Answer: Peonies are subject to a number of foliar problems as well as botrytis, often spreading from plant to plant and from season to season on old plant debris. A wet spring or rainy period can facilitate infection. The first steps in control are good hygiene, meaning cleaning up all stems and foliage each fall. Put them in the trash, not the compost pile. In the spring, before new growth appears, add a clean layer of mulch to prevent any splash up from the soil.
Immediately removing the first leaves to show signs of spotting can also help limit the spread somewhat during the growing season. Planting them far enough apart to allow for good air circulation and ample sunlight would also be a good idea. (Peonies need about a three to four foot spread and are long lived and do not transplant easily, so initial planting should allow for their mature size.)
Next, you might want to take a sample to your county extension for a more precise identification of the problem and recommendations on how to control it.
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