Yellowing Curtain Crust Identification Guide

Where You Meet It

Damp, low-lying broadleaf woodland is the place: willow carr, alder along a stream, birch on wet ground, hazel coppice with standing water in the hollows. On a fallen branch or a leaning dead stem you find a long, undulating frill of thin brackets, tiered one above another and often fused side to side into a continuous wavy shelf that follows the wood for twenty or thirty centimetres. From a distance the effect really is like a ruffled curtain hanging off the log, which is where the group name comes from.

Come close and two things stand out. The upper surface is finely furry and banded in soft concentric zones, and the underside is completely smooth, with no pores, teeth or gills of any kind. Those two observations put you in the crust and curtain crust group straight away. A third observation, made with a fingernail, names the species.

The Overall Form

Fruit bodies are thin and leathery. Individual caps span 2 to 6 cm, occasionally up to 10 cm, and are only a millimetre or two thick. The typical shape is a semicircular or fan-shaped bracket attached along a narrow line, but the fungus is flexible in habit: on the underside of a log it spreads flat against the wood, and on the flanks it turns outward as a shelf, so a single colony may be part crust and part bracket. Margins are thin, wavy, often lobed or shallowly split, and paler than the rest.

When moist the whole thing is pliable and slightly rubbery, and it bends without breaking. When dry it stiffens, curls and hardens, and the colours deepen and dull. Soak a dried specimen in water for a few minutes and it revives to something close to its fresh state, which is a useful trick when identifying old material.

The Upper Surface

Run a finger across the top. It should feel like short velvet or fine felt, a dense mat of tiny hairs that gives the species its name. That texture is often best preserved near the attachment and worn away toward the margin on old specimens.

The zoning is the other feature to read. Concentric bands alternate in tone across grey-brown, ochre, tan, buff and cream, with a paler cream to pale yellow band at the growing edge. Older colonies frequently take on a green cast where algae colonise the felt, which can be misleading but does not change the underlying banding. Look for narrow, low ridges accompanying the colour bands.

The Underside and the Yellow Reaction

Turn a bracket over, or bend one outward if it is attached low on a log. The fertile surface is entirely smooth and even, without pores, spines, folds or gills, and is coloured pale ochre, buff, cinnamon-buff or a soft pinkish-tan. It may be faintly wrinkled near the attachment on thick specimens but never develops true relief.

Now the diagnostic test. On a genuinely fresh, moist specimen, drag a fingernail or a blunt blade across the fertile surface with light pressure. Within a few seconds to a couple of minutes the scratched line flushes a clear yellow, often chrome to sulphur in tone, standing out sharply against the pale background. The same reaction can appear where a slug has grazed or a twig has rubbed.

Equally important is what does not happen: no red or reddish-brown fluid appears. Several closely related curtain crusts release a red juice from the fresh fertile surface when scratched, and that reaction is mutually exclusive with the yellow flush. Testing both possibilities with one scratch is the fastest route through the group.

The reaction requires fresh, hydrated tissue. Dried brackets, sun-baked brackets and specimens collected in a long dry spell often give nothing at all, so test on a damp day or rehydrate first.

Flesh in Section

Slice a bracket across with a razor blade and examine the cut edge with a lens. You should see a thin, tough, pale layer of flesh, with the velvet mat on top and the smooth fertile layer beneath. In many collections a fine dark line separates the hairy upper layer from the flesh proper. The tissue is fibrous and leathery rather than soft, and it does not crumble.

Spore Print on a Crust

Spore prints are less routine on bracket fungi but are worth attempting. Cut a fresh, actively growing bracket free with a slice of the wood attached, and support it fertile-side down over a piece of dark glass or black card inside a closed box with a damp tissue for humidity. Leave it overnight in a cool place. The result is a faint white bloom rather than a dense heap; tilt the glass into raking light to see it.

Under the microscope the spores are cylindrical and sausage-shaped, smooth and thin-walled, and they turn blue-grey in Melzer reagent, which is a standard confirmation for the group.

Substrate, Season and Ecology

The fungus lives on dead and dying broadleaf wood, decaying the tissue and recycling it. Willow is a strong favourite, with alder, birch, hazel and beech regularly recorded, mostly on fallen branches, standing dead stems and cut stumps in humid, sheltered situations. It is far less common on well-drained ridges than in damp hollows and stream sides.

Brackets are annual but tough, so colonies persist through the year in a dried, curled state and revive with rain. The main flush of fresh growth comes in autumn, with a secondary flush in spring, and the paler growing margin is the sign of an actively expanding colony.

Separating the Look-alikes

Stereum hirsutum is the commonest confusion. It is coarser and shaggier on top, more vividly yellow to orange overall with a bright yellow margin, occurs typically on drier oak and beech wood, and shows no yellow flush when the fertile surface is scratched.

Stereum rugosum is mostly flat against the wood with only a small turned-out edge, is thicker and cream-grey, and bleeds red when scratched. Stereum gausapatum and Stereum sanguinolentum do the same; the latter occurs on conifer wood.

Trametes versicolor forms similar tiered fans with zoned, silky-banded tops, but its underside is covered in fine white pores, which settles it in one glance. Trametes hirsuta is white and hairy with pores.

Chondrostereum purpureum has a smooth underside like a curtain crust but is distinctly lilac to purple-brown beneath.

Bjerkandera adusta has grey pores and a blackening margin, Plicaturopsis crispa has a strongly folded and wrinkled underside, and Xylobolus frustulatus forms cracked tile-like plates on old oak.

Field Notes to Take

Record the host tree, the wetness of the site, the texture of the upper surface, the exact colour of the underside before testing, and the result and timing of the scratch test. Photograph the scratched area a minute after making the mark so the yellow flush is captured while it is at its strongest.