Downy Mildews are fungus-like organisms affecting many species of plants.  Peronospora chenopodii causes galls in the form of raised thickened blotches on the leaves of fat-hen and related species.  The blotches are pale yellow-green flushing pink on the upper leaf surface; underneath the leaves have shallow depressions lined with greyish-purple down.  The down is made up of a colony of branching sporangiophores bearing the sporangia.  These disperse to form new colonies. 

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 

arable land, waste land, cultivated area, roadsides, on Chenopodium and Oxybasis (Goosefoots)

For more information on structure and life-history, see Chater, A.O et al (2020) Downy Mildews (Peronosporaceae) and and White Blister‐rusts (Albuginaceae) of Wales . publ. A O Chater