Deutzia parviflora Bunge (Hydrangeaceae), also called Mongolian Deutzia, is native to northern China and Korea. During summer and autumn of 2022, about three hundred D. parviflora of unknown cultivar planted as a flowering hedge shrub in a public garden in Wanju, Korea, were infected with powdery mildew. Although the disease incidence (number of shrubs infected) was nearly 100%, disease severity (number of leaves infected) was less than 5%. Two representative specimens were preserved in the herbarium of Korea University, Seoul, Korea (voucher nos. KUS-F33345 and F33555). Powdery mildew infection results in loss of aesthetics with reddish-brown discoloration on old leaves and white mycelial mats on young leaves and tender shoots. Conidiophores were cylindrical, 3–4-celled, 54–92 × 7–9 μm, and produced conidia singly. Conidia were ellipsoid to doliiform, 24–32 × 14–17 μm, and devoid of conspicuous fibrosin bodies. Chasmothecia were not found until early winter. These characteristics were consistent with the anamorph of Erysiphe deutziae (Bunkina) U. Braun & S. Takam. (Braun and Cook 2012). The nucleotide sequences of the internal transcribed spacer regions (ITS1 and ITS2) and the large subunit (LSU) of the rDNA gene were determined in this study according to Bradshaw and Tobin (2020) from KUS-F33555. The obtained sequences of the ITS and LSU were 100% identical to those of E. deutziae (MW600366, MW600367) in GenBank. The sequences of amplicons were subsequently deposited in GenBank (Accession Nos. OQ851757 for ITS, and OQ851758 for LSU). Although E. deutziae has been recorded on several Deutzia spp. in East Asia and Europe, to date D. parviflora has only been recorded as a host plant for this fungus in Russia (Braun and Cook 2012) and China (Nguyen et al. 2018). To our knowledge, this is the first report of E. deutziae on D. parviflora in Korea.