I have a peony, Dancing Butterflies, that gets terrible mildew, sometimes not long after it finishes blooming. I think it depends on how much rain we get. But another peony nearby isn't affected by it, & this actually blooms better than my other peonies. I don't think it did it the first couple of years I had it, just after it matured completely. I've thought about spraying with a fungicide or something, but so far I just go ahead & cut it back when it starts looking too bad.