Answer: Powdery mildew appears under certain weather conditions. Sometimes it can be rinsed off with water from the garden hose when it first appears. Good air circulation (planting them in a location with good air flow, keeping them thinned and spacing them far enough apart) and avoiding wetting the leaves in the evening are preventive steps. After it appears there is no way to spray and prevent it however. Do a thorough clean up of fallen foliage this fall to try to limit sources of reinfection next year. If you want to spray preventively next year, please contact your local county extension for recommendations on what to use and how/when to apply it for best results.
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