I have 3 that are looking sick, they have had a rusty glazed look on the leaves and are going dormant early. I have no clue what could be wrong.
I have a couple that are looking rough very early this year, I think it is all the rain we have had here. I wouldn't worry too much about it, just make sure when you cut it back in the fall to clear away all the foliage & stems so any disease will not carry over next year. If it is dormant now, I would go ahead & cut it back.
It's definately not rain here, I'd say they are a good month early going dormant. They didn't look just right last year either. The plants are; Alex Woolcot, Red Grace, and Topeka Coral.
There are a few diseases peonies are prone, but as far as I know the best care is to cut away all the stems & leaves to the ground, don't leave them there to carry the disease to next year. Don't add this debris to your compost pile either, it can live there then spread to your other plants. Adding a little compost around the soil can help rebuild them for next year, but don't add too much so that the eyes are covered too deep. Make sure there are no plants too close, the peonies need good air circulation.