Grey Knight

The Grey Knight (Tricholoma terreum) is the archetype of the grey-capped section of Tricholoma, a group of medium-sized, fleshy fungi with dry felted or fibrillose caps, notched gills and no ring. The specific epithet means earthy or soil-coloured, a fair description of the mouse-grey to slate-grey cap.

It is one of the most abundant autumn fungi of pine plantations and dunes on lime-rich ground, often appearing in large scattered troops along tracks and in short needle litter. Fruitbodies are noticeably fragile for their size, with a brittle stipe that snaps cleanly, and the thin flesh makes the cap margin liable to split radially as it expands.

The grey knights as a whole are one of the trickiest species complexes in the genus, and several closely related taxa — T. myomyces, T. scalpturatum, T. argyraceum, T. squarrulosum — have been separated from it on the basis of veil remnants, yellowing reactions and stipe ornamentation. Careful attention to those features is essential for a firm determination.

Cap

3 to 8 cm across, conical or bell-shaped when young, expanding to convex or flattened with a distinct broad umbo. The surface is dry and matt, covered in fine dark grey radiating fibrils that break up into small appressed scales, especially towards the centre, over a paler grey ground. The margin is thin, often splitting radially with age.

Gills

White to pale greyish, sometimes greying at the edges with age but not yellowing. They are moderately broad, fairly crowded and sinuate, notched where they meet the stipe. Gill edges are even to slightly eroded in older specimens.

Stipe

3 to 8 cm tall, 0.8 to 1.5 cm thick, cylindrical and equal or slightly tapered downwards, dry and white to very pale grey, smooth or minutely fibrillose. It is stuffed then hollow and characteristically brittle. There is no ring and no volva, though very young fruitbodies may show a faint fibrillose zone.

Flesh & spore print

The flesh is white, thin in the cap, and does not change colour when cut or bruised — an important point against relatives that yellow. Odour is faint or absent. The spore print is white and the spores are smooth and ellipsoid.

This is a pine specialist, occurring on soil and in needle litter under Pinus species, particularly Scots pine and Corsican pine. It prefers calcareous or sandy free-draining ground and is abundant in coastal dune pine plantations, on chalk downland plantations and along the edges of forest tracks where the litter layer is thin.

Fruiting begins in early autumn and continues late in the season, often persisting into November and December where frosts are mild. The species is common and widespread throughout Europe, with records from North Africa, temperate Asia and, under related names, parts of North America.

Tricholoma terreum is ectomycorrhizal with pines. The fungal mycelium forms a dense sheath around the short roots of the host and penetrates between the outer root cells, dramatically extending the volume of soil from which the tree can draw phosphorus, nitrogen and water. In return the fungus receives carbohydrates, which is why fruitbodies track the presence of living pines so precisely.

Because the partnership is with a long-lived host, the same mycelial patches fruit year after year, and the scattered arcs and troops seen on a track edge usually represent a small number of large individual mycelia. Spore release from the white gills is passive, relying on air movement beneath the cap.

Tricholoma scalpturatum is very similar in stature and cap texture but its gills and flesh slowly stain yellow, especially at the gill edge and around insect damage, and it grows with broadleaved trees as well as conifers. Tricholoma myomyces is often treated as a close relative or synonym and differs in retaining a fine cobwebby veil that leaves a faint ring zone on young stipes. Tricholoma squarrulosum has coarser, more upright dark scales on the cap and a stipe that is conspicuously scaly-dotted with dark grey right down to the base.

Tricholoma virgatum has a more sharply conical, silvery-grey cap with fine radial streaks rather than distinct scales, and a taller, more slender stipe. Grey-capped Entoloma species may resemble it at a glance but give a pink spore print and have angular spores, while Cortinarius species have a cobwebby cortina in youth and a rusty-brown spore deposit.