Yellowing Brittlegill
Russula puellaris
Also called yellowing russula
A small, fragile brittlegill whose entire fruiting body — cap, gills, stipe and flesh — discolours a deep honey-yellow with age or handling. It is common in damp mossy conifer woodland.
Overview
Russula puellaris, the Yellowing Brittlegill, is a small and easily overlooked mushroom with one memorable habit: everything about it turns yellow. Cap, gills, stipe and internal flesh all discolour progressively to ochre and then a deep honey or butter yellow, whether from age, damp or simple handling, so that a basket of fresh specimens looks quite different an hour later.
The fresh cap is usually a dull purple-brown, wine or violet-brown, often with olive or ochre tones at the centre and a paler, fading margin. It is thin-fleshed, easily peeling and quickly grooved at the edge, placing it among the more delicate members of the genus.
The species is characteristic of damp, mossy conifer and mixed woodland, where it fruits in small scattered groups among Sphagnum, feather mosses and needle litter. Among brittlegills its whole-body yellowing is close to unique and makes it one of the more approachable species in a notoriously difficult genus.
How to identify it
Cap
Small, 2-5 cm across, convex then flattening and often depressed at the centre. Colours are muted — purple-brown, wine, violet-brown, sometimes olive or reddish, and frequently with an ochre or yellowish disc. The cap fades markedly with rain and age, and the whole surface progressively yellows. The cuticle peels almost completely and the thin margin develops clear grooves early.
Gills
Whitish to pale cream when fresh, moderately spaced, adnexed and brittle. They discolour to ochre and finally deep yellow with age or where bruised, following the same pattern as the rest of the fruit body. Gill edges are entire.
Stipe
Slender and notably fragile, 3-6 cm long and 0.5-1.2 cm thick, white at first but soon washed with ochre and then a strong honey-yellow, most rapidly at the base. It becomes hollow and crushes easily, and bears no ring or volva.
Flesh & spore print
The flesh is white, thin and soft, turning ochre-yellow within minutes of being cut or handled — the definitive character. The spore print is pale cream to cream-ochre. Spores are broadly ellipsoid with warts joined by scattered fine lines.
Where and when it grows
The Yellowing Brittlegill is mycorrhizal, chiefly with conifers such as spruce and pine, and also with birch in mixed woodland. It favours damp, acid, moss-rich ground — Sphagnum hollows, feather-moss carpets under spruce, boggy woodland margins and shaded ditch banks.
Fruiting runs from July to October, with the heaviest flushes after periods of sustained rain. The species is common and widespread through northern and central Europe and extends into boreal forest, following the range of its conifer and birch hosts.
Ecological role
This species forms ectomycorrhizas with the fine roots of conifers and birch, sheathing the root tips in a fungal mantle and threading hyphae between the outer root cells. The partnership supplies the tree with water and mineral nutrients extracted from acid, slowly decomposing forest soils, and supplies the fungus with sugars from the canopy.
Moss-dominated conifer floors hold moisture well but release nutrients slowly, so mycorrhizal fungi of this kind are central to nutrient cycling in such forests. The small fruit bodies are short-lived and quickly consumed by slugs, springtails and fly larvae, and the pigment change that gives the species its name continues even in decaying specimens, which end as soft yellow-brown remnants in the moss.
Similar species
The Fragile Brittlegill (Russula fragilis) is a similar size and equally frail with comparable purple-brown colours, but it does not yellow, and its gill edges are finely serrated under a lens. Russula versicolor shares the small size and mixed colours but is a birch species with a cream spore print and no strong yellowing.
Russula nauseosa is another conifer species with a greyish-violet cap, but it has more distant gills and a deeper ochre spore print, and it lacks the honey-yellow discolouration. Russula odorata is a broadleaf species of oak with a similar purple-brown fading cap and an ochre print, again without the whole-body yellowing. Watching a cut stipe for a few minutes is the simplest and most reliable confirmation of R. puellaris.