Answer: Once a peony is fully involved in the fungal disease powdery mildew it's best to cut the entire plant back. Prune it down to ground level. Your peony should be healthy when it resprouts next spring. Powdery mildew attacks plants that are under strees, either from water issues, too much shade, or over crowding. It attacks plants in the late summer when days are warm and nights are cool. Some years are worse than others so with any luck your peonies will remain healthy next summer. Hope so!
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