Hoof Fungus

Hoof Fungus is the archetypal woodland conk: a heavy, hard, unmistakably hoof-shaped bracket clamped to the side of a trunk, often several metres up, with a smooth grey crust ridged like a contour map. It is the type species of the genus Fomes, a small group of perennial, crust-covered polypores in the Polyporaceae.

Its perennial habit is central to its identity. Each growing season the fungus lays down a fresh layer of tubes beneath the previous one, so a mature conk is a stack of annual pore layers, and the concentric bands on the upper crust roughly record its age. Specimens 20 years old and 40 cm wide are not unusual on large beech.

The species is also one of the most culturally storied fungi in Europe. Its soft inner flesh, the layer called amadou, was for centuries beaten and processed into a fibrous material for catching sparks from flint and steel, and pieces were carried in the kit of the Neolithic traveller found preserved in the Alps.

Fruiting body

A thick, hard bracket 5–45 cm across, classically shaped like a horse's hoof: broadly attached to the trunk at the top, bulging outward and downward to a blunt lower margin. The upper surface is covered by a tough grey to grey-brown crust, smooth but marked with concentric ridges and shallow furrows, each step corresponding to a season's growth. Old conks pale to ash-grey and may crack; young growing margins are often rusty or ochre.

Pore surface

The underside is flat to slightly convex, pale greyish-brown to cinnamon, with fine round pores at about 2–4 per millimetre. Cutting vertically reveals the diagnostic feature: multiple stacked tube layers, each a few millimetres deep, separated by thin bands of flesh. Rubbing or scratching the pore surface causes a slow darkening to deeper brown.

Flesh & spore print

The context is fibrous, felty and tawny to cinnamon-brown, distinctly softer and more suede-like than the stony crust above it. It is tough and cannot be dented easily with a thumbnail. The spore print is white to very pale yellowish; spores are long, cylindrical and smooth, and in spring a mature conk can shed enough to leave a pale bloom on the bark beneath.

In Europe the classic hosts are birch in the north and beech further south, with sycamore, poplar, willow, lime and maple also used; in North America birch and beech again dominate, along with maple. Conks form on standing living trees that have been wounded, on snags, and on fallen trunks, usually well above ground level on the main stem rather than on branches or at the base.

The species is circumboreal and widespread through temperate and northern forests of Europe, Asia and North America. Because the conk is perennial and extremely durable, it can be found in any season; spore release peaks in spring and early summer, and the empty conks persist for years after the tree falls.

Hoof Fungus causes a white rot. It degrades lignin along with cellulose, so decayed heartwood becomes pale, soft, spongy and often marked with fine dark zone lines where mycelia of different individuals meet. Infection generally starts through a broken branch, frost crack or other wound on a living tree, where the fungus behaves as a heart-rot parasite hollowing the standing stem, then continues saprophytically once the tree dies.

That hollowing role makes it ecologically important out of proportion to its abundance. Trunks softened by Fomes provide nest cavities excavated by woodpeckers and later used by owls, tits and bats, and the decayed heartwood supports a rich saproxylic beetle fauna. The conks themselves are a habitat in their own right, with several beetles and fungus gnats breeding exclusively inside them.

Birch Polypore (Fomitopsis betulina) grows on the same birch hosts but is annual, kidney-shaped rather than hoof-shaped, pale tan and smooth without concentric ridges, and has soft white rubbery flesh and a single fine white pore layer. Willow Bracket (Phellinus igniarius) is the closest match in shape: it is also hoof-shaped and perennial, but its upper surface is blackish and deeply cracked rather than smooth ash-grey, and its flesh is a hard rusty red-brown that shows a distinct paler line at the crust.

Artist's Bracket (Ganoderma applanatum) is flattened and shelf-like rather than hoof-like, has a lumpy brown crust, and its white pore surface bruises instantly dark brown when scratched — a reaction the greyish-brown pores of Hoof Fungus never show so sharply. Red-belted Conk (Fomitopsis pinicola) has a resinous, often reddish or orange-banded margin, a cream to yellowish pore surface, and pale wood-coloured flesh, and favours conifers.