Quercus aliena commonly known as oriental white oak, is an important ecological and economic species endemic to East Asia. In 2022, the leaves of tree were found to be infected with a powdery mildew fungus at two locations of Imsil, Korea. To date, Erysiphe alphitoides and Cystotheca lanestris were identified as the casual agents of powdery mildew disease on Q. aliena in Korea. Preliminary examination indicated that the fungus belongs to the section Typhulochaeta of the genus Erysiphe due to the presence of clavate ascomatal appendages, without conidial state. The presumed species, Erysiphe japonica has only been found on various Quercus species in East Asia, except Korea. Hyphae, hyphal appressoria and aerial hyphae surrounding the chasmothecia of this species were not characterized before. Therefore, this study was conducted to describe the occurrence of powdery mildew on Q. aliena in Korea, as well as to provide detailed morphological characteristics and molecular phylogeny. Under the latest taxonomic concept, E. japonica was split into two varieties: var. japonica and var. crispulae. No morphological differences were found between these varieties, only specialization on host species. The Korean samples on Q. aliena were finally identified as E. japonica var. japonica based on the morphological characteristics and molecular-phylogeny retrieved from the internal transcribed spacer region and large subunit gene of the rDNA. This is the first report of E. japonica var. japonica in Korea and the third report of this powdery mildew on Q. aliena, with the current fungus-host association spreading to China and Japan.