Platterful Mushroom

Megacollybia platyphylla is a familiar mushroom of decaying hardwood in temperate woodland, once placed in Collybia and Tricholomopsis before being given a genus of its own. The name refers to its broad gills, which are among the widest of any medium-sized woodland agaric and are spaced far enough apart to be counted at a glance.

The most useful field character is not on the mushroom itself but at its base. Teasing away the litter reveals thick, white, string-like rhizomorphs running away from the stem through the soil and rotting wood, sometimes for tens of centimetres. These cords are the fungus's exploratory strands, and their presence, combined with broad gills and a fibre-streaked grey cap, identifies the species quickly.

In North America the name has been applied to several closely related species, of which Megacollybia rodmanii is the most widespread in the east; they differ in subtle features of the cap surface and microscopic structures and are essentially identical in the field. All share the same ecology as decomposers of hardwood, including buried wood and mulch, which brings them into gardens and landscaped areas as well as forest.

Cap

5-15 cm across, occasionally larger, convex with an inrolled margin when young, expanding to broadly convex and finally flat, sometimes with a shallow depression or a low umbo. The colour is grey-brown, mouse-grey, olive-brown or sooty-brown, paler with age and in dry weather. The surface is dry and matt and is conspicuously streaked with darker radial fibrils, giving a brushed appearance. The thin margin often splits radially on older caps.

Gills

White to cream, sometimes discolouring buff, and remarkably broad, up to a centimetre or more deep, with distinctly wide spacing. They are attached by a notch or narrowly adnexed, occasionally appearing almost free on very mature caps. The edges are even and sometimes finely ragged in age.

Stipe

5-15 cm long and 1-2 cm thick, roughly equal or slightly enlarged below, white to greyish and longitudinally fibrous, dry, hollowing with age, and without a ring. At the base it connects to thick white to pale grey rhizomorphs, cord-like strands that thread away through litter, soil and rotten wood and are the single best confirming feature.

Flesh and spore print

The flesh is white, firm but rather thin for the size of the cap, and unchanging when cut. The spore print is white; spores are smooth and broadly ellipsoid.

Megacollybia platyphylla grows on and around decaying hardwood: rotting logs and stumps, buried wood, root fragments, sawdust and wood-chip mulch. Fruit bodies often appear to arise from bare soil or leaf litter, but the rhizomorphs always lead back to buried woody material. Beech, oak, birch and maple are typical hosts, and the species also fruits readily in landscaped beds and along wood-chipped paths.

The season is long, running from late spring through summer to autumn, with the heaviest flushes in warm, wet weather. It occurs across temperate Europe and temperate Asia, with the closely related Megacollybia rodmanii and other segregates occupying the same niche in North America. It is a common fungus of mature deciduous woodland and equally at home in parks and gardens where hardwood mulch is used.

This is a saprotrophic white-rot fungus. It decomposes lignin and cellulose in fallen and buried hardwood, and colonised wood becomes pale, soft and fibrous. Its rhizomorphs are the key to its success: these tough, insulated cords transport water and nutrients and allow the mycelium to bridge gaps of soil between separate pieces of wood, so a single individual can exploit several scattered fragments at once.

That reach lets the species colonise fragmented and buried resources that many wood-rotters cannot reach, including chipped mulch, and it explains why fruit bodies frequently appear well away from any visible log. In woodland it plays a substantial role in breaking down the buried component of hardwood debris, returning nutrients to the soil and creating soft, moisture-holding pockets used by invertebrates and seedling roots.

Hymenopellis radicata has similarly broad, spaced white gills but its cap is ochre-brown, greasy and radially wrinkled rather than dry and dark-streaked, and its stem tapers into a long underground rooting tail instead of ending at rhizomorphs. Pluteus cervinus grows on the same rotten hardwood and has a brown fibre-streaked cap, but its gills are entirely free from the stem and turn pink, and it drops a pink spore print.

Tricholoma and Clitocybula species can resemble it in cap colour, but Tricholoma species grow from soil in mycorrhizal association with trees and lack rhizomorphs, while Clitocybula species are smaller with crowded gills. Entoloma species with grey-brown caps have attached gills that turn pink and a pink print. Where doubt remains, excavating the stem base to look for white cords is decisive.