In a study on taxonomy and pathology of fungal species associated with oak trees showing canker, defoliation, dieback, gummosis, wilting, and decline symptoms in Zagros forests, we found 12 isolates morphologically resemble Nigrospora spp., with the same ISSR fingerprinting patterns generated by M13 primer. Thus, a representative isolate IRAN 4332C was selected for phylogenetic analyses based on ITS, tef1, and tub2 DNA sequence data. Multigene phylogenetic analyses characterized isolate IRAN 4332C as Nigrospora shadeganensis. Weak leaf necrosis and weak irregular wood necrosis were recorded in pathogenicity tests performed on leaves and stems of two-year-old Quercus brantii seedlings in vitro and under greenhouse conditions.