Gray mold or botrytis blight is caused by the fungus, Botrytis paeoniae. It is the most common disease of garden peonies. This destructive disease is very prevalent during damp, rainy seasons.

Symptoms and Diagnosis

On peonies afflicted with botrytis, the young shoots rot off at ground level when they are 5 to 8 inches tall. The stems often have a water-soaked, cankerous appearance. The leafy shoots wilt suddenly and fall over. The rotted portion of the plant will become covered with a soft brown or blackish mass of spores. Just above the ground level, the stalk will be covered with a gray mold that sheds large numbers of spores. The spores are carried by wind and insects to young leaves and flower buds and cause leaf blight and bud rot. Small buds that are affected turn black and wither. Larger buds turn brown and fail to open. During a severe outbreak of the disease, 90% of the buds fail to develop. Open flowers are affected occasionally, and they also turn brown and later develop a covering of gray mold.

Life Cycle

Botrytis fungi are both saprophytic and parasitic. The spore-producing structures of the fungus develop along the base of the rotting stalks and survive in debris left in the garden over the winter. In the spring, spores form and spread to dying, wounded, or extremely soft plant tissues. As the disease progresses, a gray mold develops. The gray mold is made up of spores that are either wind-blown or splashed onto new tissues.

Integrated Pest Management Strategies

1. Prevent infection. In early fall, cut down all old leaves and stalks to ground level. This debris should be destroyed and not composted. Apply 1 to 2 inches of mulch to bury debris.

2. Improve soil drainage. Plant peonies in well-drained soil; heavy clay soils should be lightened with organic material such as compost or peat moss. They should not be planted so that water is apt to cover their bases, nor should soil be heaped about the leaf bases.

3. Move plants to a better location. Good air circulation can reduce disease problems. Plant in full sun or at least in part sun. Space plants widely, at least 3 feet apart. Remove aging leaves from plants.

4. Use disease-free roots. When planting new plants, buy only from reputable dealers, or take divisions only from healthy, disease-free plants.

5. Use a fungicide. Spray the plants with a fungicide when young tips break through the ground. Follow 2 weeks later with another application and every 14 days thereafter until mid-June. Pesticides registered for use include copper, captan, chlorothalonil (Daconil), mancozeb, maneb, sulfur, and thiophanate methyl (Cleary 3336). Fungicides must be applied in advance of the disease as a protectant.

6. Look for varieties that show some resistance to the disease.

Organic Strategies

Strategies 1, 2, 3, 4, and 6 are strictly organic approaches. Of the fungicides listed in Strategy 5, consult the Organic Materials Review Institute (OMRI™) for appropriate organic copper or sulfur products.

Pesticide Disclaimer: 

Always follow the product's label and ensure the product is effective against botrytis. Not following the pesticide label before usage is a violation of federal law.