Abstract:Tungoil trees dieback is a serious twig disease of Aleurititis fordii Hemsl. in Siehuan. It caused a large quantity of dead twigs. According to its morphology, cuhural, and artificial inoculation, the pathogen has been identified a new species of Nectria aleuritidia sp. Nov.Perithecia are gregarious in groups of 13—19 and usually develop on bursted lenticel. They are globose, or ellipsoid with a pointed ostiolar when mature, red, later become reddishbrown in colour, with a smooth wall and measure 223.1×197.0μm in diameter.The section, the lateral wall of perithecium is 44—53μm wide and formed of two indistinct regions. The outer is 33—38μm, has thickwalled cells, globose and variation in the shape, 4—9×3—4μm. The inner is formed of 4—5 layers of thinwalled elongate cells, 9—13×2—3μm.The asci are clavate, without an apical collar, and with 8 obliquely monostichous to apically distichous ascospores, they measure 58.1×12.7μm (50.6—91.4×12.7—16.6μm).The ascospores are hyaline, ellipsoid, with non constriction or a slight constriction at the central septum. When the spores are mature, they measure 15.7×7.1μm (11.1—23.0×5.5—9.02μm.)