Pluteus thomsonii Identification Guide

First Impression and Setting

The usual encounter is unglamorous: a damp deciduous wood, a heap of rotting ash, elm or sycamore branches, and a scatter of small grey-brown mushrooms sitting on the wood or apparently on the leaf litter beside it. They are 1 to 3.5 cm across, slender-stemmed and easy to walk past. What makes you stop and crouch is the cap surface. Held at an angle to the light it is not smooth but sculptured, with a raised branching network of ribs and veins radiating from the centre and running out towards the margin, the veins darker than the spaces between them. That reticulate texture on a small grey-brown cap is the character that starts the identification.

Reading the Cap

Work on a fresh specimen, because the veining flattens as the cap dries and can vanish on old material. The cap begins convex or bluntly conical and expands to plano-convex, often with a slightly depressed centre in age. Colour is mouse-grey to dark grey-brown or sepia, usually darkest at the disc, occasionally with a slightly violaceous or olivaceous cast. The surface is dry and finely pruinose under a lens, and the venation is strongest over the central half, breaking into shorter isolated wrinkles towards the rim. The margin is even, sometimes faintly translucent-striate when very wet, and thin enough to split radially in old specimens. There is nothing sticky about the cap at any stage.

The Gills and the Free-Attachment Test

As with every Pluteus, this is the structural check that matters. Section a fruit body vertically. The gills are crowded, ventricose, moderately broad, and they terminate in a clean arc before reaching the stipe, leaving a visible channel around its apex. They pass from whitish or pale grey when young to clear flesh-pink and finally dull rose-brown as the spores mature. The edges are even, concolorous or a shade paler, and finely floccose under magnification. Note carefully that the edges are not darkened; a distinctly brown gill edge points to a different species.

The Stipe and Base

The stipe is 2 to 5 cm long and 2 to 4 mm thick, cylindrical or very slightly widened towards the base, and pale greyish to greyish-brown. The important feature is its texture: the surface is covered, especially in the upper half, with fine whitish to greyish longitudinal fibrils and a distinct pruinose or frosted bloom, so that the stipe looks streaked and slightly furry rather than polished. This granular-fibrillose finish is the visible expression of abundant sterile cells on the stipe surface, and it is a strong secondary character. There is no ring, no ring zone and no volva. The base may be slightly swollen and is bound to the substrate by white downy mycelium, often gathering wood fragments with it, so extract it whole rather than pulling the stipe out of its socket.

Flesh and Colour Change

The flesh is thin and greyish-white, more watery in the cap than in the stipe. It does not change colour on cutting, and bruised gills darken only through spore deposition and handling rather than through any chemical reaction. No latex is present.

Taking a Spore Print

Cut the cap free and set it gills-down on white paper under a small cover for six to eight hours; the caps are thin and dry out quickly, so humidity matters. The print is pink to pinkish-buff. Scrape it into a ridge with a knife blade to judge the colour properly, because a thin deposit can look almost white. Pink spores, free gills, and a stipe with neither ring nor volva place it in Pluteus without ambiguity. Microscopically the spores are smooth, broadly ellipsoid to subglobose and inamyloid.

Substrate and Season

The species is a saprotroph on well-decayed deciduous wood: fallen branches, rotting stumps, sawn timber gone soft, and especially buried fragments and woody debris mixed into leaf litter, which is why specimens frequently look terrestrial. Ash, elm, sycamore, maple and beech are all recorded hosts. It fruits from summer through autumn, singly or in small scattered groups, and favours humid, shaded, nutrient-rich woodland such as damp valley woods and old hedgerow banks. Always dig the base out to establish the wood connection; that observation is part of the identification.

Separating It From Look-alikes on Morphology

• Pluteus phlebophorus is the closest match in cap sculpture, sharing the strongly veined disc, but its cap is a warm ochre to rust-brown rather than grey-brown, and its stipe is white and smooth, without the grey fibrillose streaking.
• Pluteus umbrosus is larger and more dramatically ornamented, with a dark brown cap whose veins carry blackish fibrils, and its gill edges are distinctly dark brown, a feature absent here.
• Pluteus podospileus has a dark cap that is finely velvety, scurfy or granulose rather than raised-veined, and its stipe carries small dark dots or scales on a pale ground.
• Pluteus plautus and its allies are greyish with a granular-pruinose cap and stipe, but the cap lacks a coherent raised vein network.
• Pluteus cinereofuscus is grey-brown but has a smooth or only faintly wrinkled cap and often appears in grassy or disturbed ground.
• Entoloma species can be grey-brown with pink spores, but their gills are attached to the stipe, they usually grow from soil, and their spores are angular in profile.
• Small brown Psathyrella and Coprinellus species on the same wood have very fragile caps, dark grey to blackish spore prints, and no vein network.

Microscopic Confirmation

If you take the specimen home, three features settle it. The cap cuticle is built of erect, elongated, cystidium-like elements rather than the inflated round cells of the smooth-capped species, and this palisade is what raises the surface into visible veins. The cheilocystidia are clavate to broadly bottle-shaped and abundant. The pleurocystidia are thin-walled, without the thick walls and apical hooks of the metuloid group. Caulocystidia are plentiful on the stipe surface, matching the frosted appearance you noted in the field.

Field Checklist

Small size; cap grey-brown with a raised network of darker veins; gills free, white then pink, edges not darkened; stipe pale grey-brown with white longitudinal fibrils and a frosted bloom, no ring, no volva; flesh unchanging; spore print pink; on well-rotted deciduous wood or buried wood, summer to autumn.