Honey Mushroom Identification Guide

Building the Identification From the Cluster Outward

Most keys start at the cap. With the honey mushroom it is more efficient to start with the whole colony, because the way Armillaria mellea arranges itself is a character in its own right.

What you are looking for is a caespitose clump: many fruit bodies sharing a single point of origin, their stalks pressed together and fused at the very base, splaying outward and upward so the caps overlap in a tier. Clumps of twenty to a hundred are ordinary. On a big stump you may find several such clumps, and around a declining tree they can form an interrupted ring following the root system.

The clump will be sitting on wood, or on ground directly over wood. Typical positions: the flare of a living broadleaf trunk, the top and sides of a cut stump, a lawn above a buried root, the base of an old hedge, the soil beside a rotted-out post. In autumn, after the first cold rain, this is one of the most abundant fungi in temperate broadleaf woodland.

Overall colour of a fresh clump: warm ochre to tawny, with paler yellowish rims and a darker centre to each cap, and paler brownish stalks that darken toward the base.

Working Down a Single Fruit Body

Pull one whole specimen from the edge of the clump, base and all.

Cap. 3-15 cm, domed then flattening, sometimes retaining a broad low bump. The surface is dry or barely tacky, and over the central area it carries small erect to slightly recurved darker fibrillar scales, thinning outward until the rim is smooth. In wet weather or old age the scales may be washed away completely, so their absence never rules the species out. Ground colour spans honey-yellow, tan, olive-brown and pale tobacco; the rim goes finely lined as the cap thins with age.

Gills. Broadly attached to the stalk and running a little way down it - adnate to shortly decurrent - moderately crowded, with several tiers of shorter gills between the full-length ones. They start whitish or cream and darken to pinkish-buff, and in age they develop rusty-brown spots and stains, especially along the edge and where handled.

Stipe. 5-15 cm long, fibrous and tough, splitting lengthwise into strings when you tear it. It narrows slightly downward and fuses with its siblings in a solid mass at the base. Above the ring it is pale cream and finely powdery-floccose; below it grows steadily browner and is clothed in fine fibrils.

Ring. Look for a thick, white, membranous collar set high on the stalk, often with soft yellow woolly flecks along its edge or on its underside. It persists in most mature specimens. A ring of this substance is a genuine annulus, not merely a fibrous band, and it should be found and confirmed rather than assumed.

Base and hidden structures. Dig out the fused base and split the bark of the underlying wood with a knife. Under the bark, look for flat white to cream, papery, fan-shaped mycelial sheets, and for black or very dark brown cord-like rhizomorphs running through the wood, under the bark and into the soil. Cut a rhizomorph across: black rind, white core. These bootlaces are the fungus's transport system and their presence is powerful supporting evidence.

Flesh. White to pale cream throughout, firm in the cap, coarsely fibrous in the stalk. Slice one lengthwise and give it ten minutes: it dulls to buff at most, with no rapid colour reaction of any kind.

The Spore Print Is the Pivot

Almost every clustered brown-capped mushroom that grows on wood in autumn can be sorted by spore colour, and this is the step people skip.

Cut a fully open cap from the clump. Set it gills-down on a piece of black card, or straddle it across white paper and dark card so a pale deposit is unmistakable. Cover it with an upturned glass or bowl, keep it cool and out of a draught, and wait four to twelve hours.

The result should be white, occasionally with a barely perceptible cream tint. Spores under a microscope are smooth, hyaline, broadly ellipsoid, about 7-9 x 5-6 microns, with a small apiculus and no amyloid reaction.

You can often shortcut this. In a well-developed cluster the caps at the bottom of the tier are already frosted white with spores fallen from the caps above, and the litter beneath a big clump can be visibly dusted. Note the colour of that dust: white is Armillaria, brown is not.

Substrate, Season and Role in the Wood

Armillaria mellea is both a root pathogen and a decayer. It colonises the roots and root collar of living broadleaf trees - oak, beech, birch, apple and a long list of shrubs and ornamentals - killing bark and cambium at the collar, then continuing to break down the wood it has killed. A single genet can extend for many metres through the soil by rhizomorph and persist for decades.

The rot it produces is a white rot: the wood becomes pale, soft, moist and stringy, sometimes with darker zone lines marking the boundaries between mycelial territories. On the tree above, expect a crown that has thinned prematurely, dieback in the upper branches, loose or cracked bark at the base, and in some hosts a flow of gum or resin at the collar.

Fruiting is compressed into autumn - a single main flush after the first cool, wet spell, usually over within a few weeks and ended by hard frost. The stumps and collars that produce one year usually produce again.

Sorting Out the Look-alikes by Structure

Other Armillaria. The genus is a complex of similar species, and three characters do most of the work: the ring, the stalk base, and the host.

• Desarmillaria tabescens, the ringless honey mushroom, has no annulus whatever - the upper stalk is bare - and tends to form very tight clumps on oak.
• Armillaria ostoyae (A. solidipes) has a cap set with dark brown to blackish scales and a firm ring whose underside carries dark brown scales; it dominates on conifers.
• Armillaria gallica has a bulbous or club-shaped stalk base, few cap scales, and a thin cobwebby yellow-flecked ring that usually disappears, and it fruits in loose troops rather than crowded clumps.

Brown-spored clustered species. Pholiota squarrosa is bristling with coarse recurved scales over cap and stalk and turns rusty in the gills; Pholiota aurivella has a slimy golden cap with flattened patches; Kuehneromyces mutabilis has a smooth cap that dries in two distinct tones and a stalk sheathed in dark scales below the ring; Galerina marginata is smaller, smooth-capped and hygrophanous. All four print brown to rust, and all four are eliminated by the print alone.

Ringless clustered species. Hypholoma fasciculare is smaller, brighter sulphur-yellow, ringless, with greenish gills and a purple-brown print. Flammulina velutipes has an orange sticky cap, a ringless stalk with a dark velvety lower half, and fruits in winter on elm and willow.

Bright orange clustered species. Gymnopilus junonius forms substantial ringed clumps at hardwood bases but is a saturated orange-rust throughout and drops a vivid rusty-orange print.

A Five-Minute Field Routine

1. Confirm a dense caespitose clump on or over broadleaf wood in autumn.
2. Cap tawny to honey, dry, with darker fibrillar scales concentrated at the centre.
3. Gills whitish then buff, adnate to shortly decurrent, rust-spotting with age.
4. A substantial white membranous ring with yellow flecks, high on a fibrous stalk that browns downward.
5. Dig the base and peel the bark: white mycelial fans and black rhizomorphs.
6. Print a cap - it must be white.

Any one of these can fail on a weathered or immature specimen. Taken together, and with the negative results from the brown-spored clusterers, they make an identification you can rely on.