Abstract:Root-rot is a destructive disease in the tung-oil tree (Aleurites fordii Hemsl.) of Sichuan. The causal fungus identified as Fusarium solani (Mart.) Sacc. was isolated from diseased tung-oil tree roots.Studies on morphology and cultural characteristics indicate that there are many differences between this root-rot fungus and other 8 forma speciales of F. solani from different hosts in China in the size of macrospores and the rate of growth.Test on pathogenisity confirmed that the pathogen is pathogenic to host plant and inoculation experiments of 7 species plants were negative. Artificial inoculation with 8 forma speciales of F. solani to tung-oil tree are unsuccessful.The changes in esterase isozyme of this pathogen and other tested strains was studied with polyacrylamide gel electrophoresis technique. It has been shown that esterase isozyme band, the number and relative rate of migration of the pathogen and all 8 strains could be discriminated.Hence the pathogen of root-rot of tung-oil tree is idefied to be a new forma speciales of F. solani, and is diagnosed as Fusarium solani (Mart.) Sacc. Sp. F. aleuritidis Chen et Xiao f. sp. Nov.