Phragmidium bulbosum is a gall-causing rust fungus that affects the leaves of various Rubus species (Blackberry, Dewberry etc.). There is no host alternation.  It is usually recorded as telia, which are black and in the leaf undersides;  the teliospores have 4 - 7 cells, a long pedicel and an apiculus.  Spermogonia are on the leaf upperside, and minute yellow aecia underside.

Phragmidium violaceum on Bramble

host/telia

Photograph the rust on the host.  Ensure you have noted the host-plant species in your comments.  We recommend microscopic examination of teliospores, or confirmation by an expert.

Brambles

See species page on Plant Parasites of Europe ('Bladmineerders')