more frequently and we are moving into warmer weather, nature should take care of the rest and dry plants out enough to keep powdery mildew for spreading farther.