Unless your plant is a tree peony it will be going dormant in a couple of months and you should cut off the foliage so don't frete. Powdery mildew is a natural it lives on all of the surfaces you have around you. Look at your plants and see if they are too close to other plants. Peonies do get powdery mildew but good air circulation will help. do a search on this group for mildew one of the suggested treatments was diluted milk.

I bought a mega collection of peonies from Gilbert Wild and Son in fall 2016. Some of them bloomed last year but this year is the first year I got a really nice display. I planted about half of the roots in my yard which has clay soil and about half of the roots in my mother’s yard which has very sandy soil and is probably Zone 7b or 8a. Mine seem to be doing better. This year I hope to order from some of the peony specialty nurseries to get some more variety. These might be a little too crowded. I also have Itoh peonies that I bought as boxed roots from Lowe’s a few years ago. They have done really well, blooming in the third year I think.

Sorry Whitney, don't get over here often anymore! Cold impacts different peonies in different ways--the vast majority aren't injured by cold unless it gets down into the 20s. Once the buds have formed on most of them though the buds are more apt to be damaged by cold. The only ones that don't seem to be impacted at that point are the very early species peonies. There is also another peony forum on www.garden.org that is more active if anyone is interested. Also free and also a nice group of avid peony growers! If you don't get a response quickly on one forum try the other; there are also several great peony groups on Facebook as well. Planet Peony is a favorite and we also just started a new one--American Peony Society group. I will warn you, though--it can be fatal to your wallet to see all of the gorgeous pics shared there!

Interesting thought Mailfleur. Theoretically plants and insects being so different physiologically, it shouldn't be an issue, but there are diseases that have an insect/plant aspect/interface to them. It also floors me with my powdery mildew issues that none of the nearby other plants are affected, only the phlox for me. It's growing right next to older peonies that are unfazed, as are the coneflowers. But we have had a lot of success using milky spore to control our lawn grub problems so there is definitely an association in my yard . . . that's why I sort of love nature, science and gardening. Endless mysteries to be investigated. Although it is also sometimes why I hate them all too . . . :) I had a biology professor once who used to say, "God is in the details" which was a riff on "The Devil is in the details." A little of both sometimes. As Darwin said, "There is grandeur in this view of life." Edited to add for Rouge that the white allium red peony combination is inspired! Love it. Almost wants me to make space in my own garden for more alliums. I have stuck strictly to the edible ones so far . . .