A downy mildew affecting Zigzag Clover, Trifolium medium.  The mildew is under the leaves, and causes a corresponding yellow-green patch on the leaf upperside.  This can appear as a series of broad, angular diagonal stripes on the leaves

Other species of Downy Mildew affect similar species of clover

host

Photograph the mildew on the host plant; ensure you have identified the clover and have included a note of the host plant species in your comments

grasslands, on Zigzag Clover