Racodium Fr., Syst. Mycol. (Lundae) 3(1): 229 (1829).
Index Fungorum number: IF 9681; Facesoffungi number: FoF 06305, 5 morphological species (Species Fungorum 2022), 1 species with molecular data.
On inclined to vertical siliceous rocks in recesses. Hyphae vertically arranged, lacking interlocking corrugations, lacking lateral spines, thick-walled, fused to form elongated rectangular cells, vertically orientated cells along the axis of the algal filament, overall smooth, rather than a knobby appearance, completely smooth in SEM (adapted from Hongsanan et al. 2020a).
Type species: Racodium rupestre Pers.
Notes: Racodium is characterised by vertically arranged hyphae which are fused to form elongated rectangular cells, overall smooth, rather than a knobby appearance. Vainio (1921) mentioned that Coenogonium germanicum is a synonym of R. rupestre. Lepraria and Leproloma are often associated with Racodium rupestre, and Cystocoleus ebeneus (Hawksworth et al. 2011). Racodium can be differentiated from Coenogonium by using nitric acid, as the latter causes the filament of Racodium to twist in Coenogonium but the dark hyphae in Racodium are parallel (Hawksworth et al. 2011). Racodium is phylogenetically not related to Cystocoleus but forms a distinct lineage in Capnodiales (Muggia et al. 2008). Racodium is a distinct genus accommodated in its own family Racodiaceae. Molecular markers available for Racodium are ITS, LSU, TEF-1, RPB2 and SSU.