A month ago, I had nine buds on my peony plants. One bloomed. The other eight have turned black. What happened? They were not over-=watered, and they get plenty of sun.
You have botrytis problems. You need to clean up the area around the plant - and cut the rotted flowers and discard them. Do not recycle any part of the plant.
You may be getting too much water on them. I have an irrigation system that sprinkles from the top and all of my leaves are browning out.
I also have botryitis. As soon as this heat breaks I will cut off everything and send it to the landfill.
Then what? Should I spray the ground with something? Benomyl? Phalthan? Something else?