Ash-tree Bolete
Boletinellus merulioides
Also called ash bolete, ash-tree gyrodon
An unusual bolete found only near ash trees, with an off-centre stipe and a shallow, vein-like yellow pore surface that radiates outward like gill folds.
Overview
The Ash-tree Bolete is one of the strangest boletes in eastern North America, so much so that it is easy to overlook as a bolete at all. Instead of a neat round cap on a central stalk, it forms a lopsided kidney- or fan-shaped cap with the stipe attached off-centre or near the edge, and its underside is not a smooth pore layer but a shallow, radiating network of vein-like folds and irregular pockets.
It is the sole widespread member of Boletinellus, a small genus placed in the Boletinellaceae alongside Phlebopus. The merulioid, gill-like pore surface for which it is named recalls the wrinkled undersides of crust fungi more than the tightly packed tubes of a typical bolete.
Most remarkable is its ecology. This fungus is tied not merely to ash trees but to an insect that lives on their roots, an association almost unique among fleshy fungi and one that explains its otherwise puzzling habitat fidelity.
How to identify it
Cap
The cap is 4-15 cm across, convex then flattening and finally often depressed or irregular. Its outline is characteristically kidney-shaped, fan-shaped or lopsided rather than circular, with a wavy, lobed and inrolled margin. The surface is dry to slightly tacky, smooth or minutely velvety, and coloured yellow-brown, olive-brown, reddish-brown or dull tan.
Pores
The underside is the key character. Rather than a deep tube layer it forms a shallow, strongly merulioid surface of radiating ridges and cross-veins that create large, angular, irregular pockets, deeper near the stipe and running down onto it. It is yellow to olive-yellow, bruising blue-green to blackish when pressed.
Stipe
The stipe is short and stout, 2-6 cm long and 1-2.5 cm thick, and is attached off-centre or almost laterally beneath the cap. It is coloured like the cap or paler, often yellowish above and browner below, smooth, and lacks both a ring and a network.
Flesh & spore print
The flesh is yellowish and slowly stains blue-green where cut, especially in the tubes. The spore print is olive-brown to yellow-brown.
Where and when it grows
This bolete is found only in the vicinity of ash trees, growing on soil beneath and around them in woodland, hedgerows, parks, street plantings and suburban lawns. It fruits singly, scattered or in substantial troops and arcs, sometimes appearing in large numbers on mown grass beneath a single ash.
The season is long, running from early summer through autumn wherever rainfall is adequate. Its range is eastern and central North America, following the natural and planted distribution of ash, and it has been reported from parts of eastern Asia.
Ecological role
The Ash-tree Bolete's relationship with ash is indirect and unusual. Its mycelium forms hollow sclerotia in the soil around ash roots, and these chambers house colonies of the leafcurl ash aphid, Prociphilus fraxinifolii, which feeds on the roots. The fungus appears to benefit from carbon-rich honeydew produced by the aphids, while the aphids gain a sheltered chamber, making this one of the very few documented fungus-insect-plant triangles among fleshy fungi.
Because of this arrangement the fungus is not a classic ectomycorrhizal symbiont and does not form typical mantled root tips. Its dependence on the aphid explains why it is found under ash and essentially nowhere else, and why it fruits as readily on mown suburban lawns as in woodland, so long as an ash is nearby.
Similar species
The Alder Bolete (Gyrodon lividus) is the closest structural match, with a similarly shallow, decurrent, blue-bruising pore layer, but it grows only with alder in wet ground, has a paler buff to yellowish cap, and a more regular, centrally stalked shape. Suillus species can have angular, decurrent pores but are tied to conifers, have sticky caps, and often carry a ring or partial veil remnants.
Among non-boletes, the merulioid underside invites comparison with Paxillus involutus, which has true crowded gills that separate cleanly from the cap flesh rather than fused veins, and with wrinkled crust fungi in the genus Merulius, which form flat resupinate sheets on wood with no stipe at all. The off-centre stalk, kidney-shaped cap and strict ash association together identify the Ash-tree Bolete.