Sepia Brittlegill

Russula sororia, the Sepia Brittlegill, is one of the drab members of a genus renowned for colour. Its cap is a muted sepia, mouse-brown or greyish-olive, darker at the centre and slightly sticky when damp, and its most obvious feature at a glance is the deeply grooved and warted margin.

It belongs to a group of brown, strongly grooved brittlegills that includes Russula amoenolens, Russula pectinatoides and Russula farinipes. All are separated with difficulty, and the whole group is characterised by a pronounced odour, tuberculate-striate cap edges and cream spore deposits.

The Sepia Brittlegill is very much a mushroom of managed landscapes. It fruits along path edges, in mown grass under park trees, on woodland verges and in avenues, generally on heavy or compacted soils, which makes it one of the brittlegills most often noticed by people who are not looking for fungi at all.

Cap

4-9 cm across, convex then flattened and often depressed at the centre. Colour is dull sepia to grey-brown or greyish-olive, always darkest at the disc and paler towards the edge, sometimes with a faint ochre wash. The surface is smooth and viscid when moist, drying matt. The margin is strongly and deeply grooved with small tubercles between the ridges, a feature that develops early and becomes very pronounced in mature caps. The skin peels roughly half way to the centre.

Gills

Whitish to cream, moderately crowded, adnexed, brittle, and readily spotting brown where damaged or with age. Short intermediate gills are present, and small droplets sometimes appear on the gill faces in humid weather.

Stipe

3-6 cm long and 1-2 cm thick, white at first, soon staining greyish and then brown from the base upwards. It is cylindrical or slightly clubbed, becoming soft, spongy and eventually hollow with age, and often quite short relative to the cap. No ring or volva.

Flesh & spore print

The flesh is white, greying and browning with age, soft in the stipe. The odour is strong and unpleasantly rancid, noticeable from some distance in a group of fruit bodies, and is a helpful field character. The spore print is cream. Spores are ellipsoid with warts joined by ridges into a partial network.

The Sepia Brittlegill is mycorrhizal, most often with oak and lime and also with hornbeam, beech and poplar. It has a strong preference for grassy, disturbed and compacted ground — mown park grass beneath scattered trees, avenue verges, footpath edges, churchyards and woodland rides on clay or loam.

Fruiting occurs from July to October, frequently in troops of many fruit bodies pushing through short turf. It is common and widespread throughout Europe on lowland soils, and is one of the brittlegills most likely to be encountered in towns and cities.

Despite its urban habits this is a genuine ectomycorrhizal fungus, forming a mantle around the fine roots of its host trees and exchanging soil-derived nutrients for tree sugars. Its tolerance of compaction, trampling and heavy soils makes it one of the few brittlegills able to persist around amenity trees where most woodland fungi disappear.

Because it fruits in grass rather than deep litter, its spores are dispersed largely by air and by the many insects attracted to its strong odour. The decaying fruit bodies add organic nitrogen to the thin soil beneath park turf, and the persistent underground mycelium can outlive individual host branches, fruiting in the same rings of grass for many years.

Russula amoenolens is extremely similar and shares the grooved margin and grassy path-edge habitat; it is generally paler and more olive-brown with a different odour, and its cap centre is less blackened. Russula pectinatoides has a more ochre-brown cap with an even more strongly combed margin and a paler, less browning stipe.

Russula farinipes is paler again, a buff to straw ochre, with a mealy, powdery stipe surface and remarkably elastic gills. The Stinking Brittlegill (Russula foetens) is far larger, with a honey to ochre-brown viscid cap that can exceed 12 cm, and a hollow chambered stipe. Cap colour depth, degree of stipe browning and overall size separate these species.