What causes this? A branch, one on each side of this plant has grown white leaves.
I can't see it well enough to tell if you have powdery mildew or not. Your peony is being crowded by other plants and it looks like it is not getting full sun. Al
Actually, it gets full sun all day, no shade at all, there are plants around, but all trimmed to prevent any shade on this peony, and no root competition. It is actually about 4 feet above the juniper trunk. This morning, I see more green and pink appearing, yet nothing has changed. I have lots of PM here and there, but have never found any insects or diseases on this Moon River peony, and it is around 30 years old, this is the first 'odd' I've ever seen. Hope these pics give a better look. Thanks. First on east side, second pic on west side of the plant.
Check the stem to see if there is any damage further down on it. The third picture appears to have pink & brown bite like marks on the stems.
Who knows you might have another varigated peony. If you are near a college you might see if they have a plant specialist.