Your peony has symptoms of powdery mildew, a fungal disease that is common on these plants and has become more prevalent in recent years. Make sure there is adequate sunlight and good air circulation to reduce humidity levels around the plant(s). Allow proper plant spacing for the same reasons. Pruning for better air circulation also may help. You can cut down all infected foliage this fall as soon as the frost turns it brown (or before if the mildew has killed it.) Discard this plant material in the trash (not compost). Typically by this time in the season, the plants will have been able to generate enough stored energy in their root system to come back again next year. The powdery mildew can get very unsightly on peonies, but it does not kill them.

For next spring, you will have to decide if you want to spray a fungicide or a horticultural oil labeled for powdery mildew control or tolerate it. Fungicides are preventative--not curative. They will halt further infection, but cannot restore infected leaves to good health.

Here is our website information on powdery mildew: https://extension.umd.edu/resource/powdery-mildew-disease-flowers

And this is a good resource on peony problems and care:
https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/visual-guides/peony-problems.aspx

Christa