Abstract: The Cladosporium red spot of peony appears mainly on the leaves and stems. Small, circular or oval discolored spots may appear on the ventral surface of the leaf and will finally extend to irregular large specks. Leaf spots show different colors on the two surfaces, which are dark purple above and dull chestnut below. On the stem there are small, dark purple elliptical lesions. The pathogen of the disease has been identified as Cladosporium paeoniae Pass. The optimum temperature for fungus growth and spore germination was found to be 20-24°C. The germination of spores in tap water and distilled water was relatively poor, the germination ratio being only 57.7% and 43.3% respectively. But in 25-50% peony and tree peony decoction, the germination ratio increased to 90% or so. In artificial inoculation the fungus could invade the host either through wounds or not.Results of a series of artificial inoculation experiments conclusively showed that the incubation periods varied with temperature, for instance, an incubation period of 14 days at 8°C reduced to 5-6 days at 24-25°C. Infection spots on leaves spreaded slowly and the fruiting body formed only after two to two and half months. The process of primary infection of this disease lasted rather long, and a secondary infection would be only once. The pathogen overwintered on the stems and fruitcoats of the diseased plant remained in the fields. The source of primary infection is of vital importance to the development of the disease. The disease can be effectively prevented by cutting off the stems above the ground as much as possible without injury to the bud, then transplanting the plant to the field where no disease existed or filling with fertile soil of about 15 cm thickness during winter. After that, spray with Bavistin, at a dilution of 1:1000. the first application being at the time when new shoots start to break through the ground, followed by a second and third before blooming.