Peonies are robust and long-living plants. They produce blooms consistently, even during periods of drought. They withstand aphids, slugs and most other pests. Gray mold poses the only serious threat to peonies, and it usually attacks large collections. Small collections of peonies may never encounter gray mold, according to Martin Page's "The Gardener's Guide to Growing Peonies." If you purchase high-quality plants and maintain good hygiene, you will reduce the risk of gray mold developing on peonies.
Two types of fungi cause gray mold on peonies: Botrytis cinerea and Botrytis paeoniae. Botrytis cinerea is also known as late blight and infects a wide range of herbaceous and perennial plants. Botrytis paeoniae is often referred to as early blight or bud blast and only infects peonies. The fungi develop on the tissues of infected plants and produce spores, or conidia. The spores transfer by air currents, rainwater or garden tools. The fungi may overwinter in the soil, in mulch or on dead plant material. The disease often attacks previously damaged tissue.
The spores appear on buds, leaves and flowers during wet or humid weather, but they are often overlooked. The silvery-gray spores scatter like dust when they are disturbed. The fungi form black structures called sclerotia in dead plant tissue in late summer. The sclerotia are masses of threads that store food and enable the fungi to survive periods when host plants are scarce.
Botrytis cinerea occurs in late-blooming flowers, often infecting stressed or wounded green plants just below the flower bud. The buds and flowers droop and fail to open. Eventually, they shrivel and die. Botrytis paeoniae causes soft brown areas to develop at the base of the leaves or shoots. The infection progresses down the stem and along the foliage. The buds turn black and rot. If left untreated, Botrytis peoniae causes wilt, the plant's collapse and death.
Moisture encourages germination and the spread of infection. Do not disturb infected peony plants while they are wet with dew or rainwater. Avoid overhead watering during bloom time. Drip irrigation reduces water splash. Watering early in the day allows the leaves enough time to dry. Do not use a densely packed mulch when new growth emerges, as mulch retains moisture.
Discard blighted or faded peony plant parts. Relocate infected plants to an open spot with improved air circulation. In severe case, it may be necessary to cut infected stalks to ground level and destroy the infected plants, along with removal and replacement of the first few inches of topsoil surrounding the plants. Treat previously infected peonies with a fungicide in the spring.