HI DACONIL is a very good fungicide.

Bayer puts out a systemic fungicide. I've had no experience with this on peonies.

I tried corn meal a few years ago based on a thread in this forum. It seemed to work. Now every year, I cultivate some organic corn meal around the base of each peony before I mulch. I use about 1/2c. per plant depending on size of plant. I am rarely bothered by botrytis and then only a leaf here and there. Since it is cold and VERY wet here this year, my guess is that it does work. I would consider it a preventive and not expect miracles if you already have botrytis, though it "seemed" to cut it short on a purchased peony. Cheap, organic and worth a shot. Oh, the chipmunks do like it, so be sure to cultivate it in lightly.