What on earth happened to this healthy peony
This peony bloomed beautifully and in the last two weeks suddenly I have noticed the strange color of the leaves on about half the bush.
Freeborn County Minnesota peony diseases
There are several diseases that affect peonies. I can't tell which one is affecting your plant from a photo but a sample can be sent to the plant disease clinic.
These resources are useful for figuring out what is wrong with your plant.