Oak Curtain Crust Identification Guide

First Impressions and the Setting

Old oak stumps in coppice woodland, wood-pasture and hedge banks are the place to look. Hymenochaete rubiginosa produces a dark rusty-brown, leathery sheet that spreads over the cut face and sides of a stump, and along the upper edge that sheet lifts away into rows of small, tiered brackets. The overall effect really does look like a dark curtain hung on the wood, and once you have seen it the impression is unmistakable at ten paces.

Colour is the first cue: a deep, saturated rust-brown to blackish-brown, far darker and browner than the usual pale or ochre crusts. Texture is the second: dry material is rigid and brittle and snaps like a thin biscuit, while damp material is leathery and flexes.

Reading the Fruit Body

The growth form is effused-reflexed, which simply means mostly flat on the substrate with the upper part turning outward into a shelf. Understanding this saves confusion, because the same colony can look like a pure crust in one place and a tiny bracket colony a few centimetres away.

Individual brackets are 0.5-3 cm wide and occasionally 5 cm, and remarkably thin, usually well under 1.5 mm. They are tiered, fused laterally into rows, and often cover a whole log surface.

Upper surface: concentrically zoned and grooved, so raking light picks out narrow bands; dark brown to nearly black with age; finely velvety when young from a short tomentum, then wearing bald and crusty. The extreme margin is usually a lighter rusty-ochre or yellowish-brown and is thin and sharp, sometimes minutely fringed.

Lower, spore-bearing surface: smooth to slightly uneven or minutely warted, matt, rusty brown to cinnamon, generally a shade paler and warmer than the top. Under a lens confirm what it is not: no pores, no teeth, no gills. A featureless matt brown hymenium is the correct answer.

Flesh, Section and the Bristles

Cut or snap a bracket and look at the cut edge with a lens. The flesh is extremely thin, deep rust-brown right through, rigid, and continuous with the hymenium; there is no distinct black line separating the tomentum from the flesh, which is a useful negative character against some grey maze-pored brackets. There is no colour change on cutting or bruising and no distinctive smell.

The genus character is microscopic but easy. The hymenium is packed with setae: dark brown, thick-walled, pointed sterile cells that project like short bristles. At 100x under a microscope they are obvious as a palisade of brown spikes, and at high magnification with a good hand lens the hymenium can look faintly velvety because of them. Setae place a brown crust in Hymenochaete immediately, and their absence excludes it.

A supporting structural test: the brown flesh darkens instantly to black in potassium hydroxide solution, the standard reaction of this group of brown-fleshed fungi.

Taking a Spore Print

The spore print is worth chasing because it is counter-intuitive. Despite the rusty-brown flesh and hymenium, the deposit is white to very pale cream.

The method has to allow for a thin fruit body that grows on a vertical or overhanging surface. Cut a strip carrying an intact hymenium, lay or prop it face-down over dark glass or black card in a sealed humid box, and leave it overnight. Expect nothing more than a faint whitish film, which is why dark backing is essential. Microscopically the spores are colourless, smooth and narrowly cylindrical to slightly curved, which combined with brown setae is fully diagnostic for the genus.

Substrate, Season and Ecology

This is a saprotroph causing a white rot in dead hardwood, and it is strongly attached to oak: stumps, fallen trunks, standing dead limbs, and worked oak such as old rails and posts. Sweet chestnut is a regular secondary host, and other hardwoods are occasional.

Fruit bodies are tough and weather-resistant, so colonies persist through several seasons and can be found in any month of the year. New growth, marked by the pale ochre margin and a softer, more flexible texture, comes mainly in autumn and after prolonged rain. In summer drought the same colony is hard, black and easy to mistake for charred bark, and a fingernail is the best tool: a distinct leathery layer lifts away from the wood, which burnt bark does not.

Separating It From Look-Alikes

Stereum hirsutum is the species most often mistaken for it and the easiest to eliminate. Its brackets have a hairy grey-buff to ochre upper surface and a smooth, bright yellow-ochre to orange underside; the whole fungus is brighter, softer and more flexible, and it has no setae.

Stereum gausapatum also grows on oak and is browner, but its hymenium bleeds a red juice when scratched while fresh and damp. Hymenochaete rubiginosa never bleeds.

Hymenochaete corrugata shares the setae and the brown colour but is almost entirely resupinate, greyer, strongly cracked into a crazed pattern, and it grows on hazel and other smooth-barked hardwoods, where it characteristically glues fallen twigs to living branches.

Trametes and Trichaptum species have pored undersides, visible with a lens even when small.

Coltricia perennis is rusty-brown and zoned but has a distinct central stalk and grows on soil on paths and heaths, not on stumps.

Phellinus and Fomitiporia species are hard, perennial, thick and pored, forming hoof-shaped or cushion-like brackets rather than thin leathery frills.

Working Order in the Field

Confirm the substrate is dead oak. Confirm the fruit body is thin, leathery and mostly flat with a lifted, tiered, concentrically zoned upper edge. Check the underside is smooth rust-brown with no pores or teeth. Scratch it: no red bleeding. Snap it: uniform brown flesh, no black line. If a microscope is available, find the setae. That sequence separates it from every common brown crust and bracket on oak.

Field Checklist

Dark rusty-brown leathery sheets on dead oak; small tiered brackets along the upper edge; concentrically zoned, velvety then bald upper surface with a paler ochre margin; smooth matt brown hymenium; thin uniform brown flesh; brown pointed setae under the microscope; white spore deposit; present year-round.