Yellow Swamp Brittlegill

Yellow Swamp Brittlegill (Russula claroflava) is the brightest of the common yellow brittlegills and the one most tightly bound to a habitat. It grows in wet, mossy birch woodland and at the edges of bogs, where its clear chrome-yellow caps sit among Sphagnum and are visible at considerable distance.

The epithet claroflava means "clear yellow", and the colour really is unmixed — no olive, no ochre, no orange. Combined with the greying reaction of the flesh and the pale ochre spore print, this makes it one of the more straightforward Russula species to name with confidence, which is unusual in a genus notorious for requiring chemical tests and microscopy.

It sits close to a small group of brittlegills that grey or blacken on handling — a much milder version of what R. nigricans does — and this slow discolouration is often the clinching character when a bright yellow Russula is found away from the classic bog setting.

Cap

4–10 cm across, convex then expanding to flat or shallowly depressed. The colour is a bright, clear chrome- or lemon-yellow, sometimes with a slightly deeper tone at the centre, and it holds its intensity well rather than fading to cream. The surface is smooth and faintly shining, slightly sticky when wet, drying to a soft matt sheen. The margin stays smooth or becomes only faintly furrowed. The cuticle peels roughly halfway to the centre.

Gills

Pale cream at first, deepening to a warm ochre-cream at maturity, moderately crowded, brittle, and adnexed to almost free. Bruised or aged gills grey and then darken to slate. Short intermediate gills are present near the margin.

Stipe

5–10 cm tall, 1–2 cm thick, cylindrical or gently tapering upward, white and dry, faintly wrinkled lengthwise. It greys progressively with handling and age, especially at the base, without a ring or volva, and snaps cleanly across. The interior becomes stuffed then partly hollow.

Flesh & spore print

The flesh is white and firm, turning slowly grey and finally almost black over an hour or more where cut or damaged — the single most useful confirming character. The odour is faint and slightly fruity. The spore print is pale ochre to deep cream; spores are broadly ellipsoid with fairly tall isolated warts joined only sparsely.

This is a specialist of wet ground under birch. It fruits in Sphagnum moss at bog and mire margins, in damp birch carr, in boggy hollows and along the wet edges of birch-dominated woodland, and occasionally under aspen or willow in similar conditions. Acid, peaty, permanently moist soils are the rule; it is essentially absent from dry or calcareous ground.

It is widespread across northern and central Europe, Scandinavia, northern Asia and northern North America, becoming more frequent northwards and in upland areas where wet birch woodland is common. Fruiting runs from mid-summer through to late autumn, and the mushrooms often stand with their stipe bases buried deep in saturated moss.

Russula claroflava is ectomycorrhizal with birch, forming a fungal mantle on the tree's short roots and a Hartig net between the outer root cells. In the waterlogged, acidic, nutrient-poor peat where it lives, this partnership is central to the birch's nitrogen and phosphorus supply, since much of the available nutrient is locked in slowly decomposing organic matter that the fungal hyphae are better equipped to exploit than roots alone.

The association is strong enough that the fungus effectively maps the presence of birch on wet ground, and it is a useful indicator species for that habitat. Its fruiting bodies are heavily used by invertebrate larvae and by slugs, and their rapid breakdown returns nutrients to the surface peat. The greying of damaged tissue is a wound response, caused by oxidation of pigments in the flesh when cells are ruptured.

Russula ochroleuca is the commonest confusion but is a duller ochre or olive-yellow, grows in any woodland including dry ground, and its flesh stays white when cut rather than greying and blackening. Russula fellea is a paler uniform straw colour with gills and stipe the same tint as the cap and a distinct geranium-leaf odour, and grows with beech on drier soils.

Russula risigallina (R. lutea) is smaller, more fragile, warmer orange-yellow, has a deep ochre spore print and shows no greying. Russula puellaris also browns and yellows with age but is much smaller and more delicate, with a more purplish or brownish cap. Colour purity, habitat wetness, and the greying reaction in cut flesh separate this species reliably.