The fungus Puccinia phragmitis causes galls on the alternate host, Docks (Rumex) species and Rhubarb (Rheum) causing reddish purple blotches bearing aecia.

Later stages occur on the primary host, the Common Reed (Phragmites); the brown uredinia rupture and black telia the upper and lower surfaces of leaves causing dark brown patches often surrounded by an area of yellowing of the leaf.

Puccinia acetosae affects some species of Rumex, but does not cause galls, and aecia are not present

Rumex/gall Phragmites

Photograph both sides of leaf, showing galls and white aecia if on a dock or sorrel species.  If recording on Phragmites microscopic examination may be needed.  Ensure you have noted the host species in your comments

Alternating on Rumex (dock or sorrel) leaves and on Phragmites, Common Reed