The primary host of the rust fungus Melampsora caprearum is sallow, causing galls on the leaves; Salix caprea, S. cinerea and S. aurita; rarely Salix daphnoides, purpurea or viminalis. The leaf blade or veins become distorted with irregular thickened spots bearing orange-yellow uredinia and later on, telia.  

The alternate host bearing spermogonia and aecia is Larch; it is not a gall-causer on this host.

Several other species of Melampsora have Salix species as their primary host, bearing uredinia and telia, with various aecial hosts - e.g. Melampsora epitea complex. Only M caprearum and M amygdalinae cause galls; M amygdalinae is confined to Almond willow (Salix triandra) 

gall/host

Photograph the rust and galls on the host.   Ensure you have noted the host species in your comments.  If the rust is not gall-causing, microscopic examination is needed.

alternating Sallows and Larch