Bulbous Honey Fungus

The Bulbous Honey Fungus is the most soil-associated member of the Armillaria complex, typically pushing up singly or in small loose groups from the woodland floor above buried hardwood roots rather than in the massive trunk-base clusters of its relatives. Its trademark is the stipe base, which swells conspicuously into a club or bulb, sometimes almost onion-shaped, and gives the fungus both of its most-used names, bulbosa and "bulbous".

Separating Armillaria into distinct species took decades of mating studies, and A. gallica has accumulated several synonyms along the way, including A. bulbosa and A. lutea. It is now recognised as one of the most widespread honey fungi of temperate deciduous forest across the northern hemisphere.

The species has a place in biological history: a single clone in a Michigan hardwood forest, mapped in the early 1990s, was found to cover roughly fifteen hectares and to be many centuries old, and its description as the "humongous fungus" brought the concept of very large fungal individuals to public attention.

Cap

The cap is 3–10 cm across, convex or bell-shaped at first and expanding to broadly convex or flat, often with a low umbo. The colour is a soft pinkish-tan, ochre-brown or dull yellow-brown, generally paler and warmer than the reddish-brown of the conifer honey fungus. The surface carries fine, soft, appressed yellowish to olive-brown fibrillose scales concentrated over the centre and thinning towards the margin, which is often faintly striate and hung with pale yellowish veil fragments in young specimens.

Gills

The gills are adnate to shortly decurrent, moderately crowded, and white to cream when young. They discolour with age to pale pinkish-tan and commonly develop brownish or rusty spots and blotches.

Stipe

The stipe is 5–14 cm long and 0.8–2 cm thick at the apex, but expands markedly downwards into a distinctly bulbous or club-shaped base that may be twice the width of the apex. It is whitish at the top and yellowish to brownish below, and its most useful character is the ring: rather than the thick cottony collar of other honey fungi, A. gallica has a delicate, cobwebby, arachnoid ring zone with a fringed yellow edge, which frequently tears away entirely, leaving only a faint yellowish fibrillose band.

Flesh and spore print

The flesh is white and firm, thick in the stipe bulb. The spore print is white. Black rhizomorphs are usually abundant in the soil and leaf litter around the base and can be exposed by gently raking the surface.

The Bulbous Honey Fungus fruits from soil, leaf litter, buried roots and the bases of stumps in deciduous and mixed woodland, with oak, beech and other hardwoods the usual associates, though it also occurs in mixed conifer stands. Fruiting bodies are typically solitary, scattered or in small loose groups of a few individuals rather than in the massive fused clusters produced by other members of the genus.

It is common across temperate Europe, eastern North America and temperate Asia, and is often the most abundant Armillaria in lowland broadleaved woodland. The season runs from late summer through autumn, with fruiting concentrated after the first cool wet spells and generally slightly earlier than the conifer species.

Armillaria gallica is primarily a saprotroph, decaying dead hardwood roots, buried woody debris and stumps and causing a white rot in which the lignin is broken down to leave pale, stringy, fibrous residues. It is a much weaker parasite than the conifer honey fungus, generally establishing only on trees already stressed by drought, injury, waterlogging or age.

Its most striking ecological trait is the extent of its rhizomorph network. Flattened black cords ramify through the litter and upper soil, exploring over large areas and linking widely separated woody substrates into a single physiological unit. This exploratory strategy is what allows individual clones to reach enormous size and great age, and it also makes the species an important redistributor of carbon and nutrients through the forest soil.

Armillaria mellea has a honey-yellow cap, a thick persistent white membranous ring and grows in very dense fused clusters at the bases of living hardwoods; its stipe tapers or is equal rather than bulbous. Armillaria ostoyae is darker reddish-brown with coarse blackish erect cap scales, a thick white ring edged with dark scales, and is a conifer species.

Armillaria cepistipes also has a swollen base and a slight ring but a greyer, more finely scaly cap and a more slender build. Away from the genus, Pholiota species growing at hardwood bases have rusty-brown spore prints and much more conspicuous, dry, recurved scales, while Cyclocybe aegerita has a smooth two-toned cap, a firm membranous ring and tobacco-brown spores.