Four morphologically similar specimens of an unidentified perithecial ascomycete were collected on decaying wood submerged in fresh water. Phylogenetic analysis of DNA sequences from protein-coding and ribosomal nuclear loci supports the placement of the unidentified fungus together with Achroceratosphaeria in a strongly supported monophyletic clade. The four collections are described as two new species of the new genus Pisorisporium characterised by non-stromatic, black, immersed to superficial perithecial ascomata, persistent paraphyses, unitunicate, persistent asci with an amyloid apical annulus and hyaline, fusiform, cymbiform to cylindrical, transversely multiseptate ascospores with conspicuous guttules. The asexual morph is unknown and no conidia were formed in vitro or on the natural substratum. The clade containing Achroceratosphaeria and Pisorisporium is introduced as the new order Pisorisporiales, family Pisorisporiaceae in the class Sordariomycetes. It represents a new lineage of aquatic fungi. A sister relationship for Pisorisporiales with the Lulworthiales and Koralionastetales is weakly supported by Bayesian inference and maximum likelihood analyses. The systematic position of Pisorisporium among morphologically similar perithecial ascomycetes is discussed.
Keywords: Achroceratosphaeria; Hypocreomycetidae; Koralionastetales; Lulworthiales; freshwater; multigene analysis; systematics.