A powdery mildew on Beech.  The large chasmothecia are on the leaf undersides, and have needle-like appendages with bulbous bases; these may be seen with a low-power microscope.

The oak mildew Erysiphe alphitoides can also occur on Beech - close examination of chasmothecia needed.  

microscopy/host

Photograph the chasmothecia on the host, the straight appendages with swollen bases must be evident in your photos;  Note the host plant species in your comments

Beech