A downy mildew causing galls on various species in the Amaranthaceae family.  The affected leaves become swollen and distorted with yellowish or purplish patches on upper leaf surface.  This species has now been split in several species, depending on host: see 'Further Information' below, and Chater et al (2020) for more details.

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 

on plants in the Amaranthaceae family (Fat-hen, Oraches, Goosefoots, ec.)

The species complex includes the following species - ref. Chater et al (2020)

• Peronospora minor on Atriplex (Oraches)
• Peronospora schachtii on Beta (Beet)
• Peronospora chenopodii on Chenopodium (Fat hen, etc). and Oxybasis (Red Goosefoot).  
• Peronospora chenopodii-polyspermi on Lipandra (Many-seeded Goosefoot)