Answer: Powdery mildew is a fungus that produces the symptoms you describe. Phlox seems to be susceptible to it so at the first sign of disease cut away the affected plant parts to stop the spread of the disease. Once the disease becomes a problem: Avoid overhead watering to help reduce the relative humidity. Remove and destroy all infected plant parts (leaves, etc.). Selectively prune overcrowded plant material to help increase air circulation. This helps reduce relative humidity and infection. If cultural controls fail to prevent disease buildup or if the disease pressure is too great, an application of a fungicide may be necessary. These include: sulfur, neem oil (Rose Defense, Shield-All, Triact), triforine (Ortho Funginex), or potassium bicarbonate (Kaligreen, First Step) Follow the instructions on the fungicide label for use on specific plant species, varieties, rates to be used, timing of applications, and waiting periods before harvest. An alternative nontoxic control for mildew is baking soda combined with a lightweight horticultural oil (Sunspray). Researchers at Cornell University have discovered the fungicidal properties of this combination but it is still experimental so use it at your own risk. Best wishes with your garden!
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