The Q&A Archives: Mildew On Phlox

Question: I have two varieties of tall phlox, some of which has been divided and planted as small plantings. The small plantings do not have the mildew as the larger plantings do. Is this due to the lesser number together, the newness of the planting, or none of the above? What causes the mildew? What can I do about it?

Answer: Phlox is susceptible to powdery mildew, and while you didn't describe the symptoms, I suspect that's the problem.
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.

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