Uromyces valerianae is a gall-causing rust fungus affecting Common Valerian and Marsh Valerian. There is no host alternation.  Aecia are borne in clusters, followed by brown uredinia and (rarely) dark brown telia.

Puccinia commutata also affects Valerian; it is described as having large black telia and being 'very rare' in Redfern and Shirley, 2023. 

host

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

On Valerian

Note: there is discrepancy between the information on 'Plant Parasites of Europe', Malcolm Storey's 'BioImages' website and Redfern & Shirley 2023 - 'British Plant Galls'.  We have followed the latter source.