Powdery mildews on peonies
Thanks for the question.
As you mentioned, your peonies have powdery mildew (a fungal disease) which is very common and generally more unsightly than deadly. It infects a large number of different plants and shrubs. The warm, damp weather that you may have had this season is a significant factor. In future years, you could try spraying a fungicide with powdery mildew on the label--but you must apply it before the disease gets established. Fungicides are preventatives--they are not curatives. This will not help you present situation. Thankfully there does not appear to be serious damage done to the plants by powdery mildew. Your peonies should bounce back next season.
For the time being, I would suggest that all stems with heavily infected leaves be removed and discarded before hitting the ground. Put them in trash bags and discard appropriately. To minimize the over wintering of spores in the soil, thoroughly clean all plant debris away from the base of the peonies as soon as possible. Also dispose in trash bags.
The should give you additional information as well as other remedial procedures:
Thank you! Where did the simplest thing for you to cut the whole plant down to the ground? Would it spring back next year?
Thanks for the response.
Yes, you could do this. Generally peonies are cut back in September or October but given your current situation, it would be a good idea. The main objective is to reduce the number of spores left in the soil over this coming winter. Cutting back the stems now before any more leaves fall to the ground would be proactive. In the event that your peonies may be stressed by this infection, be sure to water them well and frequently before the ground freezes. Mulching would also be advisable around the bases of the plants. Do NOT fertilize this fall. I have high confidence that your peonies were reappear next spring.