Stem rot attacks in early summer and causes arrested growth and decay of flowers. Underground plant parts are not attacked. Small round specks on the stems and leaves develop into dark sunken brown spots. The decayed spots appear on the stems and spread. The upper parts of the plants collapse and die. Avoid excessive nitrogen applications and destroy all decayed foliage in the fall. The disease is caused by the fungus Botrytis paeoniae and is controlled with drenches of benomyl.
Anthracnose causes circular to oval, brown spots with purplish red margins. The spots are one-half inch in diameter or larger. The diseased tissue drops out and the foliage dies prematurely. Destroy diseased foliage in the fall and remove diseased plants when seen. No chemical control listed.
Leaf spot causes brown spots with purple borders on the leaves. This disease is not a problem if stem rot is controlled. No chemical control is listed. Leaf spot is caused by a Kabatiella sp. fungus.