The rust fungus Puccinia poarum galls the leaves of the alternate host, Colt's-foot (Tussilago farfara). The underside of the leaves have thickened areas bearing orange-red aecia and spermogonia. 

The primary host is the meadow-grass, Poa, bearing yellow uredinia mainly on the leaf upperside and brown telia mainly on the underside.

Coleosporium tussilaginis also affects Coltsfoot, but is not a gall-causer

Coltsfoot Poa

Photograph the rust and the gall on the host.  If recording on Coltsfoot, photograph both sides of leaf. If recording on Poa grass, microscopic examination will be needed.  Ensure you have noted the host-plant species in your comments

Alternating stages between Coltsfoot and a Poa grass

Widespread and locally frequent in Britain.

Status in Leicestershire and Rutland not known.