Bryoria (Parmeliaceae, Ascomycota) is one of the dominant genera of hair lichens in western North America and is characteristic of high-elevation conifer forest ecosystems. In areas where Bryoria is abundant, it is common to find thalli in which the thalline filaments become conglutinated, forming brittle dead zones. After sampling Bryoria thalli across western Canada and the northwestern United States at different times of the year, we found this dieback phenomenon is associated with the winter growth of a mold-forming basidiomycete. We report that this fungus belongs to Athelia (Atheliaceae, Basidiomycota) a genus known to contain lichen pathogens, most notably A. arachnoidea. By sequencing a combination of genetic markers – nuc rDNA ITS1-5.8S-ITS2 (ITS), partial nuc 28S rDNA (28S), and partial translation elongation factor 1-α (TEF1) – paired with morphometric analyses, we reveal the involvement of at least three additional lineages of lichen-associated Athelia and describe one as a new species, Athelia abscondita. A. abscondita is morphologically distinguished from other Athelia species by its basidia and basidiospores, was found to frequently infect members of Bryoria sect. Implexae, and was occasionally on other foliose and fruticose species within Parmeliaceae.