Stinking Brittlegill | Russula foetens Identification Guide

First impression: a big, dirty-honey cap with a ribbed edge

It is late summer or early autumn in broadleaf or mixed woodland, and among the litter is a large, heavy, ochre-brown cap with a slimy sheen, 8 to 15 cm across and sometimes considerably more. Fruit bodies stand singly or in small loose groups. Two things register before you crouch down: the cap edge is visibly ribbed, as though someone had run a comb around it, and the smell reaches you from a step away on a warm day. That combination of size, dirty honey colour and a deeply grooved margin is already a strong pointer, and everything after this is confirmation.

Step 1: prove the genus first

Do the brittlegill checks before you worry about species, because they take five seconds and eliminate whole families.

Snap a gill sideways with a fingernail. In this genus it shatters cleanly rather than bending, because the flesh is built of rounded cells rather than long fibres. Snap the stipe: it breaks like a stick of chalk, with a clean granular fracture and no fibrous strands pulling apart. Cut across the gills and watch for milk: brittlegills exude nothing, while their milkcap relatives release latex within seconds. Finally, check for a ring on the stipe and a sac at the base: brittlegills have neither at any stage.

Step 2: the cap, and especially the margin

The cap is 5 to 15 cm and occasionally up to 20 cm across. Young caps are almost spherical, giving the species a distinctive ball-like look before expansion; they then flatten and finally become shallowly depressed. Colour runs from ochre through dirty honey-brown to greyish tan, often paler at the edge. The surface is viscid to distinctly glutinous in wet weather and shiny when drying, and the skin peels only a short way in from the margin.

The margin itself is the signature feature. For 1 to 2 cm inward it is strongly tuberculate-striate, meaning it carries a series of grooves separated by small bumpy ridges, so the edge looks scalloped and beaded rather than smooth. In young fruit bodies the margin is also long-incurved, hugging the gills. Very few large brittlegills combine that degree of grooving with this size and colour.

Step 3: the gills

The gills are adnexed, moderately close, pale cream to dirty straw, and repeatedly forked near the stipe with cross-veins between them in places. On young fruit bodies in humid weather the edges are frequently beaded with clear watery droplets, which is a useful supporting character. With handling and age they spot rusty-brown, and the edges brown before the faces.

Step 4: the stipe

The stipe is stout out of proportion to many brittlegills: 5 to 12 cm long and 2 to 4 cm thick, often barrel-shaped or slightly clubbed. It is whitish at first and browns progressively from the base upward. The surface is uneven and slightly wrinkled rather than polished.

Cut it lengthwise. Young stipes are stuffed with white pith; as they mature the pith breaks down into irregular hollow chambers separated by thin partitions, and those chambers discolour brown. That chambered, browning interior is a strong supporting character and one that photographs badly, so it is worth cutting every collection.

Step 5: flesh, colour change and smell

The flesh is firm at first and softens with age, whitish throughout, and browns where damaged or where insect larvae have tunnelled. There is no blueing, no reddening and, importantly, no progression to grey and then black, which is what the blackening brittlegills do.

The smell is the feature the common name records. It is strong, oily and rancid, with a stale greasy edge, and it becomes more pronounced as the fruit body ages or is enclosed in a container. Two reagent tests are worth doing on the stipe flesh: iron salts give a pale salmon-pink reaction, and potassium hydroxide leaves the flesh essentially unchanged or only dingy. That last result matters enormously in the look-alike section.

Step 6: the spore print

Take the print properly, because in this genus the exact shade is data. Cut the cap free, lay it gills down on white paper or, better, on a piece of glass laid over white paper, cover it, and leave it six to twelve hours in a cool place. Scrape the deposit into a thick heap with a blade before judging the colour, because a thin film always reads paler than it is.

The print of Russula foetens is pale cream. It is not pure white and it is not deep ochre or orange-yellow, which places it low but not at the bottom of the standard brittlegill colour scale. Under the microscope the spores are roughly 7.5 to 10 by 6.5 to 8.5 micrometres, ornamented with warts up to about 1.5 micrometres high that are joined by ridges into a partial to nearly complete network, and that ornamentation stains blue-black in iodine-based reagent.

Habitat, partners and season

This is an ectomycorrhizal fungus, forming partnerships with oak, beech and birch and also with spruce and pine in mixed woods, on acid to neutral soils. It favours damp, mossy ground, banks along woodland paths and the edges of rides. The season is long, from midsummer through autumn, and it is often among the first of the large brittlegills to appear in a wet summer.

Separating it from look-alikes on morphology

Russula subfoetens. The most difficult separation, since it shares the colour, the grooved margin and the strong smell. It is generally smaller with a less deeply grooved margin, but the decisive test is potassium hydroxide on the flesh, which turns a bright chrome yellow in R. subfoetens and leaves R. foetens dull.

Russula laurocerasi. Smells strongly of cherry-laurel, a sharp benzaldehyde note quite unlike rancid oil, and its spores carry enormous wing-like ridges up to about 2 micrometres high that are unmistakable once seen under a microscope.

Russula illota. Similar stature, but the cap carries a lilac or purple-brown flush and the gill edges are flecked with dark purple-brown dots visible with a lens. Its smell also carries that same cherry-laurel note.

Russula pectinatoides and R. amoenolens. Both are much smaller at 3 to 7 cm, duller grey-brown, less viscid, with thinner flesh and a stipe that does not develop the same chambered interior, although their margins are also grooved.

The blackening brittlegills (Russula nigricans and allies). Thick, distant gills and, decisively, flesh that reddens and then blackens after cutting.

Milkcaps (Lactarius). Settle this in seconds by cutting the gills and watching for latex.

Amanita species. Check the stipe for a ring and the base for a bag-like volva; brittlegills have neither, and their brittle, non-fibrous stipe is a further separation.

Paxillus involutus. Superficially similar in colour with an inrolled margin, but the gills are soft, decurrent, easily separated from the cap flesh as a layer, and they bruise brown on pressure.

Field sequence

1. Large ochre to honey-brown cap, viscid, in damp broadleaf or mixed woodland.
2. Margin strongly grooved and bumpy for 1 to 2 cm.
3. Gills brittle, cream, forked, often beaded with droplets when young.
4. Stipe stout, whitish then browning, chambered inside on section.
5. Strong rancid oily smell; no latex; no ring or volva.
6. Print pale cream; potassium hydroxide on flesh gives no bright yellow.