Phyllactinia guttata is a powdery mildew found on Hazels (Corylus).  The sexual stage (teleomorph) produces chasmothecia in 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 chasmothecia appendages of Phyllactinia are straight and needle-like and swollen at the base; they are just visible under low magnification.  

other Phyllactinia affect different host species

host

Photograph the mildew and chasmothecia on host plant, and add host species to the comments for your record. 

Hazel