Technical Abstract: Erysiphe flexuosa, a powdery mildew parasite of Aesculus species, is believed to have originated in North America where distribution records were confined to regions east of the Rocky Mountains. The fungus recently was found in eastern Washington State and northern Idaho. The fungus can be distinguished readily from other powdery mildews attacking Aesculus species by the presence of two distinctive kinds of chasmothecial appendages. Consistent with previous reports on similar species, observations determined that subulate appendages formed first, on dorsal sides of chasmothecia. The longer, uncinate appendages formed equatorially during later chasmothecial maturation. There are no Aesculus species native to northwest North America that could have hosted an undetected population of the fungus. It appears likely that E. flexuosa became established relatively recently in the inland Pacific Northwest, possibly being introduced on infected host material, similar to the situation in Europe where several reports documented the presence of the fungus beginning in 2000.