Xerocomus silwoodensis sp. nov., a new species within the European X. subtomentosus complex

A recent analysis of the Xerocomus subtomentosus complex in Europe using rDNA-ITS sequence data distinguished four taxa in Europe. Two of these corresponded to the established taxa X. subtomentosus and X. ferrugineus, and a new taxon, X. chrysonemus, was described. The fourth taxon was noted but left undescribed owing to lack of material. Here, we describe this taxon as X. silwoodensis sp. nov. X. silwoodensis is a rare but widespread taxon known from single sites in Italy and Spain, and three in the UK. The features of X. silwoodensis basidiomes are very similar to other members of the complex but the pileus colours tend to show richer red-brown tones and the stipe often radicates deeply into the substrate. The taxon also exhibits a strong preference for associating with Populus species, whereas the other taxa are associated with either Quercus (X. chrysonemus) or generalists on broadleaved hosts (X. subtomentosus) or conifers and broadleaved trees (X. ferrugineus). Microscopically, the spore characteristics of X. silwoodensis are similar to the recently described X. chrysonemus, but differ significantly from both X. subtomentosus and X. ferrugineus. X. silwoodensis is probably overlooked due to the resemblance to other taxa within the complex. The present study on the identification and description of X. silwoodensis should reduce the confusion associated with the identification of taxa within this species complex and lead to a more accurate assessment of the geographic distribution and conservation needs of the taxa.