Gassy Webcap

The Gassy Webcap is a substantial, pale lilac mushroom of boreal and montane conifer forest, and one of the few webcaps that can be named with confidence from a combination of colour, flesh and smell. Its common name refers to the strong, penetrating, distinctly chemical odour that hangs around fresh fruitbodies and is often noticeable before the mushroom itself is spotted.

It belongs with the dry-capped, silky, veiled webcaps of the Sericeocybe group. The single most reliable character is internal: slicing the stipe lengthways reveals flesh that is not white but a marbled ochre to saffron-brown, in sharp contrast to the pale lilac exterior. This alone separates it from all the other lilac webcaps.

The species is common through the northern conifer belt and in mountain spruce forest further south, where it fruits in groups on acid, humus-rich ground beneath dense canopy.

Cap

4-10 cm across, hemispherical then convex and finally broadly flattened. The surface is dry, silky-fibrillose and often finely cracked or scaly in age, coloured pale lilac to bluish-violet when young and fading through greyish lilac to dull ochre-buff, usually starting at the centre. The margin is inrolled for a long time and hung with pale veil fibres.

Gills

Adnate to emarginate, moderately crowded, and importantly ochre to saffron or clay-brown from an early stage, never violet. They deepen to rusty cinnamon at maturity, and the contrast between lilac cap and warm ochre gills is a useful field cue.

Stipe

Stout, 5-12 cm long, thickening downward into a swollen or bulbous base, lilac on the surface and sheathed in a silky pale violet veil that ends in a woolly ring-like zone. Above the zone the cortina leaves rust-dusted fibres.

Flesh & spore print

The diagnostic feature. Flesh in the stipe, and often the cap, is ochre-yellow to saffron-brown and conspicuously marbled or streaky rather than white, unchanging on exposure. The smell is strong, penetrating and chemical. Spore print rusty brown, with almond-shaped, warted spores.

This webcap is confined to coniferous woodland, growing chiefly with Norway spruce and Scots pine on acid, humus-rich or peaty soils, and occasionally in mixed stands where birch is present among conifers. Fruitbodies appear singly, in groups or in loose clusters among mosses and needle litter, often in the deeper shade of mature plantations and old forest.

The season is late summer to autumn. It is common in Scandinavia and the boreal zone, in montane spruce forests of central Europe, and in Scottish and northern British conifer woods. It also occurs widely in the conifer forests of northern North America.

Cortinarius traganus is ectomycorrhizal with conifers, enclosing spruce and pine rootlets in a fungal mantle and forming a Hartig net through which mineral nutrients and water pass to the tree in exchange for sugars. On the acid, slowly mineralising humus of northern forests, such partnerships are central to tree nutrition, and webcaps as a group are among the most abundant ectomycorrhizal fungi in these ecosystems.

The strong volatile compounds responsible for the smell are released continuously from fresh fruitbodies and may function in signalling to insects or in deterring grazers. The mycelium is perennial and long-lived, forming persistent colonies that fruit reliably from the same ground each autumn once rain arrives.

Cortinarius camphoratus is the closest match in colour and conifer habitat, but its young gills are bluish-lilac rather than ochre and its flesh is whitish-lilac rather than saffron-marbled. Cortinarius alboviolaceus is a broadleaf-woodland species with a pearly silvery cap, whitish flesh and no strong smell.

Cortinarius malachius shares the lilac tones and conifer habitat but has a more scaly cap, violet young gills and pale flesh. Cortinarius violaceus is far darker, a deep blackish violet with a dry scaly-woolly cap and violet flesh throughout.