Xylobolus subpileatus

The caps are somewhat flat to wavy, with a dark zonate top and light underside. They are 2–20 centimetres (1–8 in) wide and 1–3.5 cm (1⁄2–1+1⁄2 in) deep.^[5] The wood-inhabiting stereoid fruit bodies form tough, crust-like basidiomes that adhere closely to the substrate, typically developing as effused patches or with reflexed margins on fallen oak logs.^[6] Basidiomes often occur adjacent to a characteristic honeycomb-like decay pattern—resulting from selective lignin degradation that leaves medullary ray parenchyma and early-wood vessels intact between pockets of degraded material.^[6]

Lookalikes include some within the same genus, as well as Cerioporus mollis, Dentocorticium portoricense, and Hymenochaete rubiginosa.^[5]

Xylobolus subpileatus is a saproxylic wood-decay fungus that colonises fallen oakwood, producing its characteristic honeycomb-like decay patterns by selectively degrading lignin. Fruiting bodies appear only on logs of Quercus species, with the likelihood of basidiome formation rising as the wood advances through decay; in old-growth oak forests in Corsica, for example, fruiting probability increases steadily from around 7 years after treefall to peak at roughly 40 years within natural canopy gaps, where deep shade and stable microclimates prevail.^[6]

Geographically, X. subpileatus has been recorded from at least 14 countries across North America, Europe, Asia and South America, although it is absent from Africa and Antarctica.^[6] In eastern North America it is known from mature deciduous forests on fallen oak logs,^[5] while in Europe its populations are often scarce and highly localised; several countries list it as endangered (e.g. Austria, Hungary) or critically endangered (Czech Republic). In the Mediterranean basin, occurrences are confined to old-growth Quercus ilex stands on Corsica, where fruiting is restricted to logs in advanced stages of decay within dated canopy gaps.^[6]