A powdery mildew affecting birch (Betula). The chasmothecia and are produced on the mycelium on the underside of the leaf, visible as minute spheres later in the season.  These are yellow or brown when developing and black when mature.  The chasmothecial appendages are bulbous at the base, straight and entire.

Erysiphe ornata var. europaea is also present on Birch; the chasmothecial appendages are branched at the tip and not swollen at the base,  Other Phyllactinia species are on other host plants.

host/microscopy

Photograph the mildew and chasmothecia on host plant, and add host species to the comments for your record.  Microscopic examination or verification by an expert before submission to NatureSpot will be needed

on Birch