Ian Morgan sent me a picture of a rust on Greater Burnet (Sanguisorba officinalis), determined by Nigel Stringer as the localised Xenodochus carbonariusat Ynys Tawela near Brynamman.
Yesterday I noticed another very conspicuous rust growing on Burnet-rose (Rosa pimpinellifolia) at Whiteford, which turned out to be the rather long-winded Phragmidium rosae-pimpinellifoliae. I also noted another on Eyebright (Euphrasia tetraquetra) that I was unable to identify.
Rusts are pathogenic fungi that affect a wide range of plant species and once you start looking it is surprising how many infected species you can find. Many are gall-forming and disperse by producing yellow, orange or brown telia that liberate aerial basidiospores, thereby spreading the infection to new host plants.