Hi, and welcome to UK Fungi.
This could be interesting!
It's unfortunate that your second photo is slightly out of focus, as I can't tell with any certainty that what I'm seeing is actually a rust, rather than the results of some insect activity. However, it does seem to show what may be "cupulate / cluster-cup" shaped aecia.
On the assumption that it is indeed a rust, it is very unfortunate that you didn't collect samples. Since, as far as I can see, there are no British records on the FRDBI, specifically in respect of rusts associated with Pastinaca sativa
According to what I've read, P.sativa
is affected by various smuts, mildews, and chitrids, - but none of them particularly resemble what is shown in your photos.
However, there are records (within Europe and elsewhere, but not Britain), of the rust Uromyces lineolatus
having association with P.sativa
Apparently it is stages 0, and 1, of U.lineolatus
which have that particular association, and stage 1 does produce cupulate aecia on the host.
See: - http://www.bladmineerders.nl/gallen/fun ... olatus.htm
An internet search brings up several websites where records of the association are confirmed,
(for example: -https://www.idigbio.org/portal/records/ ... 02ba061246
but I haven't been able to find any comparable "macro" photos specifically showing E.lineolatus
aecia on P.sativa
Hopefully Chris will see this thread, and will hopefully be able to provide some more constructive input!