Systemic infection of <i>Bryoria</i> (Lecanoromycetes, Ascomycota) by <i>Athelia</i> (Agaricomycetes, Basidiomycota) in western North America

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