Ash-tree Bolete Identification Guide

The Find

Eastern or midwestern North America, mid to late summer, a few days after heavy rain. You are near a white ash — in woodland, along a fence line, or, very often, standing on a mown lawn or in a park where an ash was planted decades ago. What catches your eye is a squat, brown, distinctly lopsided mushroom sitting low to the ground, usually with others nearby, sometimes with two or three caps fused into an overlapping clump.

From above it looks like a bolete that has been knocked sideways. The cap is not circular and the stalk is not in the middle. That single impression — a bolete built off-centre — is the beginning of the identification.

The Cap

Five to fifteen centimetres across, occasionally wider. The outline is fan-, kidney- or shell-shaped rather than round, because the cap grows out to one side of an off-centre stalk. The surface is dry to slightly tacky in wet weather, matt, minutely velvety or bald, and coloured dark yellow-brown, olive-brown or reddish-brown; in dry spells it dulls and often cracks into a coarse mosaic.

The margin is thick and strongly inrolled when young, then becomes wavy, lobed and irregular, and frequently overhangs the fertile surface. Older caps are often uneven and shallowly depressed toward the stalk.

The Undersurface — Where the Species Declares Itself

Turn it over. Instead of a neat sponge of small round tubes you find a coarse, radiating network of shallow ridges and folds: boletinoid, close to merulioid. Broad, angular, elongated compartments run outward from the stalk, cross-connected by lower veins so the whole surface looks like a shallow honeycomb that has been stretched into rays. Nothing about it resembles the fine, even pore layer of a typical bolete.

The colour is yellow to mustard or olive-yellow, dulling and browning with age. The surface runs distinctly down onto the stalk, and it is soft and easily torn. Press or scratch it: most collections stain slowly blue-green or dingy blue, then fade to brown over several minutes. Some collections in dry weather barely react at all, so a negative result is not an objection.

Stipe and Base

Short and stout — one to five centimetres long, one to two and a half thick — always off-centre and often nearly lateral, sometimes so reduced that the cap seems to sit directly on the ground. It is yellowish at the apex where the ridges run down onto it, brown to reddish-brown below, dry, and often ribbed by the descending folds.

Check for negative characters: no ring, no glandular dots, no scabers, no raised net.

Then dig. Beneath the fruit body you will often find a mat of yellowish mycelium, and among the ash rootlets in the surrounding soil, small hard black bodies about the size of peppercorns. These are sclerotia, and finding them is worth the trouble because they belong to the fungus's peculiar way of life.

Flesh and Colour Change

Slice from the cap edge down through the stalk. The flesh is soft, pale yellow to yellowish-buff, thickest above the stalk and thin toward the margin. In many collections the cut face slowly flushes bluish or blue-green, most obviously just above the fertile layer, before fading to a dull grey-brown. The reaction is variable: prompt in humid weather, slow and weak in dry, and sometimes absent. Odour is faintly fungal and unremarkable.

Taking a Spore Print

The decurrent ridges and short stalk make this a slightly awkward print. Cut the cap free with a short stub of stalk, then rest it fertile-side down on white paper with a couple of matchsticks or twigs propping it so the ridges hang clear of the paper. Cover with a bowl and leave four to eight hours.

The deposit is olive-brown to yellowish-brown. Under the microscope the spores are a genuine surprise and a strong confirmation: broadly ellipsoid and only about 7–10 by 6–7.5 micrometres — far shorter and rounder than the long spindle-shaped spores of most boletes. If your spores look almost egg-shaped, that is a point strongly in favour.

Substrate, Association and Season

This is the most interesting part of the species and the most useful for identification. Unlike the great majority of boletes, this fungus is not a mycorrhizal partner of its tree. It is associated with a root-feeding aphid that lives on ash rootlets, and its mycelium builds the small black sclerotia around aphid colonies in the soil.

The practical consequence in the field is that the fungus is tied to ash without being attached to ash roots in the usual partnership. Search under ash in lawns, parks, roadside strips, floodplain woodland and old hedgerows, especially where the soil is moist. Fruit bodies appear from June into October, peaking in the humid part of midsummer, often gregariously and in troops, and frequently reappearing in the same spot year after year.

Geographic range is eastern and central North America.

Separating It from Look-alikes

Suillus species. Several have radially arranged, boletinoid pores that could suggest this fungus at a glance. They differ in having a central stalk, a cap that is usually viscid, glandular dots or a ring or veil remnants on the stalk, and conifer partners rather than ash.

Gyrodon lividus (Europe). The closest analogue: decurrent, shallow, yellow merulioid pores that stain blue, and a similarly soft build. It is separated by tree association — alder rather than ash — a paler ochre-brown cap, and a different geographic range, so in practice the two are unlikely to meet.

Phylloporus rhodoxanthus and relatives. These look bolete-like but have true blade-like gills: thick, widely spaced, bright yellow plates that run down the stalk. Run a finger across the underside — you feel discrete blades rather than a cross-veined network — and the stalk is central.

Tapinella panuoides and Paxillus species. These do have off-centre or lateral stalks and decurrent fertile surfaces, but the surface is made of true gills that peel away from the cap flesh with a fingernail, and the spore deposits are different in colour and the spores different in shape.

Xerocomus and Xerocomellus species. Genuine tube layers with more or less round to angular pores, central stalks, and a cap that is round in outline.

Other Boletinellus. A small genus; the more southern species differ in subtle features of cap colour and spore size, and the ash association plus the ridged undersurface remains the core of the genus concept.

Field Checklist

Lopsided brown cap with a lobed inrolled margin; short off-centre or lateral stalk; coarse yellow radiating ridges rather than fine pores, running down the stalk; slow blue-green staining; olive-brown print with unusually short, broad spores; growing in soil under ash in eastern North America in summer.