Abstract:Stem rot of black locust (Robinia pseudoacacia) which caused high mortality of the trees has been observed in the most part of Jiangsu province since 1972.The pathogen attacks the trunk or roots of its host to induce a necrosis of inner bark usually around the basal or lower trunk.Some saprophytic sporodochia of Fusarivm sp.may be observed in cracks or diseased part of the bark.When a trunk was completely encircled,the tree wilted gradually. Isolation and inoculation experiments indicate that Phytophthora cinnamomi Rands is the pathogen of the disease.Infection occurs from March to October and mostly from May to September.Contaminated soils and infected plants are main sources of infection.