A downy mildew on Cleavers (Galium aparine).  A greyish brown mildew is under the leaves, and the whole plant may be pale and yellowish.

host

Photograph the host plant and the mildew under a leaf; ensure you have noted the host plant in your comments

on Cleavers (Galium aparine): hedgrerows, roadsides, field edges, waste ground, etc,