Honey Fungus

Honey Fungus is the best known member of Armillaria, a genus of white-spored, ringed, wood-inhabiting agarics in the family Physalacriaceae. What was once treated as a single variable species is now split into several biological species that differ in host preference, ring structure, clustering habit and how scaly the cap is; A. mellea in the strict sense is the tightly tufted, hardwood-loving, strongly ringed one.

The genus is famous for two features. The first is the rhizomorph - a black, cord-like strand of bundled hyphae, tough as a bootlace, that grows through soil and under bark to reach new roots. The second is scale: interconnected Armillaria clones are among the largest known organisms on earth, spreading over hundreds of hectares of forest floor over centuries.

Honey Fungus is also one of the fungi responsible for foxfire, the faint greenish glow of colonised wood at night. The mycelium is bioluminescent, and freshly infected wood taken into a completely dark room can be seen to glow after the eyes adjust.

Cap

3-12 cm across, convex with an inrolled margin when young, expanding to flat with a low broad umbo and often a wavy, faintly striate edge. Colour ranges from honey-yellow through tawny to olive-brown, typically darker in the centre, where fine dark erect scales are concentrated; the outer part becomes smoother with age.

Gills

Whitish to cream when young, ageing pinkish-buff and finally developing rusty or brownish stains, particularly where damaged. They are adnate to slightly decurrent, moderately crowded, and run a short way down the stem.

Stipe

5-15 cm long, often curved and tapering downward where fruitbodies are packed together in a tuft. It bears a persistent, thick, cottony white ring high on the stem, usually with a yellowish edge. Below the ring the stem becomes fibrous, streaky and browner toward the base, which is frequently fused with its neighbours into a solid clump.

Flesh & spore print

Flesh is white, firm in the cap and tough and fibrous in the stem, without dramatic colour change on cutting. The spore print is white to very pale cream, and a mature cluster often leaves a white dusting on the caps below it. Peeling back bark near the base reveals flat white mycelial sheets and black bootlace rhizomorphs.

Fruitbodies erupt in dense tufts from the base of living and dead broadleaf trees, from stumps, exposed roots and buried timber, and sometimes apparently from soil where they are attached to a root below. Oak, beech, birch, willow, apple and many ornamental shrubs are commonly affected, and the fungus is a familiar presence in gardens, orchards, parks and hedgerows as well as woodland.

The main season is late summer to early winter, with a strong peak in autumn after heavy rain; a single stump may produce successive flushes over several weeks. Armillaria mellea is widespread across Europe, North America and temperate Asia, with related species occupying conifer forests and cooler regions.

Honey Fungus is both a parasite and a saprophyte. It attacks the roots and butt of living trees, killing the cambium and producing a white rot in the heartwood, then continues to decay the dead stump and root system for many years. Infection often first shows as thinning foliage, dieback and resin or gum bleeding at the trunk base, with white mycelial sheets found under the bark.

Rhizomorphs are the key to its spread: these dark, protective cords grow independently through soil for metres, allowing the fungus to move from an established dead substrate to a new host without needing spores. Because a single genetic individual can occupy a large area of forest floor and persist for centuries, Armillaria is a major structuring force in woodland, creating standing deadwood and canopy gaps that in turn support hole-nesting birds, saproxylic beetles and regenerating seedlings.

Sulphur Tuft (Hypholoma fasciculare) grows in equally dense clusters on stumps but has sulphur-yellow caps, yellow-green then purplish gills, no ring, and a purple-brown spore print. The Funeral Bell (Galerina marginata) is smaller, smooth-capped and tawny, with a rusty-brown spore print and rusty gills, and a thin ring rather than a thick woolly one.

The Shaggy Scalycap (Pholiota squarrosa) also grows in tufts at tree bases and has a ring, but it is covered in coarse, dry, upturned scales on both cap and stem and has a brown spore print. Other Armillaria species (A. gallica, A. ostoyae) are separated by growth habit and veil: A. gallica tends to fruit singly or in small groups with a bulbous stem base and a yellow, cobwebby, evanescent ring, while A. ostoyae has darker brown cap scales and a thicker, dark-edged ring and favours conifers.