I think bortrytis is hard to control. At least my experience is that I can limit it a bit, but it is always lurking, waiting to strike. If I lose a few leaves, I can live with that, but I don't like it infecting buds.

It lives in leaf litter and compost. Remove any leaves at the first sign of disease, like brown spots. Throw them in the trash, and don't leave them around or compost them yourself. Clean up throughly in the fall. I don't compost my peony plants even in the fall because they are rather prone to botrytis and I am afraid of spreading it, but that may be overkill. Good air circulation is helpful; I have to stake my larger plants so they are fairly tightly bunched together and that is probably part of the problem that I have. Good luck!

I purchased Mancozeb from Merrifield garden center in Fairfax, VA. because I'd read it combats peony fungal diseases well when used as a soil drench in the spring before new shoots emerge.
I haven't had to use it yet because we've had bad drought the last couple years and the fungal diseases disappeared in the dry weather.
I've also read if you've had diseased plants to cut the stems back SLIGHTLY below ground in fall so the fungus doesn't overwinter.

ps - I also remove the old mulch in spring right over & around my peonies right before their new shoots emerge -just in case I missed pieces of diseased leaves & plant parts during the fall clean-up. I replace with fresh mulch.

Besides doing the good clean up of leaves and all plant residue I spray with a Bordeaux mix in the fall and before new growth in the spring. Al

What's a Bordeaux mix?

It is a Lime/Copper mix best made up when needed. Recipes are available, do a search of "Bordeaux mix". Al