Xylobolus frustulatus is a wood inhabiting fungus bare oak wood on coarse trunks and branches widely distributed in Eurasia. It is well-sought-after and easily identifiable species, associated to a clearly defined substrate.

The global population is estimated at 20,000-50,000 mature individuals, with an optimistic estimation of about 90,000 mature individuals. Following the calibration from Sweden and Norway (where the species has the largest subpopulations), 10 mature individuals per locality were set for the calculation (5-15 individuals). Following the same logic, since 91% of all records were equivalent to app. 2,400 localities and associated with 20,000 mature individuals, we estimate a lower bound of about 2,800 localities and 28,000 mature individuals. Considering that outside Northern and Central Europe, the population might be slightly overlooked, we estimate around 30,000 mature individuals. The global population is estimated to decrease, mostly because of its habitat’s ongoing decline. National assessments indicate declining populations across Europe, with the most correct values being 15% over 3 generations (50 years). AOO is estimated to be 7,552km2, and EOO to 33.41 million km2.

Therefore, Xylobolus frustulatus is assessed as Near Threatened.

In this assessment, only the Eurasian populations have been taken into consideration.The population in the Americas probably belong to a separate species, as a provisional analysis indicated phylogenetic separation between the American and Eurasian populations (Russell et al. 2025, https://substack.com/home/post/p-166731288). The situation for the Australian population is judged similar.

Xylobolus frustulatus is a wood-inhabiting species dependent on large decaying hardwood substrates in mature and old-growth forests. Although widely distributed, the species population is fragmented and associated with habitat decline due to forestry practices that remove deadwood and reduce forest conditions persistence.

Xylobolus frustulatus is widely distributed in Eurasia. It has a close taxon in North America. In addition, X. frustulatus has been recorded in Southeast Asia, Australia, and South America, which, However, these records could be introductions or possible other species (GBIF 2026). In this assessment, only the Eurasian populations is considered.

There are about 5000 records of X frustulatus Confirmed records from the following national databases: Austria - 56 records (Mykologische Datenbank), Denmark - 6 localities (Danish Red List 2019) and 43 approved & expert-approved records (Danish Fungal Atlas), Italy - Piedmont and Sardinia (Checklist dei macrobasidiomiceti d’Italia), Netherlands - 3 localities (NMV Verspreidingsatlas Paddenstoelen), Norway - 2,349 records (1950-2026; Artsdatabanken 2026) in 380 localities (Brandrud et al. 2021), Slovenia - 3 localities (Soklič et al. 2026 - “Boletus informaticus” database), Sweden - 1,920 (SLU Artdatabanken, 2025), Switzerland - 7 records in 4 5x5-km grid cells (Gross et al. 2024 - “Der Verbreitungsatlas” database), Ukraine - 3 records (Electronic Distribution Maps of Ukrainian Fungi). Other confirmed presences: Romania - 3 localities (personal communication, Pavel 2006).

Cao and Shuang (2020) treated X. frustulatus as a temperate species in their taxonomic framework (where they describe a new Xylobolus species from China - X. austrosinensis with a subtropical-tropical distribution), thus potentially distinguishing Xylobolus records as a different species than the one targeted in this assessment. Nevertheless, Dai et al. (2011) confirmed X. frustulatus on the checklist of corticioids in China, while the species was found in 5 localities (GBIF 2026). Also confirmed from Taiwan with one locality (Lee 2010), a possible introduction, and therefore not considered.
In India, the species is confirmed on the checklist of Aphyllophorales (Ranadive 2013) and has 6 localities mostly in the North-Western part (GBIF 2026); confirmed from North-Western Himalaya (Chander et al. 2017), Western Ghats - Maharashtra State (Ranadive et al. 2011), and North-Eastern India - Nagaland State (Roy et al. 2022).

The population size of X.frustulatus is likely large, though highly fragmented and declining. According to the GBIF database (2026-04-13), there are approximately 5,000 records from 1900 and onwards. 46% of all records are found in Sweden alone, while 45% in Norway. In Germany, the species is considered rare with several states listing the species as threatened with extinction (Hassen), extremely rare with geographic restriction (Rheinland-Pfanz), Data Deficient with restricted distribution (Saarland), and EN (Thüringen). In Slovakia, it is abundant in the southern and central parts, while rare in Czechia (Kotlaba 1967). In Finland, the species is considered regionally extinct in parts of the Southern boreal zone, including the biogeographical provinces of Southwestern Finland and Southern Ostrobothnia (Hyvärinen et al. 2019). In Sweden, the population is nationally red-listed as Near Threatened due to population decline (SLU Artdatabanken, 2025). The generation time is estimated at 16.7 years (Brandrud et al. 2021).

Xylobolus frustulatus is a wood saprotrophic fungus, although under certain conditions it can also become parasitic on living trees. It produces a distinctive white pocket rot in decorticating oaks (Otjen & Blanchette 1984; Stasińska 2008). These small, elongated cavities are due to the mycelial activity as the fungus breaks down the lignin and cellulose of the wood in patches (SLU Artdatabanken, 2020). The species is perennial as it produces long-lived basidiomes, forming extended patches on the bare wood. The typical substrate is represented by large branches, large trunks with or without ground contact, and potentially lying firewood with ground contact (SLU Artdatabanken, 2020). It is found growing almost exclusively on old decorticated stumps, logs, and branches of deciduous trees, mainly oak. In Europe, it grows on Quercus cerris, Q. frainetto, Q. petraea, and Q. robur (Stasińska 2008), while in North America, on Q. alba and Q. rubra (Otjen & Blanchette 1984). Although less frequently, it was also found on Betula sp., Liliodendron tulipifera, Pinus strobus, Pseudotsuga sp. (Stasińska 2008) or Castanea sp. (Stasińska 2008; Yamashita et al. 2017). The species is most typical of mature and old-growth forests, where it is associated with persistent habitat continuity and the presence of large coarse woody debris. Still, it has also been occasionally found in parks and avenues with old oaks and dead oak wood (Red List of Species 2021, Norway). According to the Finnish Red List (2019), the species is associated with dry and mesic herb-rich forests, and old-growth forests (including sporadic old trees and younger forests with decaying trees). As expected, the Swedish population was recorded especially from oak forests, but also from more closed deciduous and mixed forests (SLU Artdatabanken, 2020).
Outside its usual temperate range and oak host, X. frustulatus was found in India, on the bark of Bauhinia variegata, Grewia asiatica, and Syzygium cumini (Chander et al. 2017), and on Calycopteris floribunda (Ranadive et al. 2011).
X. frustulatus developed relationships with other groups of organisms than their plant host. For example, in Europe, it is the main host for a fungicolous fungus - Hypomyces xyloboli, which develops perithecia directly on the X. frustulatus hymenophore (Põldmaa 2003).

The main threats include: (a) removal of coarse woody debris through forest management; (b) loss of old-growth or mature forest stands; (c) main host decline (in Europe at least).

The species could benefit from: (a) retention of coarse woody debris, especially oak; (b) protection of mature and old-growth forests with oaks; (c) protection of veteran oaks. Mixed-aged structure must be favored for the continuity of old trees (SLU Artdatabanken, 2020).

It is essential to conduct new sampling in isolated and less studied regions. Outside the fungus’s usual temperate range, more sampling has to be conducted, followed by sequencing, in order to confirm or refute the presence of distinct Xylobolus species.

There is no known use or trade associated with this species.