Parasitic Bolete
Pseudoboletus parasiticus
Also called parasitic bolete of earthballs
A small olive-yellow bolete that grows nowhere except attached to the base of a common earthball, usually with several fruitbodies clustered around a single host.
Overview
The Parasitic Bolete is a small, unassuming mushroom made extraordinary by where it grows. Every fruitbody emerges directly from the base or side of a Common Earthball (Scleroderma citrinum), the tough, warty, buff-coloured puffball-like fungus of mossy acid woodland. Nothing else in the bolete world behaves this way.
Once placed in Boletus and later in Xerocomus, it now sits in its own genus Pseudoboletus, recognising both its unique habit and its distinct microscopic characters. Typically two to five small boletes ring a single earthball, their stipes curving outward from the host as they push through the moss.
With an unremarkable olive-yellow cap and yellow pores, it would be easy to pass over as a young Xerocomellus if it were growing on soil. Attached to an earthball, it cannot be anything else.
How to identify it
Cap
The cap is small for a bolete, generally 2-6 cm across, convex and remaining so or flattening only slightly. The surface is dry, matt and finely felted or velvety, and coloured olive-yellow, ochre-yellow, tan or dull yellow-brown. In age it may crack finely, showing paler flesh in the fissures.
Pores
The pore surface is lemon-yellow to golden-yellow when young, dulling to olive-yellow or brownish-yellow with age. Pores are relatively wide and slightly angular, and the tubes are adnate to slightly decurrent. Bruising causes little change or at most a slow brownish discolouration; strong blueing is absent.
Stipe
The stipe is 3-6 cm long and 0.5-1.5 cm thick, often curved because it grows outward from the host, and coloured yellow above with brownish or rusty tones below. It is smooth or faintly fibrillose, without a ring or a raised network, and its base is fused to the rind of the earthball.
Flesh & spore print
The flesh is pale yellow and essentially unchanging, perhaps reddening faintly in the stipe base. The spore print is olive-brown.
Where and when it grows
This bolete occurs only where its host does, which means mossy, acid, well-drained woodland soils supporting the Common Earthball, typically under oak, beech and birch, and often on sandy banks, heathy woodland edges and old mossy paths. The earthball itself may be partly buried, so the bolete sometimes appears to emerge from bare moss until the host is uncovered.
Fruiting runs from late summer into autumn, usually August to October, coinciding with the maturity of the earthball crop. The species is widespread but uncommon and locally distributed across Europe, eastern North America and parts of Asia, always following the range of Scleroderma citrinum.
Ecological role
The relationship between this bolete and its earthball host has been debated for over a century. The traditional interpretation is parasitism, with the bolete tapping the earthball's tissue; infected earthballs frequently appear stunted, aborted or prematurely decayed. Other observations suggest the interaction may be closer to a one-sided sharing of the host's ectomycorrhizal connection to nearby trees, giving the bolete indirect access to tree-derived carbon without forming its own root sheaths.
Whichever reading is correct, the fungus is not a free-living soil symbiont: it has never been found fruiting away from Scleroderma, and its distribution and abundance track the host's exactly. That dependency makes it a genuine rarity and a good example of one fungus exploiting another's established position in the forest nutrient network.
Similar species
On the ground, small Xerocomellus and Xerocomus species such as the Red Cracking Bolete (Xerocomellus chrysenteron) and the Suede Bolete (Xerocomus subtomentosus) look broadly similar in size and colour, but they grow from soil, are usually larger, and show clearer blue bruising in the pores or flesh. The Red Cracking Bolete additionally shows red flesh in its cap cracks and red streaking on the stipe.
No other bolete grows attached to an earthball, so checking the stipe base against the host settles the identification instantly. The host itself, the Common Earthball, is distinguished from true puffballs by its thick, tough, coarsely warted rind and its hard purple-black interior, and from Amanita buttons by having no internal outline of a developing mushroom when sliced.