Abstract: Pseudovalsella canker disease of Castanea mollissima was first found out in spring of 1983.It occurs commonly on the plantations of C.mollissima within the natural range of the Chinese chestnut tree in central Yunnan,and it has not been recorded in China before.This disease restricts the health and growth of the tree,being most prevalent in Chinese chestnut forest from the altitude of 1600 to 1800 meters above sea level.The canker develops either on the branches or on the main stems of C.mollissima,and Coryneum causes twig blight or even makes the whole tree dead.There are three plantations of Chinese chestnut in which the incidence of infected trees may be up 87-94.3 percent and its utmost disease index may attain to 45-47.4. Consequently,this is one of the most prevalent and destructive diseases of C.mollissima.The canker disease of C.mollissima is caused by Pseudovalsella sp.which is similar to P.modonia(Tul.) Kobayashi,and Coryneum runzei Corda var.castaneae Sacc.& Roum.,the Conidiospore can be produced below 24℃ in 12-15 days on PDA.The disease can be effectively prevented by pruning the blight twigs,excising the canker parts of the tree,and applying the medicament(1% CaO+H₂O) to the excision cuts.The measure is very economical.