Peonies are showy plants that are generally easy to care for. All they require is a support to grow on, neutral soil that drains well, and full sun. For best growth and disease prevention, each plant should have at least three feet of growing space. But sometimes, even when these precautions are taken, peonies develop disease. Two of the most common peony diseases cause gray mold to appear on the plant.
Sometimes called "bud blast" or "early blight," this type of disease produces dense gray fuzziness on stems, leaves and buds of peony plants. While botrytis can appear any time during a peony’s growing season, it’s most prevalent when the weather has been cloudy and rainy.
Young peony stalks affected by botrytis discolor near the soil, wilt and eventually topple over. Older plants with botrytis develop irregularly shaped large spots and dark brown or black buds, all covered with velvety gray fuzz–which is actually a fungus, says the Oregon State University Extension. Blackened buds may spread rot from the top of the stem down. Later, blackened stems may develop brown spots.
To prevent botrtis blight attacks, water at the base of plants and promptly remove all spent flowers. Do not mulch around peonies until the plant is more than 6 inches high. In the early spring, mancozeb or copper sulfate may be applied to protect the plant when shoots reach 6 inches high, according to the University of Minnesota Extension.
Once plants are affected, prompt removal of diseased parts can prevent the spread of the disease. Fungicides applied after infection are not effective.
Appearing as a dusty white or gray coating on plants, powdery mildew can attack peonies, stunting them. Starting out as small, typically circular spots, the mildew eventually mats the entire surface of leaves and other plant parts.
Symptoms typically appear in the late summer or fall, according to Cornell University. Usually, the mildew appears after humid weather or where plants are crowded or in shade. Left untreated, powdery mildew can lead to great decline in the vigor of the plant and could eventually kill it.
To help prevent powdery mildew, rake up and remove fallen leaves in the fall. If powdery mildew has been a problem in that area, the leaves should be burned or placed in a sealed bag and thrown away. Giving peonies plenty of room can prevent mildew, as can moving the plant into a sunnier location.
Once plants are infected with powdery mildew, a fungicide specifically for use on peonies with the disease should be applied. If applied as soon as a few spots appear, the fungicide will work best. Many fungicides should be reapplied if the weather turns humid, so read the product’s label carefully.