Powdery peony leaves
What causes this? Can it be treated? Plant gets full sun. Two peonies with less sun are shiny and dark green
Howard County Maryland
This is powdery mildew, a fungal disease. It's very common on peonies. Yes, you can treat. Here is what you need to know: https://extension.umd.edu/hgic/topics/powdery-mildew-annuals-bulbs-groundcover-perennials-and-vines
Peony varieties vary a great deal in their susceptibility or resistance to powdery mildew. It's very possible you have more than one variety of peony and, thus, they can handle it differently.
The air circulation around the different plants can have a big impact on how much disease they get also.
At the end of the season, when peony foliage browns, cut it off at soil level and dispose of it elsewhere to reduce the amount of infectious material that will spread to new foliage next year. (Don't cut it off now. It can still carry on photosynthesis and feed the roots for next year even if it looks bad.)