Recent molecular studies have shown that Xerocomus in its current circumscription is likely an artificial grouping and it is possible that it will be split at some point into smaller genera. Molecular studies also have changed our understanding about the species of xerocomoid boletes showing that morphological features are quite variable in this group. Not only microscopic study is essential for determination, but scanning electron microscope will be often needed in this “genus” as the spore ornamentation is not always seen under ordinary light microscope. Do bear in mind that macroscopic characters, such as colours, cracking cuticle, etc., tend to intergrade between the different species. Note that Boletus impolitus and Boletus depilatus that were shown to be close to Xerocomus subtomentosus and its allies, are here retained in Boletus for practical reasons. The same applies also for Phylloporus pelletieri, placed here in a genus of its own, but being also close to Xerocomus subtomentosus group.
Although large reference list will be found under most of the species, one should always consult Ladurner & Simonini (2003) having in mind that there are some new species (X. chrysonemus, X. marekii, X. silwoodensis) described after this otherwise superior book was printed. Useful keys, covering most of the European xerocomoid boletes (except some southern taxa) are provided by Knudsen & Vesterholt (2008), Hills (2008) and Kibby (2011), the later also featuring an excellent comparison chart.
Fruitbody medium to small sized, boletoid, without veil and ring. Stipe solid, often tapering towards the base. Flesh variously coloured, changing or not when exposed to air. Tubes not separable from each other, instead tearing apart. Pores usually angular.
Xerocomus fennicus (Harmaja) H. Ladurner & Simonini
Yet unknown to me. Reddish coloured bolete with truncate and striate spores, macroscopically similar to Xerocomus rubellus, X. riparielus, X. marekii, X. cisalpinus and X. pruinatus (see the similarity section below).
Habitat. Deciduous and mixed forests, parks, probably associated with birch (Betula) or alders (Alnus).
Distribution. Not entirely understood. So far known from Austria, Czech Republic, Denmark, Finland and Sweden, possibly also Belgium.
Xerocomus rubellus has characteristic orange red dots in the flesh of the stipe base and not truncate smooth spores.
Xerocomus ripariellus has striate, non truncate spores.
Xerocomus marekii also has truncate striate spores but grows in slightly different habitat, associated presumably with oaks (Quercus).
Xerocomus ripariellus has striate, but not truncate spores.
Xerocomus cisalpinus has striate, but not truncate spores and grows in warm habitats, associated with oaks (Quercus), beech (Fagus), pines (Pinus) or cedar (Cedrus).
Xerocomus porosporus has differently coloured fruitbodies and truncate non striate spores.
I am not yet hosting any images of this species, nor have I seen any identifiable images on the internet. Colour illustrations are found in Harmaja (1998), Ladurner & Simonini (2003) and Hills (2008).
Antonín V. & Vágner A. 2006. Xerocomus fennicus, first record in the Czech Republic. – Acta Musei Moraviae, Scientiae biologicae 91: 159–164.
Harmaja, H. 1998. Boletellus ripariellus, a hitherto misidentified species in Finland. – Karstenia 38: 45–48.
Harmaja, H. 1998. Boletellus fennicus, a new species from Finland. – Karstenia 39: 37–38.
Hills, A.E. 2008. The genus Xerocomus. A personal view, with a key to the British species. Field Mycology 9(3): 77–96.
Knudsen, H. & Vesterholt, J. [eds.]. 2008. Funga Nordica. Nordsvamp, Kopenhagen.
Ladurner, H. & Simonini, G. 2003. Xerocomus s.l. – In: Fungi Europaei. Vol. 8. Pp. 1–527. Edizioni Candusso, Alassio.