Bleeding Oak Crust

Stereum gausapatum is the bleeding crust of oak. It forms extensive resupinate sheets on dead oak wood and lifts its margin into narrow, hairy, tan to reddish-brown shelves, so a colonised branch appears wrapped in a ragged, ruffled skin. On the undersides of large logs it may remain almost entirely flat.

The epithet means 'shaggy' or 'woolly', referring to the felted upper surface of the reflexed portion. Like its relatives it belongs to a group of Stereum species that exude a red pigmented liquid when fresh tissue is damaged, a reaction that is immediate on well-hydrated material and absent once the fruit body has dried.

The species is closely tied to oak throughout its range in Europe, temperate Asia and North America, and is one of the principal fungi involved in the decay of oak branchwood. Where oak is dominant it is often the commonest crust present, forming long continuous colonies on fallen limbs.

Fruit body

Effused-reflexed: a resupinate sheet spreading over the wood with reflexed shelves 3–15 mm deep along the upper margin, frequently fusing laterally into a long ruffled band. The crust itself is thin, under a millimetre, and closely attached to the substrate.

Upper and fertile surfaces

The reflexed upper surface is densely hairy to felted, tan, ochre-brown, or reddish brown, with faint concentric zoning and a paler wavy edge; weathered growth greys and greens with algae. The fertile surface is smooth and featureless — no pores, teeth or wrinkles — coloured buff, ochre, or pale reddish brown, sometimes with a warmer flush toward the margin.

Bleeding reaction

A scratch across fresh, moist fertile tissue produces bright red droplets almost immediately. This reaction is the definitive separator from the non-bleeding oak crusts that share the same branches.

Flesh and spore print

Flesh is thin, tough and leathery, drying rigid and horn-like but rehydrating after rain. There is no stalk or ring. The spore deposit is white; spores are smooth, ellipsoid and amyloid.

The species is strongly associated with oak, fruiting on dead attached branches, fallen limbs, stumps and standing dead trunks, and it can also be found on chestnut and occasionally other hardwoods. It thrives in oak woodland, wood pasture and hedgerows, and colonises both fine branchwood and large-diameter limbs.

Dried crusts persist all year; fresh, bleeding growth is most reliably found from autumn through winter and into spring during wet spells. The fungus is common across temperate Europe and recorded widely in Asia and North America.

Stereum gausapatum is a saprotroph causing a white rot of oak wood, and is well known as an agent of pipe rot in the heartwood of standing oaks, entering through wounds and dead branch stubs. Decayed wood becomes pale, soft and stringy as lignin is removed.

It is typically an early coloniser, establishing in branches that die back while attached to the living tree and then continuing to fruit for years after those branches fall. Its persistent leathery crusts add new fertile layers season after season, so a single colony can occupy the same limb for a long time.

Stereum rugosum bleeds identically but is markedly thicker and almost entirely resupinate, with barely any turned-out margin, and it favours hazel and birch over oak. Stereum sanguinolentum also bleeds but is greyer, thinner-edged, and confined to conifer wood.

The non-bleeders on the same oak branches are separated by the scratch test: Stereum hirsutum has a coarsely hairy upper surface and bright yellow-orange fertile surface with no red reaction; Stereum complicatum forms small crowded orange fans and never bleeds; and Stereum subtomentosum stains yellow rather than red. Trametes and Trichaptum species growing alongside are ruled out at once by their poroid or toothed undersides.