A downy mildew occurring on speddwell, commonly on Veronica serpyllifolia, the Thyme-leaved speedwell and also on Brooklime Veronica beccabunga and other species.  The whole plant is affected, becoming pale yellow, but may also flower as usual.  The greyish mildew is seen on the leaf undersides

other downy mildew can occur on speedwells - e.g. Peronospora agrestis on various species, and P arvensis on Ivy-leaved Speedwell.  See Chater et al (2020) for more inmformatin on hosts.

host

Photograph the host plant, showing yellowed shoots; and also mildew on both sides of leaf; ensure you have noted the host plant species in your comments.

on speedwells