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. Some southern species have secondarily angiocarpic development and then a ring of coarse granules is observed on the stipe, where the cap margin was initially attached. 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 porosporus (Imler ex G. Moreno & Bon) Contu
Cap up to 7 cm, at first hemispherical then convex, ochraceous, pale brown, greyish brown, olivaceous brown, dark brown to blackish brown, dry, velvety but very soon cracking and white flesh is revealed in the cracks. Stipe cylindrical or slightly club-shaped, often curved and usually tapering at the base, pale yellow to yellow entirely or in in the upper half, downwards gradually becoming brownish to greyish brown, sometimes with reddish tint. Flesh cream to yellowish in the cap, brownish in the stipe, unchanging when exposed to air. Tubes lemon yellow, later pale yellow with olivaceous tint, blueing when injured. Pores concolorous with the tubes, blueing when bruised. Smell not distinctive. Taste not distinctive. Spores 11–17 × 5–6.5 μm, smooth, truncate. Pileipellis (the cap cuticle) a palisadoderm of septate hyphae. Hyphae composed of nearly ellipsoid to cylindrical, incrusted cells.
Habitat. In deciduous forests, mycorrhizal mostly with oaks (Quercus).
Distribution. In Europe widespread and mostly common.
Similarity. Xerocomus porosporus shows similarity to many other xerocomoid species but is easily distinguished by its truncate spores. Xerocomus marekii also has truncate smooth spores, but it has reddish cap and blueing flesh. Xerocomus fennicus is the only other European bolete with truncate spores, but these are clearly striate. It also has reddish cap.
Note. Some European authorities use for this species the name Xerocomus truncatus. It is a different American fungus and there is no evidence so far that it is found in Europe.
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