There is no host alternation with this species of rust.  On Purging or Fairy Flax (Linum catharticum) yellow uredinia are formed, on stems and both sides of the leaves; telia are present later, but there are no spermogonia or aecia.  On other species of Flax, aecia and spermogonia are present as well.  It is not a gall-causer.

There is some disagreement regarding taxonomy; some authors consider this species only affects Linum catharticum, with Melampsora liniperda being the species that affects other Linum, including commercially grown species. 

host

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

on Linum (flax) species