This rust fungus causes leaf spots bearing uredinia and telia on Sow-thistles (Sonchus). The uredinia are surrounded by paraphyses.  There is no host alternation.  The leaves may be slightly deformed, but this is not a gall-causing species.

other microfungi can cause leaf-spots on sow-thistles

host

Photograph the rust on the host.  Ensure you have noted the host-plant species in your comments.  Microscopic examination may be needed.

on sow-thistles