New species of Hohenbuehelia, with comments on the Hohenbuehelia atrocoerulea - Nematoctonus robustus species complex.

Four new species of Hohenbuehelia (Fungi: Pleurotaceae) are described in the group of Hohenbuehelia atrocoerulea and Hohenbuehelia grisea. Hohenbuehelia algonquinensis, found on Pinus in Ontario, Canada, may be distinguished macroscopically from bluish collections of H. atrocoerulea and watery grey-brown collections of H. grisea by its coal-black pileus. Hohenbuehelia canadensis, on or associated with Pinus in both Ontario and Alberta, Canada, and Hohenbuehelia nimueae, on Salix in Ontario and Abies in Wyoming, USA, have similarly dark fruiting bodies and were previously misidentified as H. approximans (which we treat as a synonym of H. grisea), H. atrocoerulea, H. mustialensis or H. nigra. The latter species is shown to be a member of Resupinatus, despite the presence of prominent metuloid cystidia in its hymenium. Hohenbuehelia carlothornii has been found in Costa Rica; collections of the sexual fruiting bodies were previously identified as H. grisea and isolates from soil nematodes were identified by the anamorph name Nematoctonus robustus. That name has been treated as a synonym of H. atrocoerulea but, given the genetic and geographic variation within this complex, we transfer it to Hohenbuehelia as a distinct species. Sequences of the nuclear ribosomal DNA internal transcribed spacer region (ITS), the D1/D2 variable region of the large subunit gene, and a portion of the translation elongation factor (TEF1) gene provide good separation and support for these new species. A key to the dimidiate species of Hohenbuehelia of North America and Europe is provided.