Melampsora hypericorum is an orange rust fungus that attacks Hypericum species, most notably Tutsan (Hypericum androsaemum), forming yellow aecia on the leaf undersides, with corresponding yellow or reddish patches above.  There is no host alternation, and no uredinia are formed; telia are rare.  It is not a gall-causer.

host

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

on Hypericum