Botrytis on peonies
IDENTIFICATION — Peony bushes can suffer from a variety of fungal diseases and botrytis is a big one. Fungal diseases thrive in warm, moist conditions like those in excessively rainy springs or humid summers. Small brown spots on the leaves will eventually spread to cover the entire leaf surface. Infected buds may shrivel up and fail to bloom.
CONTROL — Copper-based fungicidal soap sprays, such as Soap-Shield, may help. You may not need to treat the disease though; this fungus rarely kills the plant. As a preventative measure, cut the plants back to the ground after frost. Don’t compost infected material; discard it to get rid of fungus spores.