The rust affects the leaves of Birch and Alder as its primary host, producing powdey yellow uredinia on leaves, usually the under-surface.  The telia are produced within the leaf (sub-epidermal). 

The alternate aecial host is Larch.   It is not considered to be gall-causing on either host.

host

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

alternates between Larch and Birch trees