Yellowing Curtain Crust

Stereum subtomentosum is a widespread European and North American crust fungus that forms proper little brackets rather than staying flat, often in dense overlapping tiers along the flanks of a fallen trunk. The upper surface is finely felted — the epithet means 'somewhat downy' — and marked with narrow concentric bands in ochre, grey-brown, cinnamon and, on older growth, greenish tones from algae.

What identifies the species instantly is a colour reaction. Draw a fingernail or a twig across the smooth fertile underside of a fresh specimen and the scratched line turns clear chrome or lemon yellow within seconds. No red liquid appears, distinguishing it from the several bleeding Stereum species.

Like all Stereum, it has no pores, gills or spines; the entire spore-producing surface is flat and unbroken. Its combination of a wavy, curtain-like margin, well-formed reflexed brackets and yellow-bruising underside makes it one of the more approachable members of a difficult genus.

Fruit body

Brackets are 2–6 cm wide and 1–3 cm deep, thin, leathery and semicircular to fan-shaped, attached broadly along a narrow base. They form dense overlapping tiers and frequently fuse laterally into long undulating curtains running along a log; on undersides of wood a resupinate portion is often present behind the reflexed edge.

Upper surface

Densely velvety to finely tomentose, at least when young, with clear concentric zoning in ochre-brown, tawny, greyish and buff bands. The margin is thin, wavy or lobed, and typically paler cream to yellowish. Aged specimens go greyish and algal-green and lose much of their felt.

Fertile surface and bruising

The underside is smooth, even and without any pores or teeth, coloured pale ochre, buff or greyish cream. Scratched or rubbed while fresh, it discolours a distinct chrome yellow along the damaged line. No red or coloured juice is exuded.

Flesh and spore print

Flesh is thin, tough and flexible, buff-coloured, drying hard and rigid but rehydrating readily. The spore deposit is white; spores are small, smooth and cylindric-ellipsoid.

The fungus grows on dead broadleaf wood — fallen trunks, stumps, large branches and standing dead poles — with birch, alder, willow, hazel and beech among its most frequent hosts. It favours damp woodland, carr, streamsides and riparian scrub, and does well on wood that retains some bark.

Dried fruit bodies persist year-round, but fresh growth and spore release peak from late summer through autumn and into winter. The species is common across temperate Europe and is recorded from temperate Asia and North America.

Stereum subtomentosum is a saprotroph causing a white rot in broadleaf wood, degrading lignin as well as cellulose so that decayed wood becomes pale, soft and stringy. It commonly colonises branches and stems that die while still standing and continues to fruit for years after they fall.

Individual fruit bodies are annual to short-lived perennial in effect, drying to hard, weather-resistant leather and reactivating during wet spells. Long fused curtains along one trunk usually represent a single extensive mycelium working through the sapwood from multiple points.

Stereum hirsutum is the most frequent confusion: it has a coarser, shaggier upper surface, a brighter yellow-orange underside, and — critically — does not stain yellow when scratched. Stereum ostrea is larger and more flamboyantly zoned but likewise lacks the yellow bruising reaction.

The bleeding species are separated by a different reaction: Stereum rugosum, S. gausapatum and S. sanguinolentum all release a red juice from a scratched fresh fertile surface, and none turn yellow. Trametes versicolor is superficially similar in outline and zoning but has a finely poroid underside instead of a smooth one, and Chondrostereum purpureum has a smooth underside in violet to lilac-brown tones with no yellow reaction.