Powdery mildew on cherry bark oak (Quercus falcate var. pagodifolia) collected in Tennessee, USA, was determined to be Erysiphe abbreviata, a species confined to North America. The diagnostically important anamorph of this species is described for the first time. Sequence analyses of the rDNA ITS region and D1/D2 domains of the 28S rDNA were used to obtain phylogenetic data for and taxonomic conclusions about this species. The structure of the anamorph (Oidium subgen. Pseudoidium) and the molecular data support the placement of this species in Erysiphe emend. (including Microsphaera) as a species separate from the Eurasian Erysiphe alphitoides.