Host: Populus
ID based on host association. According to my research, Liu and Braun (2022) comment that "Erysiphe adunca s. str. is confined to Populus spp. as hosts and is also widespread and common in North America"
iNaturalist observation

Reference
Liu, M. & Braun, U. 2022. Powdery mildews on crops and ornamentals in Canada: a summary of the phylogeny and taxonomy from 2000 – 2019, Canadian Journal of Plant
Pathology, 44:2, 191-218, DOI: 10.1080/07060661.2021.1986744