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 subtomentosus (L.: Fr.) Quél.
Cap up to 12 cm, at first hemispherical, later convex to flattened, colour very variable, yellowish ochraceous, ochraceous, sometimes with olivaceous tint, becoming pale to dark brown or brick coloured, sometimes bright yellow or red, dry, felty, later smooth, sometimes cracking and yellowish or whitish flesh is then seen in the cracks. Stipe cylindrical, spindle-shaped to almost club-shaped, often tapered towards the base, pale yellow or yellow, downwards sometimes becoming brownish, sometimes striate, often covered with scattered fine brownish, pinkish or reddish granules, sometimes forming dotted network, unchanging when bruised; basal mycelium whitish. Tubes pale yellow to yellow, more olivaceous with age, faintly blueing or not blueing when injured. Pores concolorous with the tubes, blueing when bruised. Flesh whitish or yellowish in the cap and in the stipe, pinkish or pinkish brown in the lower parts of the stipe, blueing faintly in the cap or not blueing at all when exposed to air. Smell somewhat acid. Taste acid. Spores 10–15 × 4–6 μm, smooth. Pileipellis palisadoderm of septate hyphae of cylindrical, smooth or nearly smooth cells.
Habitat. Mainly in broadleaf or mixed forests, or in parkland, mycorrhizal mostly with with oaks (Quercus), but also with beech (Fagus), hornbeam (Carpinus) or birch (Betula).
Distribution. In Europe widespread, but out of any doubt in the past confused with Xerocomus ferrugineus.
Similarity. Xerocomus subtomentosus is most similar to Xerocomus ferrugineus, but the later has whitish (including in the stipe base) and unchanging flesh, yellow basal mycelium and is found mostly under conifers. Forms of Xerocomus subtomentosus with red-coloured caps might be confused with the reddish coloured species around Xerocomus rubellus, but again, the pinkish or pinkish brown flesh in the stipe base, together with the weakly incrusted or not incrusted cells of the cap cuticle, are usually easily setting Xerocomus subtomentosus apart. Xerocomus silwoodensis, a reddish or reddish brown coloured ally of Xerocomus subtomentosus is distinguished by the whitish flesh (incl. the stipe base) that becomes somewhat yellowish when exposed to air. The recently described Xerocomus chrysonemus is another similar entity. From X. subtomentosus it is distinguished by the bright yellow (not pinkish or brownish) flesh in the stipe base that is usually not changing when exposed to air (instead of the flesh of X. subtomentosus that is normally blueing in the cap). Also the basal mycelium is yellow to bright yellow (whitish in X. subtomentosus).
Alessio, C.L. 1985. Boletus Dill. ex L. (sensu lato). – In: Fungi Europaei. Vol. 2. Pp. 1–705. Libreria editrice Biella Giovanna, Saronno.
Breitenbach J. & Kränzlin F. 1991. Pilze der Schweiz. Bd. 3(1). Röhrlinge und Blätterpilze. Verlag Mykologia, Luzern.
Engel, H., Dermek, A., Klofac, W., Ludwig, E. & Brückner, T. 1996. Schmier – und Filzröhrlinge s. l. in Europa. Die Gattungen Boletellus, Boletinus, Phylloporus, Suillus, Xerocomus. Verlag Heinz Engel, Weidhausen b. Coburg.
Estadès, A. & Lannoy, G. 2004. Les bolets européens. – Bulletin Mycologique et Botanique Dauphiné-Savoie 44(3): 3–79.
Galli, R. 1998. I Boleti. Atlante pratico-monographico per la determinazione dei boleti. Edinatura, Milano.
Gelardi, M. 2011. A noteworthy British collection of Xerocomus silwoodensis and a comparative overview on the European species of X. subtomentosus complex. – Bolletino dell’ Associazione Micologica ed Ecologica Romana 84: 28–38.
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.
Lannoy, G. & Estadès, A. 2001. Les Bolets. Flore mycologique d’Europe. Documents Mycologiques Mémoire Hors série no. 6. Pp. 1–163. Association d’Écologie et de Mycologie, Lille.
Šutara, J., Mikšík, M. & Janda, V. 2009. Hřibovité houby. Čeled’ Boletaceae a rody Gyrodon, Gyroporus, Boletinus a Suillus. Academia, Praha.
Watling, R. & Hills, A.E. 2005. Boletes and their allies (revised and enlarged edition). – In: Henderson, D.M., Orton, P.D. & Watling, R. [eds]. British Fungus Flora. Agarics and boleti. Vol. 1. Royal Botanic Garden, Edinburgh.