Q. cerris, Q. tourneyi, and Q. laceyi added per abstract of Bradshaw et al..

See this observation on iNaturalist, showing this powdery mildew on cultivar oak, along with DNA test results and comment by Senna Robeson that "dense, thick, rough brown growth is characteristic of [Cystotheca]."

According to Swiecki & Bernhardt, A field guide to insects and diseases of California oaks (2006), "Cystotheca lanestris (= Sphaerotheca lanestris) is widespread and has been reported from most oaks in California."