Splendid Webcap

Splendid Webcap (Cortinarius splendens) is one of the most colourful members of subgenus Phlegmacium, the large webcaps with viscid caps and dry stems. Everything about it is yellow: the cap is bright sulphur to chrome yellow, the gills are yellow, the stem is yellow and even the flesh is yellow throughout, usually with a scatter of rusty-brown blotches developing on the cap disc.

The second key structural feature is the stem base, which swells into a flattened bulb with a sharp horizontal rim — a marginate bulb — as though the base had been pressed into a shallow dish. Marginate bulbs are common in the calcareous-woodland Phlegmacium group and are worth digging carefully to see, since they are lost if the stem is snapped off.

The species is a warm-climate calcicole, largely confined to beech woodland on chalk and limestone, and is regarded as a good indicator of ancient, unimproved calcareous forest in the northern parts of its range where it is scarce.

Cap

Roughly 4–9 cm across, convex then flattening, with a viscid to slightly glutinous surface when moist that dries shiny. The colour is a striking bright sulphur-yellow to chrome- or golden-yellow, usually developing rust-brown to olive-brown patches and blotches at the centre with age. The margin stays paler and is inrolled at first.

Gills

Crowded and adnate to notched, bright lemon- to sulphur-yellow when young, then dulling to olive-yellow and finally overrun by rusty brown as the spores mature. The bright yellow gill colour of a young specimen, with no trace of violet, is an important character. A yellowish cortina veils them in youth.

Stipe

4–9 cm tall and 1–2 cm thick, yellow throughout, silky-fibrillose, and ending in a prominent flattened bulb with a distinct raised rim or margin. Rusty veil fibrils commonly stain the upper stem where the cortina collapsed.

Flesh & spore print

The flesh is yellow in cap and stem, sometimes a little paler in the bulb, firm, with a faint indistinct smell. There is no strong bruising change. The spore print is rusty brown; spores are almond-shaped and warted. Alkali applied to the flesh produces a darkening reaction in this group, which specialists use alongside macroscopic features.

Cortinarius splendens grows with beech, occasionally with oak and other broadleaves, on chalk, limestone and other strongly calcareous soils. It is a species of warm, well-drained slopes in mature beech woodland, often on thin soils over rock, and fruits singly or in scattered groups in the leaf litter.

The season runs from late summer into autumn. Its distribution is centred on central and southern Europe, extending north patchily, and it becomes markedly rarer and more localised towards the northern limits of beech woodland, where it is treated as a notable indicator species of long-established calcareous forest.

This webcap is ectomycorrhizal with beech and related broadleaves. Its hyphae form a mantle over fine root tips and a Hartig net between the outer root cells, exchanging soil-derived nitrogen, phosphorus and water for photosynthate. On thin, free-draining calcareous soils this partnership is particularly important for the tree's water relations during summer drought.

The brilliant yellow pigments of the fruitbody are anthraquinone-type compounds widespread in Cortinarius, the same broad chemical family that gives other webcaps their reds and olives. Because the mycelium is perennial and sensitive to soil disturbance and nutrient enrichment, populations are closely tied to undisturbed ancient woodland, and the species features on conservation lists in several countries.

Several yellow Phlegmacium webcaps are close. Cortinarius elegantissimus is a warmer orange-yellow with a more olive cap and is likewise marginate-bulbed on chalk; Cortinarius sulfurinus and C. citrinus differ in the shade of yellow and in microscopic detail. Separating them reliably relies on spore shape and alkali reactions rather than field colour alone.

Cortinarius rufoolivaceus has purple and olive tones absent here, and Tricholoma species such as T. sulphureum are yellow but have white spore prints, dry caps, no cortina and no rimmed bulb. Any yellow-capped mushroom with a rusty brown spore print, a cobwebby cortina and a sharply rimmed basal bulb belongs in this group.