Plasmopara angelicae

A downy mildew affecting Angelica.  It causes yellow blotching on the upper leaf surface and a whitish downy mildew on the lower surface.  This is not considered to be gall-causing.

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 

Angelica

This is part of the Plasmopara crustosa s.lat. species complex of downy mildews that affect host plants in the Apiaceae (umbellifer) family.  The compex has recently been split into several species depending on host.  See https://www.aber.ac.uk/waxcap/downloads/Chater20-DownyMildewsWales.pdf