Remarks: Stereodiscus is characterized by smooth, thin-walled, cylindric, broadly ellipsoid to citriform basidiospores up to 25 μm long and absence of any of the typical cystidia in Aleurodiscus s. lat., i.e., acantho-cystidia, acanthophyses, gloeocystidia, and dendrohy-phidia, but with skeletocystidia present in three species (S. antarcticus, S. parmuliformis, and S. trivialis). The hyphal system is monomitic with simple-septate hyphae, except in the recently described S. patagonicus (as Aleu-rodiscus), which features scattered clamps on the hyphae and at the bases of basidia (Phookamsak et al. 2019). The latter is consistent with its phylogenetic position, as it is excluded from the main cluster of species (FIG. 1). Otherwise, the five species grouped with strong support.
Stereodiscus is most closely related to Gloeosoma (FIG. 1). Gorjón et al. (2013) suggested the cluster of Aleurodiscus antarcticus, A. parmuliformis, and A. trivialis warranted segregation at the genus level characterized by possession of simple-septate hyphae and presence of skeletocystidia and geographic distribu-tion occurring only in the Southern Hemisphere. Our analyses show that two other taxa are included in this group, widening its morphological definition, and it indeed deserves generic rank.