Alder Bolete Identification Guide

The Habitat Does Half the Work

The Alder Bolete is one of the few boletes you can begin identifying before you have looked at the mushroom. Gyrodon lividus is a mycorrhizal partner of alder and it is a strict one. The classic setting is wet ground under Alnus glutinosa: a streamside carr, a boggy ditch edge, a flush at the foot of a slope, a damp alder belt around a pond. The soil is often waterlogged, the ground layer a mix of grass, sedge and moss, and the mushrooms push up among alder litter or half-hidden in the vegetation.

If you are under alders with wet feet in late summer or autumn and you find a smallish, dull ochre bolete with a soft, floppy appearance, this species should be at the top of the list. Away from alder, it should not be.

First Impression

Nothing about the cap is dramatic. It is a soft, slightly greasy, buff to clay-yellow cap on a short, often curved stalk, and the whole fruitbody feels flimsy compared with the firm boletes people picture. What makes it memorable is the underside, so get there quickly.

Reading the Cap

The cap is 4 to 10 cm across, occasionally larger, convex then flattening and frequently becoming irregular, wavy or lobed. Colour ranges through ochre, clay-buff, dingy yellow-brown and olive-brown, sometimes with faint rusty or reddish blotches on older specimens.

Texture depends on the weather. In damp conditions the surface is tacky to slightly viscid; drying out, it becomes finely felted or matt. The margin is inrolled for a long time and often stays slightly incurved even at maturity, and it may carry small adhering flecks of tissue. The cap flesh is thin relative to the cap width, which contributes to that soft, slightly floppy handling.

The Pore Surface: The Key Character

Turn it over. Three things should register at once.

First, the pore layer runs down the stalk. The tubes are clearly decurrent, sloping onto the stipe rather than stopping at it or being notched away from it.

Second, the tube layer is extraordinarily shallow — often only one to three millimetres deep, so shallow that the underside looks less like a bolete and more like a wrinkled, folded surface. Boletes normally offer a thick sponge of tubes; this one offers a thin veneer.

Third, the pores themselves are wide, angular and irregular, and near the stalk they stretch out radially into elongated, maze-like slots. Colour is bright lemon to golden yellow when young, ageing olive-yellow and finally brownish.

Then press it. Touch the pore surface with a fingertip and it discolours quickly to blue-green, then to a darker blue-black or dingy blue-grey, before fading over some minutes toward brown. That prompt colour change on a shallow, decurrent, radially elongated yellow pore layer is the identification, and no other alder associate reproduces it.

Stipe, Base and Flesh

The stipe is short, 3 to 6 cm long and 1 to 2 cm thick, usually tapering downward and very often curved or off-centre because the fruitbodies emerge among tangled roots and litter. It is yellow at the apex where the tubes descend onto it, becoming olive-brown, dingy ochre or reddish-brown lower down, sometimes with darker blotches.

Check what is absent as carefully as what is present: there is no ring, no fibrillose veil zone, no reticulate net, and no covering of glandular dots. The base is simply attached to the soil, often with pale yellowish mycelial threads binding litter.

Slice it lengthwise. The flesh is soft and pale lemon-yellow, more watery in the cap and firmer in the stipe. On exposure it blues weakly and slowly in the cap, more distinctly around the tube layer and stipe apex, then fades to a dull brownish tone. The reaction is real but gentler and slower than the immediate flush on the pore surface.

Taking a Spore Print

Remove the stipe, set the cap pore-side down on white paper, and cover it. Because the tube layer is so shallow, the deposit takes longer and is thinner than for a typical bolete; leave it overnight and scrape the dust into a ridge before judging the colour.

The print is olive-brown to ochre-brown with a yellowish cast. Under a microscope the spores are a surprise: small, only about 5 to 7 by 3.5 to 4.5 micrometres, smooth and broadly ellipsoid. Most boletes have distinctly longer, spindle-shaped spores, so these squat little spores are a useful confirmation if you have a scope.

Substrate and Season

Fruiting is from late summer into autumn, and the species is strongly tied to wet ground. Look in alder carr, along slow streams, in damp woodland hollows and in the wet margins of lakes and ponds. Small troops are common, and fruitbodies frequently occur where the alders themselves are standing in seasonally flooded soil.

Separating It From Look-Alikes

Paxillus rubicundulus is the other alder specialist and often grows a few metres away. It has true gills, not pores — crowded, decurrent, soft, and easily rubbed off the cap flesh with a thumb — and they bruise reddish-brown rather than blue. Its cap is scaly-cracked and warmer brown, and it produces a brown spore print.

Boletinellus merulioides in eastern North America looks structurally almost identical: shallow decurrent pores radiating into maze-like folds, blue-green bruising, a short off-centre stalk. Separate it on the tree. It grows with ash, not alder, usually on drier ground, and its cap is darker olive-brown to reddish-brown with a longer, more strongly eccentric stipe.

Suillus species such as S. luteus and S. grevillei also have yellow, sometimes decurrent pores, but they grow with conifers, have viscid caps, deep tube layers, and either a distinct ring or resinous dots on the stipe.

Xerocomellus chrysenteron and its relatives have thick tube layers, round pores that do not stretch into radial slots, cracked cap surfaces revealing red flesh, and no decurrent attachment.

Hygrophoropsis aurantiaca can be confused at a glance in the same litter but has narrow, forking, gill-like folds rather than pores.

Field Checklist

Under alder, on wet ground; soft ochre to olive tacky cap with an inrolled wavy margin; very shallow decurrent pore layer with wide angular pores that elongate radially; instant blue-green bruising; short curved yellow-brown stipe with no ring or dots; olive-ochre print of small ellipsoid spores. That set is unambiguous.