Orange aecia cause galls on the leaf blade and petioles of the alternate hosts, Buckthorn and Alder Buckthorn, causing them to become twisted and distorted.

The primary hosts, where uredinia and telia are produced, are grasses. 

This is likely to be a complex of species.

Several rust species affect grasses.

Buckthorn grasses

Photograph the rust on the host.  Ensure you have noted the host-plant species in your comments.  If you are recording this species on the primary host - a grass - microscopic examination will be needed.

Alternating generations on Buckthorn (Rhamnus cathartica) and a wide range of grasses.