The peony plant is from the genus Paeonia and is native to Asia and parts of Europe and North America. Peonies are shrub-like herbaceous plants that produce large, fragrant flowers in late spring. The plant is often disease free when grown in a well draining soil with full sunlight conditions and good air circulation. Wet soil and humid weather are the most common contributors to disease infections in the plant.
Botrytis is a fungus infection that attacks the plants' leaves, buds and stems anytime during the growing season. Cloudy and wet weather are conditions that promote the spread of this infection. The symptoms include wilted young stems, large irregular shaped spots on the leaves and fuzzy fungus covering the flower buds. Apply copper sulfate for plants early in the growing season when the shoots are approximately 6 inches in height as a preventative measure. Remove and dispose of fungal growth immediately from the plant to prevent the infection from spreading.
Phytophthora blight is a fungal infection that is sometimes mistaken for a Botrytis infection. The main difference in that Phytophthora blight does not produce a fuzzy spore growth on the buds. A blight infection is often caused by wet soil and produces dark colored spots on the leaves; buds and stems with the plant shoot eventually turning black and dying. Cankers may also grow on the stems of the plant. Phytophthora blight spreads to the roots of the plant causing the entire plant to be infected. Remove and destroy the entire plant and amend the soil with a mixture of sand and organic compost to improve the water draining ability.
Verticillium wilt is a fungus that originates in the soil making it difficult to kill and prevent. An infected plant may show signs of curled, wilted leaves with infected branches wilting and dying off. Verticillium wilt infections start in the roots and gradually move up the stem of the plant. The disease is often mistaken for drought or excessive water damage or a bacterial wilt infection. Cutting through an infected stem shows an internal yellow-brown or red-brown ring. Remove and destroy infected plants and do not plant another peony in the area.
Powdery mildew is a fungal disease that forms a white fungus powder on the leaves of the peony late in the growing season. The plant tissue under the fungus powder will eventually die and cause the leaves to drop. The disease is controlled by removing fallen leaves in fall and destroying them to prevent the fungus from infecting the plant the following spring. Apply fungicide at the first sign of white spots as an attempt to eliminate the problem before it destroys the plant. Prevent a powdery mildew infection by planting resistant varieties if there is a continued problem with the disease.