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Genus Xylobolus P. Karst.

[previously in Stereum]

Summary: Xylobolus was split out of Stereum and some species continue to be moved to other genera based on molecular phylogenies and morphological characters. There are two or three species in North America. The two Xylobolus here in the Midwest follow the distribution of oak (Quercus) across the continent.

Taxon Comparisons

  1. Xylobolus frustulatus (Pers.) Boidin 1958

    This is a distinctive crust separated into small grayish tan ceramic blocks (frustules). If you look closely you can see the dark upperside against the wood and the multi-layered structure of this perennial.

    The ceramic parchment is common in the the Chicago Region and the Upper Midwest. Look for patches of X. frustulatus on the undersides of older oak logs that are missing bark.

    Gray blocks with black edges on wood.
    Xylobolus frustulatus, Illinois
  2. Xylobolus subpileatus (Pers.) Boidin 1958

    As its name implies, this bracket has an effused-reflexed structure like a Stereum, but X. subpileatus is a perennial with a hard and thicker texture. Upper surface is zoned and sulcate (concentric ridges) with dark brown toward blackish colors. Growth can be separate discs or fused into long wide rows. Hymenium underneath pale gray to light brown. Context (inside) pale ochre with a black layer under the surface tomentum. The acanthocystidia (acanthophyses, acanthohyphidia), having thick walls with bumps along their length, will separate this from a Stereum.

    It is rare in the Chicago Region with eight records from four counties: Cook and McHenry Counties, Illinois; LaPorte and Porter Counties, Indiana. It is found on a variety of dead hardwoods but clearly prefers oak (Quercus). In the western Mediterranean, this fungus prefers oak logs in a stage of advanced decay. (Taudiere et al. 2017)

    Bracket with hard gray underside.
    Xylobolus subpileatus, Illinois; underside
    Bracket with very dark brown upperside.
    Xylobolus subpileatus, Illinois; topside

Taxon Information

Mushroom Observer has two unconfirmed records of Xylobolus illudens from California. Index Fungorum (Jan 2021) listed eleven species for Xylobolus; but some are synonyms. Two were added since 2017 but I.F. is missing the following species from 2020.

Cao and He (2020) published X. austrosinensis from southern China. They exclude 4 species from the genus based on ITS analysis and morphology. They accept six species of Xylobolus. But the disposition for Xylobolus illudens is not clear (possibly basal to [Xylobolus + Stereum]).


  1. Xylobolus annosus, type: Sri Lanka; or a synonym of X. princeps.
  2. Xylobolus austrosinensis, type: China.
  3. Xylobolus frustulatus, type: America.
  4. Xylobolus gongylodes, type: Paraguay.
  5. Xylobolus princeps, type: West Java. (synonym X. hiugensis, type: Japan.)
  6. Xylobolus subpileatus, type: South Carolina, USA.

Species excluded by Cao & He (2020)

  1. Xylobolus brasiliensis, type: Brazil; possibly an Aleurodiscus.
  2. Xylobolus illudens, type: Western Australia; poor resolution of position in ITS phylogeny.
  3. Xylobolus peculiare, type: Russia; possibly a separate genus.
  4. Xylobolus spectabilis, Indo-Pacific, China; close to Stereum.

Species previously excluded

  1. Xylobolus ahmadii, now Acanthofungus ahmadii.
  2. Xylobolus apricans, now Acanthophysium apricans.
  3. Xylobolus areolatus, now Amylostereum areolatum.
  4. Xylobolus repandus, now Radulomyces repandus.
  5. Xylobolus thoenii, now Acanthophysium thoenii.
  6. Xylobolus tumulosus, now Peniophora cinerea.

Taxon Details and Links


  • Xylobolus P. Karst., Meddelanden af Societas pro Fauna et Flora Fennica 6: 11 (1881).
  • Thelephora frustulata Pers., Synopsis methodica fungorum: 577 (1801).


Xylobolus was recently part of Stereum but differs by the ellipsoid spores, numerous acanthophyses, and distinct pocket rot, which is unknown in Stereum (Ryvarden 2010).

Note that two genera have type species that are in the clade with Xylobolus: Acanthophysellum lividocoeruleum and Acanthophysium apricans. It would be more stable to combine both into a somewhat enlarged Xylobolus based on DNA data rather than trying to retain split genera based on a mosaic of other characters. However there are many other species assigned to these genera that do not belong in this clade and they will require new genus names regardless. Further DNA analysis will hopefully pin down whether Acanthofungus rimosus and its related species, including Aleurodiscus senticosus (Wu et al. 2019), represent an independent clade near Xylobolus and Stereum.

Acanthophysellum: Miller et al. (2006) in their phylogeny next to Xylobolus frustulatus shows a Xylobolus lividocaeruleus; it is otherwise not mentioned in the paper. Besides the misspelled epithet, Acanthophysellum lividocoeruleum does not have a published combination in Xylobolus. The two species do group together in this paper as well as in Wu et al. (2001) and Larsson and Larsson (2003). Larsson and Larsson (2003) discuss this relationship but decided not to declare Acanthophysellum to be a synonym of Xylobolus in order to avoid further taxonomical upheaval before more species are sequenced. Given this relationship, the species is suprisingly absent in the more recent study of Cao and He (2020). There seems to be two different species concepts judging from the few photos online. Acanthophysellum lividocoeruleum is the type species for the genus Acanthophysellum so its relationship to other species in that genus and to Xylobolus should be investigated further.

Acanthophysium has the type species A. apricans, which is missing from DNA phylogenies. Larsson and Larsson (2003) discuss A. apricans. They say Wu et al (2001) regards Acanthophysium as a synonym of Xylobolus based on similarities in nuclear behavior, the negative phenoloxidase reaction, and white-pocket rot (Boidin et al 1968). This sentence with double attribution is confused. Wu et al (2001) do not discuss Acanthophysium at all; the name is only listed in the table of source strains, and in the phylogeny figures they list these all as Aleurodiscus. Wu et al. (2000) officially combine A. apricans into Xylobolus, thus sinking Acanthophysium (they also combine various species into Acanthophysellum, some of which turn out not to belong there).

Acanthofungus: Wu et al. (2000) describe this new genus, based on Acanthofungus rimosus, as separate from Xylobolus because of clamped hyphae and heterocytic nuclear behavior. The newer species Aleurodiscus senticosus (Wu et al. 2019) is sister to Ac. rimosus with strong support in the phylogeny but excluded from Acanthofungus because Acanthofungus has smooth spores and causes a white pocket rot. But nowhere in the paper is given the rot type for Al. senticosus, simply stating on angiosperm branches; the species has aculeate spores (very weakly so; surface roughened with extremely tiny spines, barely discernible in the drawing of spores in IKI; not shown in drawing of spores in KOH).

The lists and synonymy on this page are my best attempts based on available references. I am curious to see the recent papers by Ryvarden (2020).

Description links

  1. Gary Emberger: Xylobolus frustulatus.
  2. Michael Kuo: Xylobolus frustulatus.
  3. Wikipedia: Xylobolus subpileatus.


  1. Cao, Y.-F., S.-H. He. 2020. Xylobolus austrosinensis sp. nov. (Stereaceae, Russulales) and notes on the genus. Phytotaxa, [S.l.], v. 452, n. 3, p. 200–208. doi: 10.11646/phytotaxa.452.3.2.
  2. Larsson, E., K-H. Larsson. 2003. Phylogenetic relationships of russuloid basidiomycetes with emphasis on aphyllophoralean taxa. Mycologia 95(6): 1037-1065. doi: 10.1080/15572536.2004.11833020
  3. Martini, Elia. Crusts & Jells: Acanthophysellum lividocoeruleum, Nomenclature.
  4. Martini, Elia. Crusts & Jells: Acanthophysium thoenii, Nomenclature.
  5. Martini, Elia. Crusts & Jells: Xylobolus princeps, Nomenclature.
  6. Miller, S. L., E. Larsson, K-H. Larsson, A. Verbeken, J. Nuytinck. 2006. Perspectives in the new Russulales. Mycologia 98(6): 960-970. doi: 10.1080/15572536.2006.11832625
  7. Ryvarden, Leif. 2010. Stereoid fungi of America. Synopsis Fungorum 28. [sold by]
  8. Ryvarden L. 2020. The genus Stereum - a synopsis. Synopsis Fungorum 40 : 46-96 [Not seen by me. Not yet in print?]
  9. Ryvarden L. 2020. Xylobolus P. Karst., a synopsis. Synopsis Fungorum 41: : 26-32 [Not seen by me. Not yet in print?]
  10. Taudiere, A., J.-M., P.-A. Moreau, C. Carcaillet, A. Christophe, T. Læssøe, P. Christophe, F. Richard. 2017. Xylobolus subpileatus, a specialized basidiomycete functionally linked to old canopy gaps. Canadian Journal of Forest Research 47: 965-973. doi: 10.1139/cjfr-2016-0521.
  11. Wu, S.-H., J. Boidin, and C.-Y. Chien. 2000. Acanthofungus rimosus gen. et sp. nov., with reevaluation of the related genera. Mycotaxon 76: 153–161. Mycotaxon volumes with page images.
  12. Wu, S.-H., D. S. Hibbett, and M. Binder. 2001. Phylogenetic Analyses of Aleurodiscus s.l. and Allied Genera. Mycologia 93: 720-731. doi: 10.2307/3761826
  13. Wu, S.H., Wei, C.L., Lin, Y.T., Chang, C.C. & He, S.H. (2019) Four new East Asian species of Aleurodiscus with echinulate basidiospores. MycoKeys 52: 71–87. doi: 10.3897/mycokeys.52.34066.

Taxon links for 18770 Xylobolus

Cite this page as: Leacock, P.R. (2021 Jan 05). Xylobolus - MycoGuide. Retrieved from

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