Yellow Webcap

Yellow Webcap (Cortinarius delibutus) belongs to subgenus Myxacium, the webcaps in which both cap and stem are coated in a glutinous veil. In wet weather the whole fruitbody is slippery and gathers leaves and needles; in dry weather the gluten sets to a shiny varnish that still shows as a gleam in raking light.

Its field impression is of a clean, warm ochre-yellow to buff mushroom of birch woodland. The surprise comes from looking underneath: young gills are washed pale bluish-lilac, and the very top of the stem carries the same cool blue tint, an unexpected contrast with the yellow above. Both fade as rusty spores mature.

The genus Cortinarius is defined by the cobwebby cortina and by rusty brown spores, and C. delibutus shows both features clearly. Together with the glutinous surfaces and the birch association, they make it one of the more tractable species in a famously difficult genus.

Cap

Typically 3–8 cm across, hemispherical then convex and finally almost flat, remaining rather rounded in outline. The surface is smooth and distinctly glutinous when moist, drying shiny, and is coloured ochre-yellow, honey-yellow or warm buff, often a little deeper at the centre and paler at the margin. The margin is inrolled at first and may carry pale veil fibrils.

Gills

Crowded, adnate to notched, and importantly pale bluish-lilac or greyish-violet in young fruitbodies before turning clay-buff, cinnamon and finally rusty brown. The gill edges are slightly paler and finely uneven. A whitish cortina spans cap margin to stem in buttons and leaves a rusty band later.

Stipe

5–10 cm tall, roughly 0.8–1.5 cm thick, cylindrical with a slightly clubbed base. It is whitish to pale yellow with a distinctly bluish-violet apex when young, and is sheathed in a glutinous veil that dries as irregular yellowish or ochre bands and a slimy zone where the cortina collapsed.

Flesh & spore print

The flesh is white to pale cream, sometimes faintly bluish in the stem apex, firm in the cap, with a mild, indistinct smell. There is no dramatic colour change on cutting. The spore print is rusty brown; the spores are broadly ellipsoid to almost round and warted.

This webcap grows chiefly with birch, and also with beech and occasionally hazel or conifers, on damp, acid to neutral soils. It is characteristic of moist woodland with a thick litter or moss layer, woodland tracksides and the edges of boggy hollows, appearing scattered or in loose troops.

The season runs from late summer well into autumn. Cortinarius delibutus is widespread and locally common across northern and central Europe, thinning southwards, and is also reported from birch and mixed hardwood forest in northern North America.

C. delibutus is ectomycorrhizal, forming sheathed root tips on birch and beech. The fungal mantle and the emanating hyphae explore soil volumes the roots cannot reach, mobilising nitrogen and phosphorus from litter and mineral grains and delivering them to the host in return for sugars.

The glutinous veil that coats cap and stem is thought to protect the developing fruitbody from drying and from small grazing invertebrates, and it is a repeated evolutionary theme in the Myxacium webcaps. Underground, the mycelium is perennial and can persist for decades, which is why productive patches recur in the same part of a wood season after season.

Cortinarius trivialis (Girdled Webcap) is also glutinous but darker olive- to ochre-brown with conspicuous snake-skin girdles low on the stem, and it favours willow and poplar on damp calcareous ground. Cortinarius mucosus and C. collinitus are larger conifer species with browner caps and lack the bluish gill tints.

Cortinarius vibratilis is smaller with a paler ochre glutinous cap and a slender stem without blue tones. Among other genera, slimy Hebeloma species have a clay-brown spore print and never show violaceous young gills, while Suillus species growing in the same damp woods have pores instead of gills. The pairing of a yellow glutinous cap with blue-lilac young gills is the signature to look for.