The fungus Puccinia polygoni-amphibii causes galls on the leaves of the alternate host, a member of the Geraniaceae family.  Spermogonia and aecia are borne on the leaves.

The primary hosts are in the family Polygonaceae (Persicaria, Fallopia and Polygonum), the leaves bearing uredinia and telia.  These are not considered to be gall-causers on this host.

host

Photograph the rust on the host.  Ensure you have noted the host-plant species in your comments.  On some hosts, microscopic examination may be needed.

Alterating stages between plants in the Cranesbill (Geraniaceae) family, and plants in the Knotgrass (Polygonaceae) family