Nameko

Nameko is the East Asian member of Pholiota best known outside mycological circles, largely because it is produced on a very large scale on inoculated logs and sawdust blocks in Japan, China, Taiwan and the Russian Far East. The name means roughly 'slippery mushroom' in Japanese, a direct reference to the thick layer of clear gel that coats the cap.

Taxonomically it has moved names more than once. Pholiota nameko is the name under which it appears in most horticultural literature, but the currently favoured binomial for the wild and cultivated East Asian populations is Pholiota microspora, the epithet referring to the unusually small spores that distinguish it from relatives such as P. aurivella.

In morphology it is a typical Pholiota: clustered fruit bodies on dead hardwood, amber to tawny caps, gills that darken from pale yellow to cinnamon, and a rusty-brown spore deposit. What sets it apart is the extreme development of the gluten layer, which in young specimens can be several millimetres thick and gives the whole cap a glazed, wet-varnish appearance even in dry weather.

Cap

Usually 2-8 cm across, hemispherical or bluntly conical at first and expanding to convex or nearly flat. The colour is a warm amber, orange-brown or tawny, often paler and more yellow towards the margin. The defining feature is the thick, clear to slightly yellowish gelatinous coating, which makes the cap glisten and feel distinctly slimy; the underlying surface is smooth, though a few pale veil fragments may cling near the margin in young buttons.

Gills

Adnate, sometimes with a short decurrent tooth, crowded and fairly narrow. They are pale cream to yellowish when young and mature through ochre to a warm cinnamon-brown. The gill edge is typically whitish and finely uneven.

Stipe

3-8 cm long and 5-12 mm thick, cylindrical, often curved and joined to neighbouring stipes at a common base. It is pale yellowish-brown at the apex, browning downwards, and coated below the ring zone in the same slime as the cap when fresh. The partial veil forms a thin, gelatinous ring or ring zone on the upper stipe that soon collapses and is stained rusty by falling spores.

Flesh & spore print

The flesh is whitish to pale yellow, firm in the cap and more fibrous in the stipe, and does not change colour when cut. The spore print is rusty brown; the spores are smooth, ellipsoid and notably small for the genus, which is the source of the epithet microspora.

In the wild the fungus fruits in dense clusters on dead hardwood, principally beech and Japanese oak, colonising fallen trunks, stumps and large branches in cool temperate forest. It is native to East Asia, with a natural range covering Japan, Korea, China, Taiwan and the Russian Far East, and it favours humid, shaded valley woodland.

Fruiting is chiefly in autumn and continues into early winter, since the species is tolerant of low temperatures and often produces its heaviest flushes after the first cold nights. The same substrate preferences are used in commercial production, where hardwood logs and sawdust-based blocks are inoculated with the mycelium and fruited in shaded, humid conditions; this practice has made the fungus familiar far beyond its natural range.

Pholiota microspora is a saprotrophic wood decomposer causing a white rot. Its mycelium spreads through the sapwood and heartwood of fallen hardwoods, degrading lignin along with cellulose so that the residual wood becomes pale, soft and stringy. Colonisation typically follows the death of the tree or limb, and a single large trunk can support fruiting for several consecutive seasons until the wood is exhausted.

The heavy gluten layer is thought to protect the developing fruit body against desiccation and possibly against insect grazing, an advantage in the fluctuating humidity of temperate forest autumns. Ecologically the species is part of the deadwood recycling community of East Asian broadleaf forest, releasing nutrients bound in fallen timber back into the soil and creating soft, decayed wood that supports invertebrates and mosses.

Pholiota aurivella is larger, with a cap that reaches 15 cm, and although it is also viscid it bears conspicuous flattened rusty scales and typically fruits from wounds high on standing broadleaf trunks. Pholiota lubrica is similarly slimy but has a redder brown cap centre with a distinctly paler margin and a dry, fibrillose-scaly stipe.

Kuehneromyces mutabilis forms comparable tufts on hardwood but has a dry, strongly hygrophanous two-tone cap and small recurved scales on the stipe below a distinct ring, with no gelatinous coating anywhere. Galerina marginata is smaller and duller honey-brown with a thin membranous ring and a dry to only faintly tacky cap. Among Hypholoma species the caps are dry and the spore print is purple-brown rather than rust.