Uromyces geranii

Uromyces geranii is a rust fungus that galls the leaves of various Geranium and Erodium species (Crane's-bills and Stork's-bills).  There is no host-plant alternation.  Aecia are orange, on leaf undersides and petiole; later pale brown uredinia and darker telia appear on the leaf underside. 

The aecial stage of Puccinia polygoni or polygoni-amphibiii galls some species of Cranesbill, including Geranium dissectum.  The galls have  cylindric aecia mostly on leaf undersides, with narrow recurved margin split into small segments on intensely reddened parts of the leaves. initially within pale green leaf spots.  (ref.  'Plant Parasites of Europe').   

host

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

on plants in the Geraniaceae family