Yes I am sure the constant wet foliage in the warm summer temperatures have contributed to your mold problem. The mold is producing spores which is or will infect more and more of your garden. Plants vary in their resistance to the growth of mold so some plants will be OK. If your peonies are mostly consumed with the mold I would go ahead and cut them down, removing all cut material to the trash to be hauled away. They will come back next year though not with the usual vigor because you will be removing the foliage that is normally feeding the roots through the summer. Peony roots contain large storage capacity so your plants will be OK next year. After cutting down the peonies I would spray the ground well with a Bordeaux formula to try and prevent the spores from living over to next years growth. Al