Uromyces polygoni-avicularis affects plants in the family Polygonaceae, usually being recorded on Knotgrass (Polygonum aviculare). There is no host-plant alternation.  Aecia are borne in the leaf undersides; pale brown uredinia and dark brown telia mainly under leaves but also on stems.  It causes some leaf distortion but is not considered to be a gall-causer.  (ref Redfern & Shirley 2023)

Knotgrass

Please provide a photograph and state the host species on which the rust was found.  On some hosts, microscopic examination may be needed.

Knotgrass, and other speces of Polygonum, Persicaria and Rumex in the Polygonaceae family.