Not sure on ID, on the exact same host tree (Quercus agrifolia) as previous observation with Cystotheca lanestris.
Coast Live Oak
On Quercus agrifolia leaf underside.
Maybe same taxa as seen and photographed under microscopy here:
https://www.inaturalist.org/observations/190763572
This is a picture of Podosphaera pannosa on a multiflora rose at Sandy Point State Park near Annapolis, Maryland.