Japanese spiraea (Spiraea japonica L.f., Rosaceae) is native to Japan, China, and Korea. It is naturalized throughout North America and Europe and has the potential to become an invasive weed. Nevertheless, many varieties are widely planted for their ornamental value worldwide. The structures were typical of the powdery mildew Euoidium anamorph of the genus Podosphaera. To confirm the identity of the fungus, the complete internal transcribed spacer (ITS) region of ribosomal DNA from KUS-F28608 was amplified and sequenced using universal primer pair ITS1 and ITS4. The resulting sequence of 670 bp was deposited in GenBank (Accession No. KT290556). A pathogenicity test was confirmed through inoculation by gently pressing a diseased leaf onto five healthy leaves of potted 2-year-old seedlings. Five noninoculated leaves served as controls. The fungus presented on the inoculated plants was morphologically identical to that originally observed on diseased plants. Our field observations suggested that powdery mildew infections pose a serious threat to health of this plant, especially in shady areas of the gardens.