Podosphaera ferruginea is a powdery mildew which affects Sanguisorba officinalis (Great Burnet) and Poterium sanguisorba (Salad Burnet and Fodder Burnet), 

host

Photograph the mildew on host plant, and add host species to the comments for your record. 

Great Burnet, Salad Burnet and Fodder Burnet (Sanguisorba and Poterium)

There is some taxonomic confusion between this species and Podosphaera fuliginea.  They have been treated as synonyms in the past. In Woods et al (2024) they are separated according to host: P ferruginea on Burnets (Sanguisorba and Poterium) and P fuliginea on speedwells (Veronica).  As we are following Woods et al for taxonomy, we have separated the two species; there are also now two separate species on the NBN.