Answer: Powdery mildew fungi spread quickest when the humidity is high and temperatures are between 50F and 90F. To discourage the disease, plant on sites with good air drainage, give the plants plenty of space, water during the morning only, and allow the soil to dry out between waterings. Remove any affected leaves, and if your plant is heavily covered with powdery mildew you should probably remove the entire plant so the infection doesn't spread. For heavy infestations of powdery mildew, sulfur or copper sprays can be applied weekly, starting in July in your area. To obtain these treatments, check your local garden center, and follow the directions carefully.
Phlox is known for its susceptibility to powdery mildew. Here are a few that are supposed to be resistant: 'David,' 'Eva Cullum,' 'Bright Eyes,' and 'Franz Schubert.' Don't crowd phlox as good air circulation helps prevent mildew. Also, clean up all plant material and discard it at the end of the season, and rotate where you plant species every year to help prevent build-up over time. Good luck!
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