Corrugated-cap Milkcap Identification Guide

The Setting and the First Impression

This is a mushroom of warm, humid summer weather in the deciduous woods of eastern North America - oak and oak-hickory forest, mixed hardwood slopes, the edges of woodland trails - in the days after a heavy thunderstorm, when the leaf litter is soaked and steaming. Nothing about the first sighting is subtle. What you see is a large, heavy, dark chestnut to chocolate-brown cap sitting on the litter, matt rather than shiny, and even at a few paces the surface looks textured, as though the cap had been moulded over a network of ridges.

Pick it up and it feels solid and dense for its size. The stipe is thick and coloured much like the cap. Break a gill with a fingernail and white liquid immediately wells out and runs. That last observation is what puts you in the milkcaps and starts the real identification.

Confirming the Genus

Milkcaps announce themselves in two ways. First, the flesh is brittle and granular in exactly the way brittlegill flesh is - it snaps cleanly rather than tearing in fibres, because it is built partly from rounded cells rather than purely from threads. Second, and definitively, cut tissue exudes latex. Nick a gill, the cap flesh and the stipe surface with a knife, and watch for a few seconds; in this species the latex appears fast and in quantity.

Register what is not present: there is no ring, no cortina, no volva, and no veil remnant of any kind on the cap margin or stipe.

Reading the Cap

A full-grown cap runs 6 to 15 centimetres across, with vigorous specimens reaching 20. It begins convex with a strongly inrolled margin, then flattens and finally becomes shallowly depressed or funnel-shaped at the centre as it ages.

The surface is dry and matt, never viscid, and finely velvety to the touch - young caps often carry a faint whitish or greyish bloom that rubs away on the fingers. The colour is deep and warm: dark reddish-brown, chestnut, cinnamon-brown, chocolate or occasionally a rich rusty tan, and it is generally darker than most people expect from a large milkcap.

Then there is the character the name is built on. The cap is corrugated: the surface is thrown into a network of coarse wrinkles, ridges, veins and shallow pits, most strongly developed over the outer half and toward the margin, though on some collections it runs right in to the centre. Feel it with your fingertips as well as looking at it, because in low light the wrinkling is easier to feel than to see. Young caps can be relatively smooth and only develop full corrugation as they expand, so judge this on a mature specimen.

Gills and Latex

Turn the cap over and the gills provide the second strong character. They are close to crowded, narrow, adnate to slightly decurrent, and - unusually for a milkcap - distinctly dark. Rather than the white or pale cream you might expect, they are a warm golden-brown, cinnamon, ochre-tan or dull orange-brown even when young, deepening with age.

They also stain. Bruise a gill face with a thumb, or simply handle a specimen for a few minutes, and dark brown blotches appear where the tissue has been damaged and where latex has dried on it.

The latex itself is the third character. It is copious - more so than in most milkcaps, flowing freely from a single nick and continuing for some time - and it is white and remains white. Put a drop on a white handkerchief or a piece of paper and watch it: it does not turn yellow, and it does not turn lilac or pink. What it does do is stain: as it dries, both the latex and the tissue around it discolour to a dull brown, marking gills, flesh, fingers and cloth alike. Unchanging white latex that browns everything it touches is the signature.

Stipe, Base and Flesh

The stipe is 5 to 10 centimetres long and 1 to 3 centimetres thick, solid, firm, and either equal or tapering somewhat downward. It is coloured like the cap or a shade paler - reddish-brown to cinnamon or tan - and its surface is dry and finely velvety or pruinose, matching the cap texture. There is no ring or veil zone, and the base is simply a rounded or slightly narrowed foot sitting in leaf litter, without a bulb, sac or rooting extension. Handling browns the stipe surface just as it browns the gills.

Slice the fruit body lengthways. The flesh is white and firm, brittle in the way of the whole family, and slowly discolours to brownish on the cut surface, especially where latex has spread. It does not turn yellow, blue or red.

Odour is worth noting. Mature specimens develop a strong, unmistakable fishy or oily smell, sometimes compared to a fishing net drying in the sun, and it strengthens as the fruit body ages. Freshly emerged specimens may smell almost neutral, so an absent odour on a young button is not evidence against the identification.

Taking a Spore Print

There is one practical wrinkle here: the latex will glue a cap to paper. Set the cap gills down on a glass slide, a piece of foil or a ceramic tile rather than directly on absorbent paper, cover it with a bowl, and leave it four to eight hours. Then scrape the deposit together with a knife edge and view it against both a white and a dark background.

The print is white to pale cream with a faint yellowish cast. Under the microscope the spores are broadly ellipsoid to nearly round, roughly 7 to 10 by 7 to 9 micrometres, and ornamented with a network of ridges and warts that stain blue-grey in iodine reagent - a strongly reticulate, amyloid ornament typical of this group.

Habitat, Partners and Season

The species is mycorrhizal, forming root partnerships principally with oaks and to a lesser extent with other hardwoods such as hickory and beech. It grows from soil among leaf litter, never on wood, and it is characteristic of well-drained slopes and ridges in mature deciduous forest.

Its range is centred on eastern North America, from the Gulf states north into the Great Lakes and southern Canada, and it is most abundant in the warm humid south-east. The season is a summer one - roughly July through September, extending into October in the south - and fruiting is tied tightly to warm heavy rain. Fruit bodies appear singly or in scattered groups rather than in clusters or rings, and they often share a patch with the closely related Lactarius volemus. Note that modern classifications place this species and its relatives in the segregate genus Lactifluus, so you will meet it in newer literature as Lactifluus corrugis.

Separating It From Its Closest Relatives

The one that matters is Lactarius volemus, which shares the abundant unchanging white latex, the brown staining and the fishy odour, and often grows a few metres away. Split them on two features. First, the cap: volemus has a smooth to slightly uneven, often slightly cracked surface without a real network of coarse wrinkles, and its colour is brighter - orange-brown, tawny, apricot or golden-rust rather than dark chestnut or chocolate. Second, the gills: volemus has pale gills, whitish to pale cream or faintly yellowish, which brown where damaged but start out light. Corrugated-cap has dark golden-brown to cinnamon gills from the outset. Cap colour, cap texture and gill colour all shift in the same direction, so the two species usually look quite different once you have seen both.

Lactarius hygrophoroides also has copious unchanging white latex and an orange-brown cap, but its gills are conspicuously distant - widely spaced, thick, and easily counted - and it is a smaller, paler mushroom with a smoother cap.

Lactarius gerardii has a dark brown velvety cap that can suggest this species from above, but its gills are white to pale cream and distinctly distant, and its stipe is dark and slender.

Lactarius luteolus is smaller, pale buff to whitish, with white latex that browns quickly and a strong odour - but nothing like the size, the dark corrugated cap or the brown gills here.

Finally, keep the family boundaries in mind. A brittle-fleshed mushroom of similar size with a dark cap but no latex at all is a Russula, and one with a fibrous stem or a ring is neither.

The Short Version

Dark chestnut cap with a coarse corrugated network, dry and velvety; crowded golden-brown gills that stain darker; copious white latex that stays white but browns the tissue; a brown velvety stipe; white flesh browning slowly; strong fishy odour when mature; white to cream print; oaks in eastern North America in warm summer rain.