Soapy Knight Identification Guide

The Problem, And The Setting

The Soapy Knight is one of those species that beginners meet several times before they realise they have been finding the same fungus all along. Its cap colour is genuinely all over the place - olive-grey, sage green, grey-brown, olive-brown, tan, straw, slate, and in some forms nearly blackish or almost white - and the varieties have been split and lumped repeatedly. So the identification cannot rest on colour. It rests on a combination of gill attachment, staining behaviour and smell that stays constant across every form.

You will be standing in woodland from late summer into late autumn. It is ectomycorrhizal and catholic in its partners: beech and oak woods, birch scrub, spruce and pine plantations, and mixed woods on both acid and neutral soils all produce it. It fruits from soil and litter, singly, in scattered groups, or in loose arcs, and it is often abundant where it occurs.

The first impression is of a solid, dry-looking, medium-sized mushroom with a slightly greasy sheen, a dull greenish or greyish cap, and a stout white stem - a mushroom that looks unremarkable in a way that is itself a clue.

Reading The Cap

Size is 4-12 cm, occasionally larger. Young caps are convex with an inrolled margin; mature ones spread out broadly and often develop a low, blunt umbo, and they are frequently lopsided or wavy rather than neatly circular. The surface is dry to slightly greasy, smooth, and in dry spells it commonly cracks into small flattened scales or fine areolate patches toward the centre while the margin stays smooth.

Colour, as noted, is variable, but there are patterns: the centre is usually darker than the rim, olive and grey-green tones are the commonest, and the margin often pales toward buff or cream. Look also for pinkish or reddish flushes appearing on the cap surface where slugs have grazed or where it has been knocked - the same staining reaction that runs through the whole fruit body.

Gills

Turn the cap over. The gills are sinuate, that is, they run toward the stem, swell, and then abruptly notch upward before attaching - the diagnostic gill attachment of the genus Tricholoma. Get down and look at the junction in profile against the light rather than trying to judge it from below.

They are moderately distant to fairly well spaced, thick, and rather deep, white to cream or with a pale greenish or greyish tinge. Their edges are often slightly uneven or eroded in older material. The critical behaviour is staining: bruise a gill with a fingernail, or look at gills that insects have chewed, and they discolour pinkish, salmon or rusty-reddish. Old caps often show this as a general dirty pink flush across the gill faces.

Stem, Base And Flesh

The stem is 5-12 cm long and 1-3 cm thick, firm and solid at first, white to greyish-white, sometimes with fine grey to olive fibrils or small scales in the lower half, and it may be more or less equal, swollen in the middle, or tapered into a short rooting point pushed into the litter.

Dig it out and examine the base, because the base is where the diagnostic reaction is strongest. In most collections the lower stem and the base show a distinct pink, salmon or reddish flush, either on the surface or revealed as soon as you cut.

Now slice the whole fruit body from cap centre to stem base and watch. The flesh is white and firm; within a few minutes the cut surface, especially in the lower stem, develops a pale pink to salmon or reddish-brown tone. It is not a fast or dramatic reaction like the blueing of a bolete, but it is consistent, and it is the single most useful confirmatory test in the field. There is no latex.

The Smell

Crush a fragment of flesh between thumb and forefinger, warm it, and sniff. The odour is unmistakable once known: soapy, like a bar of household soap or laundry, sometimes with a mealy or slightly fruity edge, occasionally described as reminiscent of coal tar. It is stronger in the stem flesh than in the cap and stronger in fresh material. This smell, combined with sinuate gills and pink staining, is effectively conclusive.

Spore Print

Sinuate gills print cleanly. Cut the stem flush with the cap, set the cap gill-side down on black or half-black paper, cover it, and leave it six hours or overnight.

The deposit is white. Under the microscope the spores are small, smooth, broadly ellipsoid, about 5-6.5 by 3.5-4.5 micrometres, and inamyloid. There are no cystidia of note on the gill faces, and clamp connections are absent - all consistent with the genus.

Substrate, Season And Ecology

The Soapy Knight forms ectomycorrhizas, sheathing the fine root tips of its host and trading mineral nutrients and water for sugars. Because it is not fussy about partner or soil chemistry, it turns up in a wide spread of habitats where more specialised knights do not, from chalk beechwood to acidic conifer plantation. It always emerges from soil or litter, never from wood, and it favours slightly disturbed edges - path sides, ride margins, bank tops - as much as closed woodland floor.

Fruiting runs from August to November, peaking in October in most temperate regions.

Separating It From Look-alikes By Structure

Tricholoma sejunctum has a yellowish to olive-yellow cap densely streaked with dark radiating fibrils, whiter and more crowded gills, and no pink staining anywhere; its flesh stays white on cutting.

Tricholoma portentosum has a grey to grey-violet, distinctly greasy cap with black radial fibrils, and its gills and stem apex develop yellowish rather than pinkish tones. Its flesh does not flush pink.

Grey Knight (Tricholoma terreum) is smaller, thinner-fleshed, dry and woolly-fibrillose ash-grey, with grey-tinged gills, no pink staining and no soapy smell.

Tricholoma virgatum and Tricholoma sciodes are grey and conical with radially fibrillose caps, and their gill edges tend to spot greyish-black rather than pink.

Tricholoma equestre and its allies have distinctly yellow gills throughout.

Russula and Lactarius species may sit alongside it with similar colours, but their gills are adnate to decurrent and never notched, their flesh is brittle and crumbly rather than fibrous, and Lactarius exudes latex when cut.

Melanoleuca species have crowded gills, tall straight stems and a rougher, warted spore ornament, and they lack both the pink staining and the smell.

Field Checklist

Woodland soil, late summer to late autumn; variable olive, grey or brown cap, often cracking; sinuate notched gills that stain pinkish; white to greyish stem with a pink to reddish flush at the base; white flesh slowly flushing pink on cutting; soapy odour; white spore print.