Botrytis Blight of Peony

Botrytis blight is a common fungal disease that confronts the peony grower each spring. The fungus Botrytis cinerea blights stems, buds, and leaves and can cause plants to look unsightly. This fungus causes disease on a wide variety of herbaceous and woody ornamentals. It is sometimes referred to as "gray mold" because of the conspicuous, fluffy, gray fungal growth that forms on infected plant parts.

#1. Tips of peony stalks blighted by Botrytis cinerea.
(Photo by M.A. Hansen)

Symptoms

In early spring young stalks may suddenly wilt and fall over. Young buds turn black and dry up (Fig. 1). Larger buds that become infected later in the spring turn brown and become covered with a brown or gray mass of fungal spores (Fig. 2). Flowers may fail to open. Usually the stalks below infected buds and flowers are rotted for short distances below the necks. Large, irregular, dark brown blotches may also occur on the leaves. In severe cases, crown and root rot may occur; however, these symptoms are not as common as aboveground symptoms. In wet weather the diseased plant parts soon become covered with a grayish, felty growth of fungus spores. Small, black sclerotia may form on the base of infected stalks or in other invaded portions of plants that have fallen to the ground. The causal fungus overwinters in this sclerotial stage.

#2. Typical fluffy, grayish sporulation of Botrytis cinerea on diseased plant tissue.
(Photo by M.A. Hansen)

Bud and flower symptoms are often confused with injury from the sucking insect, thrips. If thrips are the cause of bud or flower symptoms, these insects can usually be shaken from among the petals onto a piece of paper. The presence of tiny, orange, scurrying insects barely visible to the naked eye is evidence of thrips injury. Thrips do not cause the leaf blotches or stem rot associated with Botrytis blight.

Control

Sanitary measures are the most effective means of control. Because the fungus sporulates so prolifically on infected plant parts, it is important to remove and destroy all infected parts as soon as they appear. In the fall, cut all stalks at ground level or below, removing as much of the stalk as possible without injuring the bud.

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