Botrytis Blight (fungus – Botrytis paeoniae): Most common disease of peony. Young shoots in all stages of growth, including the buds may suddenly wilt and fall over. Upon examination, a brown or blackish rot is seen at the base of the stem. This discoloration may extend down through the roots. Small buds attacked by the fungus turn black and dry up. Larger buds turn brown and may be covered with a brown spore mass. Large irregular dark brown lesions may occur on the leaves. Cool, rainy weather favors disease development. Splashing rain and insects spread the disease. Regular applications of a foliar fungicide, a drench, and destruction of infected plant material are recommended control measures.
Root and Stem Rot (fungus – Phytophthora cactorum): Infected parts are dark brown to black and leathery. Cankers appear along the stems and may cause them to fall over. May also cause a watery crown rot often destroying the entire plant. Roguing and destroying all infected plants is the only means of control.
Wilt (fungus – Verticillium albo-atrum): Plants gradually wilt and die during the blooming season. Brown discoloration of the water conducting tissues may be seen in cross sections of the roots or stems. Obtain disease-free plants, remove and destroy infected plants. Potting soil should be sterilized.
Mosaic (virus): Circular areas consisting of concentric bands of alternating dark and light green on leaves. Small necrotic spots may also form. Plants are not dwarfed. No control other than destroying infected plants.
Cotton Root Rot: (See the section on Cotton Root Rot)
Southern Blight: (See the section on Southern Blight)
Leaf Spot (fungus – Alternaria spp.): Leaves may have brown purplish or reddish irregular shaped spots. Leaves may yellow, wither and fall early.
Crown Gall (bacterium – Agrobacterium tumefaciens): (See the section on Crown Gall)
Powdery Mildew (fungus – Erysiphe polygoni): Powdery, white mold on leaves. Leaves may be deformed, turn yellow and drop early. Collect and destroy fallen leaves and provide adequate spacing of plants.
Ring Spot (virus): Plants may be severely stunted with lemon-yellow to orange-amber spots, blotches or zoned rings. Young leaves may be distorted and plants may not flower. Do not propagate from infected plants.