Rust of linseed and flax (autoecious rust) is caused by

Rust of linseed and flax (autoecious rust) is caused by Melampsora lini.
The pathogen is an obligate biotroph meaning that each stage of its cycle is dependent on having a living host. The direction between reproducing sexually or asexually is dependent on the availability of a host.
Sexually, the life cycle starts when basidiospores land on flax leaves and form pycnium during December and through January.