Puccinia arenariae affects the leaves and sometimes the stems of a wide range of plants, most often recorded on Campions (Silene), where it is not considered to be a gall-causer.  Redfern & Shirley (2023) list Chickweed (Stellaria media) and Pearlwort (Sagina) as being galled by this species.  Only brown telia are produced, on the leaf undersides.   The teliospores are two-celled, with a persistent pedicel and pointed tip; the cell wall at the tip is clearly thickened.

Puccinia behenis on campion is amost identical in the field.  Teliospores need to be examined to tell these two species apart on campions

host/microscopy

Photograph the rust on the host.  Ensure you have noted the host-plant species in your comments.  If the rust is on campion, microscopic examination is needed.

Various plants in the Caryophyllaceae family, including Campion (Silene) Mouse-ear Chickweeed (Cerastium), Pearlworts (Sagina) and Stitchworts or Chickweeds (Stellaria) - see Plant Parasites of Europe ('Bladmineerders') website for full host list.