The rust fungus Puccinia glechomatis galls the leaves of Ground-ivy (Glechoma hederacea) affecting the stem, petiole or underside of the leaf veins. The yellow, brown or black swellings bear telia.  There is no host-plant alternation, and only telia are produced.

host

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

on ground ivy; hedgrows, woodlands, shady places