P.tristisis an ectomycorrhizal, corticioid fungus whose name is frequently assigned to collections of basidiomata as well as root tip and soil samples from a wide range of habitats and hosts across the northern hemisphere. Despite this, its identity is unclear; eight heterotypic taxa have in major reviews of the species been considered synonymous with or morphologically similar toP.tristis, but no sequence data from type specimens have been available.With the aim to clarify the taxonomy, systematics, morphology, ecology and geographical distribution ofP.tristisand its morphologically similar species, we studied their type specimens as well as 147 basidiomata collections of mostly North European material.We used gene trees generated in BEAST 2 and PhyML and species trees estimated in STACEY and ASTRAL to delimit species based on the ITS, LSU, Tef1α and mtSSU regions. We enriched our sampling with environmental ITS sequences from the UNITE database.We found theP.tristisgroup to contain 13 molecularly and morphologically distinct species. Three of these,P.tristis,P.umbrinaandP.atrofusca, are already known to science, while ten species are here described as new:P.sciastrasp. nov.,P.tristoidessp. nov.,P.umbrinascenssp. nov.,P.pinophilasp. nov.,P.alnophilasp. nov.,P.alobatasp. nov.,P.plurilobasp. nov.,P.abundilobasp. nov.,P.rotundisporasp. nov.andP.mediasp. nov.We discoveredP.rhizopunctataandP.atrofuscato form a sister clade to all other species inP.tristiss.l. These two species, unlike all other species in theP.tristiscomplex, are dimitic.In this study, we designate epitypes forP.tristis,P.umbrinaandHypochnopsisfuscataand lectotypes forAuriculariaphylacterisandThelephorabiennis. We show that the holotype ofHypochnussitnensisand the lectotype ofHypochnopsisfuscataare conspecific withP.tristis, but in the absence of molecular information we regardPseudotomentellalongisterigmataandHypochnusrhacodiumas doubtful taxa due to their aberrant morphology. We confirmA.phylacteris,TomentellabiennisandSeptobasidiumarachnoideumas excluded taxa, since their morphology clearly show that they belong to other genera. A key to the species of theP.tristisgroup is provided.We foundP.umbrinato be a common species with a wide, Holarctic distribution, forming ectomycorrhiza with a large number of host species in habitats ranging from tropical forests to the Arctic tundra. The other species in theP.tristisgroup were found to be less common and have narrower ecological niches.
