This rust fungus affects the alternate hosts, Lords-and-Ladies (Arum maculatum), Ramsons (Allium ursinum) and sometimes Orchids, causing yellow to orange circular patches on leaves. On the underside it produces raised orange aecia.  It is not considered to be a gall-causer.

The primary host is the Canary-grass, Phalaris, and other grasses, where yellowish-brown uredinia and small black telia are produced.

Spermosporina aricola or Cercospora (Ramularia) ari cause pale brown or yellow spots on Arum leaves, bearing small conidia.

Arum/Ramsons grass

Photograph the rust on the host; if on Arum or Ramsons, photograph both sides of leaf, showing the aecia.  If the host is a grass, microscopic examination of a specimen may be needed.  Ensure you have noted the host-plant species in your comments.

Primary stage on grasses; more usually recorded in the aecial stage on Arum or Ramsons (Wild Garlic)