Puccinia chaerophylli is a gall-causing rust fungus that affects the leaves and stems of umbellifers such as Cow Parsley and Sweet Cicely.  Leaves, stems, petioles  and shoots are swollen and distorted by yellow aecia and spermogonia; rust-coloured uredinia and brown telia also occur, usually on the leaf undersides or stems

On some umbellifer hosts, the gall-causing Uromyces lineolatus may be present but has not been recorded in VC55

host

Photograph the rust on the host.  Ensure you have noted the host-plant species in your comments.

on umbellifers - including Cow Parsley, Upright Hedge-parsley and Sweet Cicely