Mossy Maze Polypore Identification Guide

First Impression

The usual encounter is on storm damage. A maple or birch has shed a big limb, or a trunk lies split in a wood edge, and along its flank runs a rank of thin, wavy shelves, stacked and often fused into rippled tiers. From a few metres away the colour reads green, which is misleading: the fungus itself is grey and white, and the green comes from algae and moss colonising a hairy upper surface. What draws a second look is the underside, which even from below looks torn or chewed rather than neatly perforated.

These are flexible, tough, papery-leathery brackets rather than woody hoofs. You can bend a fresh one almost double without snapping it, and dried specimens on old logs stay in place through a winter, curling and bleaching.

Reading the Cap

Each bracket is 2 to 10 cm wide, only 3 to 10 mm thick, semicircular to fan-shaped or irregularly kidney-shaped, and either sessile or narrowed to a small attachment point. Fresh growth is velvety to distinctly hairy, and the surface is arranged in concentric zones of white, ivory, ash-grey and pale ochre. As it ages the hairs mat down, the zones blur, and the green film of algae takes over the older half. The margin is thin, often wavy or lobed, and pale when actively expanding.

Reading the Spore-bearing Surface

This is where the species names itself. The underside is whitish to ash-grey when young, ageing to a dull grey-brown or smoky tan, and it is never a field of neat circles. The pores start at 2 to 4 per millimetre but immediately elongate and run together into sinuous, radially aligned slots, so the surface looks like a maze or a set of brain-like furrows. In older brackets the walls between slots break down further and the surface tears into flattened, irregular teeth. The grey tone matters as much as the shape: a maze-pored bracket with a white or buff underside is a different fungus.

The Cut: Finding the Black Line

Carry a razor or a sharp knife for this genus. Slice a bracket vertically from the margin back to the attachment and look at the cut face with a lens. You should see a thin, sharply defined dark line lying just under the hairy upper layer, separating that layer from the white flesh below. That black line is one of the most useful characters in the whole group of thin shelf fungi.

The context itself is white to off-white, 1 to 4 mm thick, and duplex: a looser, softer upper zone and a denser, tougher lower zone. The tubes below it are 1 to 4 mm long and distinctly grey-brown, so the cut face shows a clean two-tone contrast of white flesh over grey tubes. Nothing changes colour when the cut is exposed to air, and there is no bruising reaction to handling.

Taking a Spore Print

Choose a bracket with a pale, actively growing margin, cut it off, and lay it pore-side down on dark card, covered, with a damp tissue nearby for humidity. Overnight is usually enough in the growing season. The deposit is white, which is worth confirming because the grey pore surface leads some people to expect a coloured print. Under the microscope the spores are small, smooth, colourless, and cylindrical to narrowly ellipsoid, roughly 5 to 7.5 by 2.5 to 4 µm.

Substrate, Season and an Ecological Aside

Cerrena unicolor is annual, appearing from early summer through autumn and then persisting as weathered, bleached brackets well into the following year. It is a white-rot fungus of dead broadleaved wood, especially maple, birch and beech, and also appears on wounds and cankers of living trees.

The ecology is unusually interesting. In North America the species is intimately linked with the pigeon horntail wood wasp, Tremex columba. Female horntails carry Cerrena spores and mycelial fragments in specialised abdominal pouches and inoculate the wood as they lay eggs, and the resulting rot softens the timber for the developing larvae. Those larvae in turn are hunted by giant ichneumon wasps of the genus Megarhyssa, which drill through the wood with enormously long ovipositors. Finding fresh Cerrena on a dead maple is therefore also a reason to watch the same trunk for wasps.

Look-alikes, Separated on Morphology

Daedaleopsis confragosa shares the maze-like underside but is thicker and tougher, its upper surface is smooth and warm reddish-brown with strong zoning, its pale pore surface bruises reddish-brown when scratched, and a vertical cut shows no black line. It favours birch and willow.

Trametes versicolor has round pores at 3 to 5 per millimetre that stay round, a silky rather than shaggy zoned cap, and a white underside.

Trametes hirsuta does have the black line and a hairy grey cap, but its pores remain round and its white context is noticeably thicker.

Bjerkandera adusta has a grey to almost black pore surface, which can mislead, but its pores are minute and round, and the dark line in section sits at the junction between white context and grey tubes rather than beneath the hairy top layer.

Lenzites betulina has true radiating gills rather than elongated pores, and a white to buff underside.

Daedalea quercina is far thicker, corky and buff throughout, with deep maze-like pores, and grows on oak.

Irpex lacteus is white and toothed and mostly spread flat on the wood with only small shelving caps.

The safest field triad is a hairy, algae-greened, zoned cap, a grey maze-like underside, and a black line under the tomentum on a clean vertical cut.