I think one of my Japanese Tree peonies contracted botrytis . I also grow Tuberous begonias and they almost always get botrytis near the end of the season although in the case of begonias it's usually more a case of mechanical abrasion which allows the fungus a foothold and it does not generally kill the tuber. As I understand it Peonies have their own form of botrytis.
So I did a little research and beyond the obvious means of control which include sanitation issues and removal of infected parts this is what I found/noted -
1. My peonies are in a mixed bed and I have been using a composted mulch that is mostly wood chips. I noticed that when I see pictures of peonies in my various books and some of the members here that the ground around the peonies looks barren. When I say barren, I mean crusty, dry soil with no visible organic matter at all. I suspect this is no accident.
2. Avoid the use of overhead watering systems.
3. Use a compost tea which strengthens the plant immune system.
4. Use of fungicides. I have seen many brand names mentioned. Of course this will not cure an existing problem but can be part of a preventative program. If any one can suggest something that's easy to get a hold of that would be great.
5. Rootshield - This is apparantly getting rave reviews. It is mostly intended to applied during watering but apparently it does have a systemic effect. It's fairly cheap at about $17. I have seen conflicting reports as to it's effectiveness on botrytis.