Peronospora conglomerata is a downy mildew which affects  Dove's-foot Cranesbill, Geranium molle.  The underside of the leaves have greyish-white down, with a corresponding yellowing patch on the leaf upper surface.

host

Photograph both sides of the leaf and the downy mildew on the host plant; ensure you have noted the host species in your comments 

Dove's-foot Cranesbill