Arthur, (1906) Melampsora medusae is a fungal pathogen, causing a disease of woody plants.

The disease affects mostly conifers, e.g. the Douglas-fir, western larch, tamarack, ponderosa, and lodgepole pine trees, but also some broadleaves, e.g. trembling aspen and poplars.

[1] Symptoms usually are contained to a single year on conifers, shedding the affected needles in fall.

To survive the winter Melampsora medusae remain as teliospores on the dead leaves of the host, coming back in the spring to be spread by the wind as basidiospores, and infecting new conifers.

Those spores serve as inoculum for an infection in live trembling aspen and other poplar trees in another two weeks.