Cercospora mercurialis

A fungus causing circular or irregular pale brown or greyish spots on the leaves of Dogs Mercury.  The spots have a darker border and are usually on the under-surface of the leaf.  Eventually they drop out of the leaf, leaving shot-holes.  On the surface of the leaf-spots are clusters of conidiophores, visible as minute brown specks 

host

Photograph the host plant leaf, and a close-up of the leaf spot on the leaf underside, showing conidiophores. Note the host plant species in your comments

on Dogs Mercury - woodlands, hedgerows, shady places.