Various species of Poplar are the primary hosts, bearing yellow uredinia mainly on the leaf undersides; there are corresponding yellow spots on the upper side.  These are considered to be gall-causers.

The alternate host is Larch, bearing the spermogonia and aecia.   It is not considered to be a gall-causer on either host.

Melampsora allii-populina also can affect Black poplars, Balsam poplars and theri hybrids.

host.microscopy

Photograph the rust on the host.  Ensure you have noted the host-plant species in your comments.  Microscopic exmaination is needed to spearate the two species on Black and Balsam poplars and hybrids.

alternates beween Larch and Poplars (Black Poplar and hybrids, and Balsam Poplar plus hybrids and related species)