Mossy Maze Polypore
Cerrena unicolor
Also called mossy maze bracket, hairy maze polypore
A thin, tough shelf fungus with a coarsely hairy grey-white cap that is usually stained green by algae, and a distinctive greyish maze-like pore surface that breaks up into blunt teeth with age.
Overview
The Mossy Maze Polypore (Cerrena unicolor) is a common annual bracket of hardwood forests throughout the temperate Northern Hemisphere. It sits in the Polyporaceae near Trametes, and at a glance it can be mistaken for a weathered Turkey Tail, but its pore surface tells a completely different story: instead of tidy round pores it develops elongated, sinuous, maze-like slots that eventually erode into flattened teeth.
The common name captures both of its most memorable features. The cap is coarsely hairy and, because that hairy layer holds moisture, it is almost always colonised by green algae and sometimes by true mosses and liverworts, giving old brackets a distinctly furry green appearance. The maze refers to the labyrinthine underside.
A useful structural detail sets Cerrena apart from its relatives: in vertical section a thin but sharply defined black line runs between the hairy cap layer and the white flesh beneath it. This feature is shared with only a handful of other brackets and is easy to check with a pocket knife.
How to identify it
Cap
Annual, thin and tough, 3-10 cm across and rarely more than 1 cm thick, forming semicircular to fan-shaped brackets that are often fused in overlapping tiers or grow in effused-reflexed sheets along the underside of a log. The upper surface is coarsely hairy to shaggy, faintly concentrically zoned in white, grey and buff, and typically stained green over much of its area by algal growth. The margin is thin, wavy and often paler.
Pores
The pore surface is whitish to pale grey, ageing dull greyish brown. Pores start out angular and unequal, quickly elongating into sinuous maze-like slots, and in mature brackets the walls break down into irregular flattened teeth. Roughly 2-4 pores per millimetre where they are still recognisable as pores. The tube layer is shallow, only 1-3 mm deep.
Flesh and spore print
The flesh is white to cream, tough, leathery and thin. Cut a bracket vertically and look at the junction between the hairy surface and the flesh: a narrow but distinct black line is visible there, which is the single most reliable field character for the genus. The spore print is white.
Where and when it grows
The Mossy Maze Polypore fruits on dead hardwood, especially maple, beech, birch and poplar, appearing on logs, stumps, fallen limbs and the exposed dead wood of wounded living trees. It also occurs on the cut faces of stacked timber and on structural hardwood in damp situations. Occasionally it is found on conifers, but hardwood is the norm.
It is widespread across North America, Europe and temperate Asia. Fresh brackets are produced mainly from late spring through autumn, but the tough fruit bodies weather well and remain attached, greener and greyer, right through winter, so the species can be recorded in any month.
Ecological role
Cerrena unicolor is a white-rot saprotroph and opportunistic wound coloniser. Its enzymes degrade both lignin and cellulose, leaving the wood pale, soft and fibrous, and it is one of the more aggressive early colonisers of freshly exposed hardwood surfaces.
The species has a remarkable ecological partnership with the giant horntail relatives in the genus Tremex and with the ichneumon wasps that parasitise them. Female horntails carry Cerrena spores in specialised abdominal pouches and inject them with their eggs into weakened trees; the fungus then softens the wood so the wood-boring larvae can feed. This makes the polypore both a decomposer and an insect symbiont, and it is one of the classic examples of fungus farming outside the ants and termites.
Similar species
Turkey Tail (Trametes versicolor) shares the thin tiered habit and zoned cap, but its cap surface is finely velvety rather than shaggy, its zones are sharply contrasting and often richly coloured, and its underside has small, evenly round white pores with no trace of maze-like slots or teeth. It also lacks the black line beneath the cap surface.
The Blushing Bracket (Daedaleopsis confragosa) has a genuinely maze-like underside, but its cap is smooth to finely wrinkled and reddish brown rather than hairy and grey-green, and its pore surface stains reddish where rubbed. Smoky Bracket (Bjerkandera adusta) shares the internal black line but has a smooth to finely downy grey cap and a smoke-grey to blackish pore surface with tiny round pores. Trichaptum biforme is thinner and more flexible, with violet tints on the toothed underside that Cerrena never shows.