The fungus Phytophthora, common in most soils, initially attacks either the roots or the developing shoots at the soil level, causing a blackening and decay of stem tissue. These black often sunken areas, usually several inches long, may appear on upper stem tissue as well.
Stems tend to fall over at the stem lesions.
Flowers, buds, and leaves may also turn a dark brown or black color. The tissue will appear somewhat leathery. The disease is most serious in soils that are poorly drained. The disease can be spread by splashing rain or contaminated tools, soil, or plant material. To prevent Phytophthora blight, plant peony in well-drained soils and thin crowded plantings.