See:
Peony (Paeonia spp.) - Leaf Blotch
Cause The fungus Mycocentrospora acerina has been found many times by the OSU Plant Clinic and in the PNW. The fungal chlamydospores can persist in soils and decayed plant tissues and propagules are rain splashed. It may also be moved with infected planting material but has yet to be proven.
Symptoms Small, elongate red spots with light-colored centers occur on leaves and stems that turn brown to black under cool moist conditions. Multiple lesions may appear close to each other imparting a streaked reddened look. Lesions do not coalesce and remain separate. Infections on leaf margins cause twisting and cracking of the leaf as it expands. Lesion on roots begin as orange, water-soaked spots, and then turn black.
Cultural control
- Select and propagate only healthy-looking plants and roots.
- Avoid overhead irrigation or any practice that keeps plants wet for extended periods of time.
- Remove and destroy diseased plant parts both during the season.
- Space plants for good air circulation.
Chemical control None known.
Reference Garfinkel, A.R. and Chastagner, G.A. 2018. Diseases of Peonies. In McGovern, R.J. and Elmer, W.H. (eds.) Handbook of Florists' Crops Diseases. Springer Int.