Bolete Eater

The Bolete Eater is not a mushroom in its own right but a fungus that grows on other mushrooms. Hypomyces chrysospermus belongs to the Hypocreaceae, an ascomycete family whose members specialise in colonising the fruiting bodies of larger fungi, and its chosen hosts are almost always boletes — species of Boletus, Xerocomellus, Suillus, Leccinum and their relatives.

What a finder actually sees is a bolete in the process of being taken over. The mould begins as a thin white bloom on the pore surface, then spreads over the stipe and cap until the whole mushroom is wrapped in a soft, mealy skin. As the parasite matures the coating turns cream, then egg-yellow, then a vivid golden ochre, and finally darkens toward reddish brown as the tissue collapses.

Its distinctiveness lies in the transformation rather than any single feature. A bolete under heavy attack loses the sponge-like pore layer entirely, the cap surface becomes lumpy and swollen, and the whole fruiting body takes on a chalky, deformed look quite unlike any independent species. Because the golden stage is so conspicuous, it is one of the easiest fungal parasites for a beginner to recognise in the field.

Overall form

There is no cap, gill or stipe of its own — the Bolete Eater takes the shape of whatever bolete it has colonised. Infected hosts are usually squat, swollen and misshapen, with the cap margin thickened and the stipe often bloated at the base. Advanced infections reduce the mushroom to a bumpy, unrecognisable lump.

Surface and colour stages

The first stage is a fine white felt, soft and slightly downy, which spreads fastest across the pore surface. This whitish stage passes into a granular, mealy coating that ripens to bright golden yellow, the colour that gives the species its scientific name. A final stage may appear as scattered reddish-brown to purplish pimples embedded in the crust; these are the flask-shaped perithecia of the sexual stage.

Pore surface and flesh

The host's tubes and pores are obliterated early, so a fully colonised specimen shows only a smooth or minutely pitted underside instead of the expected sponge. Cutting through reveals the host's flesh becoming soft, damp and discoloured, often brownish and marbled, with the outer few millimetres converted into powdery parasite tissue.

Spores

No spore print in the usual sense can be taken. The golden powder that rubs off on the fingers consists of thick-walled, warted asexual spores (chlamydospores); the later dark pimples release colourless, spindle-shaped, two-celled ascospores with pointed ends.

The Bolete Eater occurs wherever boletes fruit, which in practice means woodland of all kinds — oak, beech, birch and pine forests, wooded parkland, and mossy forest tracks. It is found across Europe, North America and much of temperate Asia, and appears in the same summer-to-autumn window as its hosts, most abundantly in warm, humid spells after rain.

It is nearly always encountered on the ground among leaf litter or moss, because that is where boletes emerge. Repeat visits to a productive bolete patch often turn up several parasitised fruiting bodies at once, since spores released from one infected mushroom readily reach neighbours pushing up through the same litter.

This is a mycoparasite: a fungus whose entire life cycle depends on the tissue of another fungus. Hyphae penetrate the host fruiting body, absorb its nutrients and rapidly replace its outer tissues, converting the mushroom into a spore-producing platform for the parasite instead of for itself. Colonised boletes usually fail to release their own spores.

The fungus reproduces in two phases. The powdery golden chlamydospores are asexual and are produced in enormous numbers, dispersing by wind, rain splash and passing invertebrates. Later, embedded perithecia produce sexual ascospores, which persist in the soil and litter and are positioned to infect the next season's bolete crop. Because boletes are themselves mycorrhizal partners of forest trees, the parasite sits at a third level in the woodland nutrient web, feeding on the fruit bodies of a fungus that feeds on sugars from a tree.

Other Hypomyces species colonise mushrooms in the same way but pick different hosts and different colours. Hypomyces lactifluorum, the lobster fungus, coats Russula and Lactarius hosts in a hard, pimpled orange-red rind rather than a soft golden powder, and the host beneath has gills rather than pores. Hypomyces hyalinus attacks Amanita species, producing a smooth whitish to pinkish sheath around a club-shaped, gill-less form that still shows a volva at the base — a structure boletes never possess.

In its early white stage the Bolete Eater can be mistaken for ordinary white saprophytic mould on a rotting bolete, but common moulds stay thin and grey-white and never ripen to a uniform egg-yellow crust. Sepedonium moulds are microscopically similar and equally golden; separating them reliably needs a microscope, though the classic soft golden bloom on an intact bolete in woodland is by far the commonest cause. Uncolonised boletes themselves are told apart at a glance by an intact, spongy pore layer and firm, dry cap skin.