The alternate or aecial hosts are Allium or Arum species.  The primary hosts are Populus species, including Black Poplar and hybrids, White and Grey Poplar and Balsam Poplar.   Orange uredinia are formed on the leaves.  It is not listed as gall-causing on either host. 

Uredinia of other species of Melampsora can be found on Grey and White Poplars and Aspen, including Melampsora laricis-tremulae in the Melampsora populnea species complex.  Uredinia of Melampsora laricis-populina are present on Black Poplars and their hybrids, and on Balsam Poplars.

Allium and Arum can be the alternate or aecial hosts of several other species of Melampsora

host/microscopy

Photograph the rust on the host-plant; ensure you have note the host species in your comments.  Microscopic examination of aecia or uredinia will be needed, or confirmation by an expert before submission to NatureSpot.

Alternating Allium/Arum and Populus