A rust on Fool's Parsley, Aethusa cynapium, and other plants in the umbellifer family - see 'Plant Parasites of Europe' for full list of hosts.  There is no host plant alternation, and no aecia.   Light brown uredinia on the leaf undersides are folllowed by darker spots that bear a mixture of teliospores and urediniospores.  The teliospores are darker brown, two celled, with a short hyaline pedicel. The plant may be yellowed and stunted. 

Simialr rusts affect other specesi n the Apiaceae or umbellifer family; however, this is the only rust species listed for Fool's Parsley (ref. Plant Parasites of Europe) 

host

Photograph the rust on the host plant; ensure you have noted the host species in your comments.

recorded on Fool's Parsley in VC55; also said to affect some other umbellifers 

https://bladmineerders.nl/en/host-plants/plantae/spermatopsida/angiosperma/eudicots/superasterids/asterids/campanulids/apiales/apiaceae/apioideae/selineae/aethusa/aethusa-cynapium/