Melampsora medusae is a fungal pathogen, causing a disease of woody plants. The infected trees' leaves turn yellowish-orange. 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. Coniferous hosts are affected in late spring through early August, and trembling aspens and poplars from early summer to late fall. It is one of only two foliage rusts that occur naturally in British Columbia.