Erysiphe adunca is found on various species of willow and sallow (Salix) and Polar (Populus).  The white fibres of the mycelium are on the surface of living leaves.   The small spheres (chasmothecia) contain the asci and spores; colours range from white when very immature through yellow, orange, reddish brown, dark brown and finally black when fully mature.    

Erysiphe capreae is found on Salix caprea (Goat Willow or Common Sallow) but not on other Salix species.

host

Photograph on host plant, and add host species to the comments for your record. If this is on Salix caprea, the Common Sallow or Goat Willow with broad leaves, microscopic examination is needed.

On the leaves of Salix and Populus.

August to October.