Dark Honey Fungus

The Dark Honey Fungus is the conifer-associated member of the Armillaria complex and the most robust and darkly pigmented of the group. It fruits in large dense clusters at the bases of standing trunks, on stumps and directly from buried roots, sometimes producing scores of caps in a single tight tuft.

Armillaria is a genus of white-spored, ringed, wood-inhabiting fungi that were long lumped together as a single variable "honey fungus" until careful work on mating compatibility and morphology separated them into a series of biological species. A. ostoyae — also published under the name A. solidipes — is distinguished by its dark scales, its thick ring with a brown-scaled edge and its strong association with conifers.

The species is famous for producing some of the largest individual organisms ever documented: a single genetic individual in the Malheur National Forest of eastern Oregon covers several square kilometres of forest soil, all connected by a network of black cord-like rhizomorphs radiating from the parent mycelium.

Cap

The cap is 4–15 cm across, convex to bell-shaped when young, expanding to broadly convex or flat with a wavy margin. The ground colour is a warm pinkish-brown, reddish-brown or dark tan, and the surface is covered — especially over the centre — with prominent, erect to recurved dark brown or blackish scales that thin out towards the rim. The scales are dry and persistent, giving the cap a rough, bristly texture quite unlike the smooth-capped members of the genus.

Gills

The gills are adnate to slightly decurrent, moderately crowded, and whitish to cream when young. With age they discolour to dull pinkish-tan and often develop rusty or dark brown spotting, particularly along the edges and where bruised.

Stipe

The stipe is 5–15 cm long and 1–2.5 cm thick, firm and fibrous, whitish above and darkening to brown below, often tapering or fused with neighbouring stipes at the base. It carries a thick, persistent, cottony white ring high on the stipe, and the diagnostic feature is the ring's underside and edge, which is decorated with brownish to blackish woolly scales or a distinct dark band. Below the ring the stipe frequently shows further bands of brownish scales.

Flesh and spore print

The flesh is firm and white, sometimes browning slightly where cut, with a somewhat astringent smell. The spore print is white — an important separator from the many brown-spored fungi that fruit on the same wood. Black, flattened, bootlace-like rhizomorphs can usually be found under the bark of colonised roots or trunk bases.

The Dark Honey Fungus is overwhelmingly a conifer species, associated with spruce, fir, Douglas fir, pine, hemlock and larch, and only occasionally found on hardwoods. It fruits in dense caespitose clusters at the base of living and dead trunks, on stumps and from the soil directly above infected roots, so clusters may appear a metre or more from the nearest visible trunk.

It is widespread across the boreal and montane conifer forests of North America, Europe and Asia. Fruiting is concentrated in late summer and autumn, generally beginning after the first substantial autumn rains and continuing until hard frosts, and in a good year a single infected stand can carry clusters at the foot of many trees at once.

This is a facultative parasite as well as a wood decayer. Its rhizomorphs — tough, black, cord-like structures resembling bootlaces — grow through the soil in search of new roots. On contact, the rhizomorph can penetrate the root bark and establish in the cambium, producing sheets of white mycelium between bark and wood that spread up into the root collar and lower trunk. Once the tree dies, the fungus continues as a saprotroph, causing a white rot of the roots and butt.

Because the mycelium can persist in stumps and root systems for decades and expand outwards year after year, individual clones reach extraordinary sizes and ages, with the Oregon A. ostoyae individual estimated at thousands of years old. Ecologically the species is a major agent of natural thinning in conifer forests, creating canopy gaps, standing dead trunks and coarse woody debris that many other organisms depend on.

Armillaria mellea has a honey-yellow to olive-brown cap with only sparse fine fibrillose scales concentrated at the centre, a thick white ring without a dark scaly edge, and fruits in very dense clusters chiefly on hardwoods. Armillaria gallica has a paler tan cap with fine yellowish scales, a markedly bulbous stipe base and a fragile, cobwebby yellow-fringed ring that often disappears entirely.

Pholiota squarrosa is superficially similar with a dry scaly cap and clustered growth at tree bases, but its cap is dry ochre-yellow with greenish-tinged gills and it gives a rusty-brown spore print rather than white. Galerina marginata on the same conifer wood is far smaller, smooth-capped, with a thin ring and rusty spores.