Gassy Webcap Identification Guide

The moment you spot it

You are nearly always standing in conifer forest when this one turns up. Spruce and pine on acid ground, a floor of moss, bilberry or a deep spongy mat of old needles, often in the cooler and more northerly or upland parts of the range. What catches the eye is a chunky, opaque, pale violet mushroom that looks as though it has been dusted in lilac chalk. It is not a translucent or glossy lilac; it is matt, dry and slightly bluish, spread over a heavy, self-contained body with a thick cap and a thicker stem. There is nothing elegant about the silhouette. Fruitbodies usually appear in scattered troops rather than singly, and older ones fade to a dirty buff with only a memory of violet at the margin.

The most useful thing to do at this point is to pick one, split it lengthways from cap to stem base, and look at the interior. That single action separates this species from almost every other pale violet mushroom growing beside it, and it is worked through below.

Reading the cap

The cap runs roughly 4 to 12 cm across. Young ones are strongly convex, almost hemispherical, with the margin curled tightly inwards; with age they expand to broadly convex or nearly flat, often staying slightly irregular rather than becoming a clean disc. The surface is dry to the touch at every stage. Never expect a viscid or greasy feel here, even after rain.

Under a hand lens the skin is finely silky-fibrillose, with faint radial fibres running from centre to margin. The colour is a clear pale lilac to bluish violet when young and fresh, and this is a surface pigment only. As the fruitbody ages, ochre and tawny tones bleed in from the disc outwards, so a mature specimen may be violet only at the rim, or scarcely violet at all. Because of this, always look for the youngest button in the troop when you are trying to read colour.

The gills and the cortina

Turn the cap over. Very young gills carry a violet or greyish-lilac wash, but they never stay that way. They shift quickly through clay-buff and ochre to a dull cinnamon or rusty brown as the spores ripen. Gill attachment is adnate to slightly notched, the spacing is moderately close, and the gill edges are usually a shade paler than the faces.

Stretched across the young gills you should find the cortina: a cobwebby veil of fine silky threads running from the cap margin to the stem, which is the diagnostic feature of the whole genus. It is fugitive. Within a day or two it collapses onto the upper stem, where escaping spores stain it into a rusty-brown fibrillose band or ring zone. If you find a ring zone the colour of dried rust on a violet stem, you are in the right genus and most of the way to this species.

Stipe and base

The stem is short and thick relative to the cap, typically 5 to 12 cm tall and 1.5 to 3 cm wide, often noticeably swollen through the middle or clavate towards the base, sometimes almost barrel-shaped. It is solid, firm and fibrous, coloured the same pale lilac as the cap in youth and covered with fine silky fibrils.

The base is important. It broadens into a rounded club rather than ending in a bulb with a sharp rim, and there is no volva, no sac, no free skirt-like ring. Excavate it gently rather than snapping it off, because the club shape and the way white to lilac mycelial threads bind the needle litter are both part of the picture.

The cut, the flesh and the odour

This is the decisive step. Slice a whole fruitbody in half from the top of the cap down through the stem base. In this species the flesh inside the stem is not white. It is a distinct saffron, ochre-brown or dull yellowish-brown, sometimes marbled, and it contrasts sharply with the lilac exterior. Cap flesh is paler, often whitish to pale lilac near the skin, but the stem interior gives the game away. There is no dramatic instantaneous colour change on cutting; the point is that the flesh was already brownish before you cut it.

The odour is strong, persistent and unmistakable once learned: pungent, gassy and goaty, sometimes described as acetylene-like or like an old billy goat. Cup the cut fruitbody in your hands for a moment to warm it and the smell intensifies. Combined with the brown stem flesh, it is essentially conclusive.

Taking a spore print

Cut the cap free of the stem, set it gills-down on white paper (dark paper is a waste of time here, since the print is not white), cover it with a bowl or cup to keep the humidity up, and leave it several hours or overnight. Mature caps drop heavily. The print is rusty brown to cinnamon-brown, sometimes with a faintly ochre cast.

That rusty print is the fastest way to eliminate the entire white-spored and pink-spored crowd of lilac mushrooms. If your print comes out white, cream or pink, you are not looking at a webcap at all.

Substrate, season and partners

Gassy Webcap is ectomycorrhizal, forming root partnerships with conifers, particularly Norway spruce and pines. It fruits from soil and litter, never from wood, and it is a creature of mossy, acidic, often nutrient-poor forest floors. Bilberry-dominated spruce forest is classic ground for it. The season runs from late summer into autumn, peaking after the first properly wet spell that follows warm weather, and it is far commoner in boreal, montane and northern temperate forest than in lowland broadleaf woods.

Separating the close look-alikes on structure alone

Cortinarius camphoratus is the nearest problem: a violet webcap of the same conifer habitat, with a similar stature. Its flesh tends to be paler and whitish to lilac in the stem rather than saffron-brown, and its odour, while equally strong, reads as sickly-sweet and rotten rather than gassy and goaty. When in doubt, trust the stem flesh colour first.

Cortinarius alboviolaceus is silvery whitish-violet rather than solidly lilac, distinctly more slender, with white flesh in the stem and a stem base that is often clavate but capped by a marginate swelling. It leans strongly to broadleaf woodland, especially beech, oak and birch, and it has no notable smell.

Cortinarius malachius and its relatives share the lilac tones but generally have a scaly-fibrillose cap surface and whitish stem flesh.

Lepista nuda is the commonest false alarm for beginners. It has no cortina at any stage, its gills stay lilac then turn pale pinkish-buff, they are sinuate and easily separated from the cap flesh, and the spore print is pale pink, not rust. It also fruits in leaf litter and compost-rich ground rather than conifer moss.

Laccaria amethystina is a fraction of the size, with thick, widely spaced, deep violet gills that stay violet, a fibrous wiry stem and a white spore print.

A working sequence

In practice: confirm conifers overhead and soil below; check the cap is dry and silky, not slimy; find a rusty ring zone or cortina; cut the fruitbody in half and look for saffron-brown stem flesh; smell it; then take a rust-brown print to confirm the genus. Five steps, one of which is free.