Caused by the fungus Botrytis paeoniae, this is the most prevalent peony disease, particularly in cool, wet spring weather. The fungus infects young shoots as they emerge from the soil, causing them to wilt and collapse. It also produces dark brown, leathery lesions on leaves and buds. Infected buds often turn black and fail to open, or if they do, the blooms are distorted. A tell-tale sign is the fuzzy gray mold that appears on decaying tissue under humid conditions. To control this organically, ensure excellent air circulation through proper plant spacing. In fall, cut stems back to ground level and remove all plant debris from the area to eliminate overwintering fungal spores. Avoid mulching directly over the crown, and water at the base of the plant to keep foliage dry.
This disease, caused by water mold pathogens like Phytophthora cactorum, is a serious root and crown rot. It thrives in poorly drained, waterlogged soils. The plant's vascular system is attacked, leading to sudden wilting, blackening of the stems at the soil line, and rapid death of the entire plant. The roots will appear dark, mushy, and rotten. Organic control is primarily preventative and cultural. The most critical step is to plant peonies in well-draining soil and avoid sites where water pools. There are no effective organic fungicides that can cure an advanced infection. If a plant is infected, it should be dug up and destroyed immediately to prevent the pathogen from spreading to healthy plants.
While less devastating than fungal rots, powdery mildew (often caused by Erysiphe spp.) affects the plant's photosynthetic capabilities and aesthetic appeal. It appears as a white, powdery coating on the upper surfaces of leaves and stems, typically later in the growing season. Severe infections can cause leaves to curl, twist, and prematurely yellow. To manage this organically, again focus on promoting good air flow. If detected early, you can spray affected leaves with a diluted solution of neem oil or a homemade mixture of one part milk to nine parts water. These treatments can help suppress the fungus's spread. Removing severely infected leaves can also reduce spore load.
This disease, caused by the fungus Cladosporium paeoniae, is identifiable by its distinctive symptoms on the foliage. It produces glossy, dark purple to brown irregular blotches on the upper surfaces of leaves. On the undersides, the spots appear chestnut-brown. As the disease progresses, the spots can coalesce, causing large sections of the leaf to yellow and die back, weakening the plant. Organic management involves meticulous sanitation. All infected fallen leaves must be raked up and disposed of in the fall, as the fungus survives winter in this debris. Avoid overhead watering to keep the leaves as dry as possible.
