Plant Disease Clinic--Highlights

Peony diseases have been a common problem this spring. Wet conditions have favored the development of several diseases.

Leaf blotch and stem spots. Leaf blotch is the most common disease of peony. The fungus Cladosporium paeoniae causes reddish- purple spots on leaves and stems. Often spots coalesce so that leaves appear irregularly blotched. On stems, infection appears as reddish-brown spots or streaks.

Anthracnose. Anthracnose is caused by the fungus Gloeosporium. The fungus can cause lesions on stems, leaves, bud scales, and flowers. Stem lesions range from small spots to elongated cankers several inches long. These cankers are often gray to black in color, but may be covered with the growth of salmon colored fungal spores. Cankers can girdle and kill stems. The anthracnose fungus also may cause irregular brown spots on leaf tissue, resulting in leaf curling and puckering. Infection of bud scales and petals interferes with flower develpment.

Botrytis Blight. The fungus Botrytis cinerea may also infect stems, leaves, and buds. In wet weather, diseased plant parts become covered with a brown to gray coat of fungal spores. Stems, leaves, and buds when infected suddenly wilt and turn black color. In severe cases crown and root tissue may also show symptoms of rot.

Controlling Peony Diseases. Sanitary measures are the most effective means of controlling peony diseases. Remove infected plant parts when they appear. In the fall, remove as much of the stalk as possible without injuring the buds.

Fungicide sprays, such as Bordeaux mixture, basic copper sulfate, maneb, or mancozeb, are directed at protecting healthy plants. Diseased plants cannot be cured. Sprays need to be started in the spring as new shoots appear through the ground.

This article originally appeared in the June 16, 1993 issue, p. 93.

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