In 2015, a powdery mildew caused by a fungus belonging to Erysiphe sect. Uncinula was recorded on two species of ash, Fraxinus excelsior and F. pennsylvanica (Oleaceae), from Ukraine (Kyiv, two localities). Based on the comparative morphological analysis of Ukrainian specimens with samples of Erysiphe fraxinicola and E. salmonii collected in Japan and the Far East of Russia, the fungus was identified as E. salmonii. This identification was confirmed using molecular phylogenetic analysis. This is the first report of E. salmonii not only in Ukraine but also in Europe. It is suggested that the records of E. fraxinicola from Belarus and Russia could have been misidentified and should be corrected to E. salmonii. In 2016, the fungus was found not only in Kyiv but also outside the city. The development of the fungus had symptoms of a potential epiphytotic disease. Thus, it may become invasive in Ukraine and spread to Western Europe in the near future.