Answer: Phlox paniculata are very susceptible to powdery mildew. Your best defense is a good offense...buy powdery mildew resistant varieties of phlox. It sounds like you have good cultural practices: good air circulation, no crowding, etc. The most criticalof these is good air circulation. You must also rake up and remove any fallen foliage as that will spread the disease. Here is a good home remedy: Make a baking soda solution and spray it onto the affected plants. To make this solution mix 1 tablespoon of baking soda and 2 1/2 tablespoons of ultra-fine horticultural oil with a gallon of water. The oil is slightly fungicidal and acts to help the baking soda coat the leaf and hang on longer. Apply as a spray as soon as the mildew appears and every 10-14 days thereafter. Be sure to coat all surfaces!
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