The taxonomic status and phylogenetic placement of Sporidesmium pachyanthicola in Capnodiales (Dothideomycetes) are revisited based on a specimen collected on the petiole of a dead leaf of Sabal palmetto in south Florida, U.S.A. New evidence inferred from phylogenetic analyses of nuclear ribosomal DNA sequence data together with a broad taxon sampling at family level suggest that the fungus is a member of Extremaceae and therefore its previous placement within the broadly defined Teratosphaeriaceae was not supported. A new genus Castanedospora is introduced to accommodate this species on the basis of its distinct morphology and phylogenetic position distant from Sporidesmiaceae sensu stricto in Sordariomycetes. The holotype material from Cuba was found to be exhausted and the Florida specimen, which agrees well with the original description, is selected as epitype. The fungus produced considerably long cylindrical to narrowly obclavate conidia in culture strongly resembling those of Sporidesmajora pennsylvaniensis, another sporidesmium-like, capnodiaceous anamorph. However, phylogenetic analyses show that they are not congeneric and the latter belongs to the family Phaeothecoidiellaceae.