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 parasiticus (Bull. : Fr.) Quél.
Fruitbodies solitary or clustered, attached to fruitbodies of earthballs (Scleroderma citrinum). Cap up to 5 cm, at first hemispherical then convex, straw, ochraceous or brown, sometimes with olivaceous tint, dry, felty to smooth, sometimes finely cracked. Stipe cylindrical, often curved and usually tapering towards the base, concolorous with the cap or sometimes more rusty coloured, but paler at the base. Flesh yellowish, often brownish in the stipe base and under the cuticle, unchanging when exposed to air. Tubes lemon yellow or ochraceous, sometimes rusty tinted, unchanging when injured. Pores more or less concolorous with the tubes, but often rusty and even reddish when old, unchanging when bruised. Smell not distinctive. Taste not distinctive. Spores 11–20 × 4–6 μm, smooth. Pileipellis (the cap cuticle) a trichodermium of septate hyphae of cylindrical, smooth cells.
Habitat. Parasitic on fruitbodies of earthballs (Scleroderma citrinum).
Distribution. In Europe relatively widespread, but seldom.
Note. Xerocomus parasiticus is instantly recognized due its unique for the European boletes parasitic habit. It is sometimes placed in a genus of its own, namely Pseudoboletus.
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