The rust fungus Puccinia iridis affects both surfaces of the leaves of various types of the primary host, Iris, producing brown uredinia and blackish telia.  These are not listed as gall-causers in Redfern & Shirley (2023)

The alternate host, producing white aecia and spermogonia, is either Valeriana (Valerian) or Urtica (nettle); it is not gall-causing on these hosts.

host

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

On Iris leaves, frequently recorded on Stinking iris (I foetidissima); also Yellow Flag-iris (I pseudoacorus) and Bearded Iris in gardens (I x germanica); may be present in VC55 on other species.