Numerous taxa of Hebeloma have been reported in association with Salix, Dryas, and Betula in arctic-alpine habitats. However, species are notoriously difficult to delineate because morphological features overlap, and previously there was little reliable molecular data available. Recent progress in ITS-sequencing within the genus, coupled with an extensive database of parametrically described collections, now allows comparisons between species and their distributions. Here we report 16 species of Hebeloma from the Rocky Mountain alpine zone from some of the lowest latitudes (latitude 36°–45°N) and highest elevations (3000–4000 m) for arctic-alpine fungi in the northern hemisphere. Twelve of these species have been reported from arctic-alpine habitats in Europe and Greenland and are now molecularly confirmed from the Middle and Southern Rockies, greatly expanding their distribution. These are: Hebelomaalpinum, H.aurantioumbrinum, H.dunense, H.hiemale, H.marginatulum, H.mesophaeum, H.nigellum, H.oreophilum, H.subconcolor, H.spetsbergense, H.vaccinum, and H.velutipes. Hebelomahygrophilum is known from subalpine habitats in Europe, but was never recorded in arctic-alpine ecology. Three species recorded from the Rockies, but as yet not reported from Europe, are H.alpinicola, H.avellaneum, and H.excedens. The last two have never previously been reported from an arctic-alpine habitat. For all three of these species, the holotypes have been studied morphologically and molecularly, and have been incorporated into the analysis.

Nomenclatural type definitions are one of the most important concepts in biological nomenclature. Being physical objects that can be re-studied by other researchers, types permanently link taxonomy (an artificial agreement to classify biological diversity) with nomenclature (an artificial agreement to name biological diversity). Two proposals to amend the International Code of Nomenclature for algae, fungi, and plants (ICN), allowing DNA sequences alone (of any region and extent) to serve as types of taxon names for voucherless fungi (mainly putative taxa from environmental DNA sequences), have been submitted to be voted on at the 11th International Mycological Congress (Puerto Rico, July 2018). We consider various genetic processes affecting the distribution of alleles among taxa and find that alleles may not consistently and uniquely represent the species within which they are contained. Should the proposals be accepted, the meaning of nomenclatural types would change in a fundamental way from physical objects as sources of data to the data themselves. Such changes are conducive to irreproducible science, the potential typification on artefactual data, and massive creation of names with low information content, ultimately causing nomenclatural instability and unnecessary work for future researchers that would stall future explorations of fungal diversity. We conclude that the acceptance of DNA sequences alone as types of names of taxa, under the terms used in the current proposals, is unnecessary and would not solve the problem of naming putative taxa known only from DNA sequences in a scientifically defensible way. As an alternative, we highlight the use of formulas for naming putative taxa (candidate taxa) that do not require any modification of the ICN.

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Cercopemyces is described as a new genus based on collections of a newly discovered agaricoid species from the arid Cercocarpus forests of Utah and Colorado. The new genus is near Ripartitella and Cystodermella based on nLSU, rpb1 and ITS molecular sequences but distinctly different from these taxa. The ornamented basidiospores and inflated cells in the scales of the pileus surface of Cercopemyces indicate a close relationship to some species of Ripartitella, and the molecular analyses support the sister group relationship. Morphologically Cercopemyces crocodilinus is reminiscent of the genus Amanita in the field, but the inamyloid, cyanophilic, ornamented basidiospores and lack of acrophysalidic hyphae in the trama indicate otherwise. A rare eastern USA species, Ripartitella ponderosa, is transferred to Cercopemyces based on morphological and molecular data.

Recently collected material was used to evaluate the phylogenetic position of the secotioid fungus Araneosa columellata. We amplified the nuclear ribosomal RNA gene (ITS1, ITS2, and LSU) and found Ar. columellata to be a member of the genus Agaricus (Agaricaceae, Agaricales), necessitating its transfer to that genus as Agaricus columellatus comb. nov. A detailed description is provided along with illustrations, and a lectotype is designated.