Yellow Webcap Identification Guide

The Setting and the First Glance

You meet this species in damp deciduous woodland, most often where birch grows on heavy or clay-rich ground, sometimes with beech or oak nearby. It is an autumn mushroom of wet weather, and wet weather is exactly when it looks its best. The first impression is of a glossy, egg-yolk to honey-yellow dome that has clearly been varnished, with a fringe of stuck-on leaf fragments and grass blades adhering to the slime. The stipe is white and stout, which makes the yellow cap look brighter by contrast, and older caps in a troop will have a dusting of rusty-brown powder over the upper stipe where spores have dropped from the caps above.

Before anything else, confirm the genus. Look for a cobwebby veil. In a young button, fine silky threads run from the cap margin to the stipe like a spider web stretched across the gill chamber. As the cap expands those threads collapse onto the stipe and catch falling spores, leaving a rusty fibrillose girdle. That web, called a cortina, plus a rusty spore deposit and growth from soil under trees, places a collection in the webcaps.

Reading the Cap

Caps run 4 to 8 cm, occasionally 10 cm, convex at first and expanding to broadly convex or flat with a low umbo. The defining texture is glutinous rather than merely tacky: in humid conditions the surface carries a thick, slippery, slightly rubbery layer that pulls away as a transparent skin if you tease it with a fingernail. Colour is ochre-yellow to chrome-yellow, often warmer or slightly browner over the centre and paler toward the margin. In dry spells the gluten sets to a shiny varnish and the cap can look almost polished.

The margin is even, incurved for a long time, and not striate. Older caps sometimes develop faint radial streaks under the gluten where the flesh has thinned.

The Gills and Their Colour Sequence

The gill colour changes through the life of the fruit body, and catching it early is worth the effort. In genuinely young specimens the gills are pale bluish-lilac, clay-lilac or violaceous grey. That tint fades quickly, often within a day, passing to pale clay, then to dull ochre and finally to a rich cinnamon-rust as the spores mature. Attachment is adnate to slightly notched, spacing is moderately crowded, and the gill edge is often a shade paler than the face.

If you find only mature caps, split one down the middle and check the gill edge with a lens; residual pale violaceous tones sometimes linger there when the faces have gone fully rusty.

Stipe, Veil Layers and Base

The stipe is 5 to 10 cm long and 1 to 2 cm thick, white, cylindrical or slightly swollen downward, and rounded to bluntly clavate at the base. It never has a sharply rimmed, marginate bulb.

The critical detail is that the stipe is also glutinous, sheathed by a slimy universal veil that is colourless to pale yellowish. As the fruit body extends, that sheath breaks into slippery bands or leaves an oblique gluey ring zone partway up, above which the surface is dry, white and often finely fibrillose. A cap that is slimy over a dry stipe is a different group entirely; both surfaces slimy is the signature of the subgenus this species sits in.

The extreme apex of the stipe in fresh young material may carry the same faint bluish-lilac wash as the young gills. It is subtle and fugitive but supportive when present.

Flesh and Spore Print

Section the fruit body from cap centre to stipe base. The flesh is white and firm, occasionally with a lilac tinge in the upper stipe, and shows no dramatic colour change on cutting. The stipe flesh stays solid and does not become chambered.

For a spore print, choose a cap that has flattened and lay it gill-side down on white paper under a cover for a few hours. The deposit is rust brown to cinnamon brown, thick and obvious. Under the microscope the spores are subglobose to broadly ellipsoid and coarsely warted, roughly 7 to 9 micrometres long, which is another routine check for the genus. Wipe the gluten off the cap before printing or the paper will stick.

Substrate, Season and Ecology

This is a root-partner species, forming ectomycorrhizal associations rather than rotting wood. Look on moist soil among leaf litter and moss under birch above all, and secondarily beech and oak in mixed woodland. It favours damp, heavy or clay soils and low-lying woodland where water sits. Fruiting runs from late summer through autumn, peaking after prolonged rain, and fruit bodies are typically scattered to gregarious, sometimes in loose arcs following a root system. Because the fungus depends on living trees, it recurs in the same spots season after season.

Separating the Nearest Relatives

Cortinarius triumphans is the species most often confused with it. It is larger and more robust, its cap is only weakly viscid rather than truly glutinous, and its stipe is dry with conspicuous ochre-yellow girdles or belts of veil tissue. Its young gills are pale clay to greyish and never show the lilac cast.

Cortinarius mucosus and Cortinarius collinitus are also fully glutinous on cap and stipe, but they grow with conifers, their caps are orange-brown to red-brown rather than yellow, and the stipe sheath in the collinitus group often breaks into bluish or violet-tinted bands.

Cortinarius vibratilis is a slighter mushroom with a paler yellow, strongly viscid cap and a slimmer white stipe, and it lacks the lilac gill phase. Cortinarius croceocaeruleus is smaller with a whitish-lilac cap and pronounced lilac gills.

Outside the genus, Hebeloma species have a brown spore print and no cortina, Pholiota species are scaly and grow in tufts on wood, and Gymnopilus species have rusty spores but dry, often fibrillose caps and grow on wood. Checking for the cobwebby veil, the double gluten layer and the terrestrial habit sorts these out quickly.

Field Notes Worth Taking

Record the state of the gluten in relation to the weather, the exact gill colour on the youngest cap present, the tree species within a few metres, and whether a marginate bulb is present or absent. Photograph a young button in section, because the lilac tones and the intact slimy sheath are the two characters most likely to be gone by the time you get home.