This disease of peonies and many other plants is caused by a fungus (Phytophthora cactorum) that is common in most soils. Like gray mold, this fungus is favored by the cool, wet conditions of early spring. It can survive in the soil and in plant debris for many years in the form of oospores. Initially the fungus attacks either the roots or the developing shoots at the soil level, causing shoot wilting and a dark decay of the stem tissue. Wherever the fungus is splashed onto the plant, it may cause lesions, spots, and a typical brown, leathery decay. This disease is spread by splashing rain or water, and by contaminated plants, soil, and tools. It is most serious in heavy, poorly drained soils.
Remove and destroy plants with decayed roots. Pick off and destroy infected plant parts. Clean up plant debris. Spray the foliage and drench the base of the plant with a fungicide containing mancozeb. Spray 3 times at intervals of 5 to 10 days. Reapply the spray if infection recurs. Thin out overcrowded plants. Plant peonies in well-drained soil.
Plant Care for Paeonia