A bright orange rust fungus affecting some non-native members of the Sorrel family, including those with pink flowers (Oxalis rosea, O articulata, O debilis, O latifolia).  Yellow uredinia are borne on the underside of the leaves. Telia are minute and rarely seen; aecia and spermogonia unknown in Europe.   It does not cause galls, and does not affect the native Wood-sorrel Oxalis acetosella or the small-leaved yellow sorrels O corniculata and O exilis.

host

Photograph the rust on the host.  Ensure you have noted the host-plant species in your comments

parks, gardens, waste ground