Parasitic Bolete Identification Guide

First Impressions and the Setting

Almost no other bolete is identified this way. You are walking through mossy, acidic woodland, oak, beech or birch on sandy or gravelly soil, in late summer or autumn, and you notice the tough, ochre, coarsely scaly ball of a common earthball sitting half-buried in the moss. Growing out of its flank, curving upwards and outwards, are one, two or several small yellow-olive boletes, their stipes bent because they had to turn to find the light.

That image is the identification. Pseudoboletus parasiticus is found attached to Scleroderma citrinum and essentially nowhere else, and once you have seen the attachment there is nothing else it can be. The rest of the examination is confirmation and good record-keeping.

Confirming the Host

Start with the host, because getting it right anchors everything. Scleroderma citrinum is a firm, rounded to flattened ball 3-10 cm across, sitting directly on moss or thin soil, sometimes with a short pinched rooting base bound with mycelial cords. Its rind is thick, tough and leathery, dull ochre-yellow to buff, and broken into coarse, angular, darker scales that look like cracked mud. Slice one across and the interior is firm and purplish-black to nearly black at maturity, marbled with pale veins, and enclosed by a rind several millimetres thick. That thick rind and dark marbled interior are what distinguish a true earthball from the soft, pale-fleshed puffballs.

Reading the Cap

The bolete itself is small: caps 2-5 cm across, occasionally to 7 cm. They are convex and stay convex, flattening only slightly with age, with an incurved margin that persists for a long time. The colour is yellow-olive, olive-ochre, dull mustard or a brownish-olive, sometimes with warmer tan tones over the centre.

The surface is dry and finely felted or velvety, exactly the matte texture of the Xerocomus group, and with age it often develops a fine network of shallow cracks. In this species the flesh showing in those cracks is pale yellowish rather than red, which is a small but useful point of separation.

Reading the Pore Surface

Turn a cap over. The tubes are shallow and the pores comparatively large and angular, giving a coarse, honeycombed look quite unlike the fine, near-solid pore surface of a young Boletus. Colour begins bright golden yellow to lemon-yellow and dulls with age to olive-yellow, then to a dingy ochre or rusty-brown. The tubes are adnate to slightly decurrent at the stipe.

Bruise the pores firmly with a fingernail and wait several minutes. Typically there is little reaction, or at most a slow dulling to brownish or a faint bluish-green tinge in some collections. What you should not see is the prompt, vivid blue flash of the strongly blueing boletes. Record what happens either way.

Stipe, Base and Flesh

The stipe is 3-6 cm long and 5-15 mm thick, usually curved or eccentric because it emerges from the side of the host, tapering downwards and often flattened where it presses against the earthball. It is yellowish at the apex, becoming streaked with olive-brown or rusty fibrils below, and it is never reticulated, so there is no raised net anywhere on it.

The base is the crucial part and it must be excavated whole. Work the moss and soil away with your fingers and follow the stipe down until you can see it joining the earthball. Frequently several boletes share one host, arising in a rough ring around it, and the host is often already discoloured or decaying where the bolete has taken hold.

Split a fruit body lengthways from cap to base. The flesh is pale yellow to whitish, firm, and either unchanging or slowly flushing pinkish, brownish or faintly bluish in the stipe base. It is not dramatic. Leave the halves for ten minutes before recording.

Taking a Spore Print

Cut the cap free, lay it pore-side down on white paper, cover it, and leave it overnight; small boletes can be slow to deposit. The print is olive-brown to olive-ochre, the standard result for the family and useful mainly as a genus-level confirmation.

Substrate, Season and Ecology

The ecology is the most interesting thing about this mushroom, and it is unlike almost any other bolete. Rather than forming mycorrhizal partnerships with tree roots, Pseudoboletus parasiticus develops in association with the fruit body and mycelium of Scleroderma citrinum, drawing on it to fruit. Whether the relationship is straightforwardly parasitic or a more complex biotrophic interaction has been debated, but the practical consequence is fixed: no earthball, no bolete.

Look for it from late summer to mid autumn in acidic, sandy or peaty woodland with a good moss layer, under oak, beech and birch, on banks, path edges and heathy clearings where earthballs are common. It is uncommon and patchy, and it can appear in numbers one year and be absent the next. Because it depends on the host, the way to find it is to check every earthball you pass rather than to scan for the bolete itself.

Separating the Look-Alikes on Morphology

Nothing else grows out of an earthball, so the look-alike problem only arises when the attachment is missed, for example when someone has snapped the stipe off above the host.

Xerocomellus chrysenteron has a similar size and a cracking cap, but the cap is darker brown to olive-brown with distinctly reddish flesh visible in the cracks, the pores bruise blue-green fairly readily, the stipe is usually reddish-streaked, and it grows from soil.

Xerocomus subtomentosus is larger, with a dull olive-brown velvety cap, bright yellow pores that are large and angular, a stipe often with fine ribbing, and mycorrhizal habit on soil under broadleaves.

Chalciporus piperatus is small and slender, but its pore surface is a striking rusty cinnamon to reddish-brown rather than yellow, and its stipe base flesh is chrome yellow. It grows from soil, often in the company of fly agarics under conifers.

Aureoboletus species have viscid caps and bright yellow pores, and again grow from soil.

In every case the deciding evidence is the same: dig the base and look for the earthball.

A Compact Field Checklist

Attached to Scleroderma citrinum; small convex yellow-olive velvety cap, often finely cracked; large angular golden to olive-ochre pores with little or no blueing; curved unreticulated yellow-brown stipe; pale yellow, nearly unchanging flesh; olive-brown print; mossy acidic broadleaf woodland, late summer to autumn.