Piptoporus betulinus (Bull.) P. Karst. - Birch Polypore or Razor Strop Fungus

Piptoporus betulinus - Birch Polypore or Razor Strop Fungus

Taxonomy

Phylum: Basidiomycota

Class: Agaricomycetes

Order: Polyporales

This large polypore develops from a small white spherical swelling on the side of dead or living birch trees. Barbers used to 'strop' or sharpen their cut-throat razors on the tough, leathery cut surfaces of these polypores, and so they became known as the Razor Strop Fungus.

This is a tough, bitter fungus. Although young specimens are reportedly edible they are of poor quality and not worth collecting. The 5,000 year old mummy found in the Tyrol and nicknamed Ötzi the Iceman was carrying a sample of this fungus.

A young fruitbody of Piptoporus betulinus - Birch Polypore or Razor Strop Fungus

In 1753 Linnaeus described this fungus and referred to it as Boletus suberosus, and later the French mycologist Jean Baptiste Francois (Pierre) Bulliard changed the specific epithet to betulinus - a reference to the birch trees (Betula spp.) on which it occurs.

It was also Bulliard who, in 1821, transferred this very common and widespread polypore to the genus Polyporus, where it rested in peace for another sixty years. Then, in 1881, the Finnish mycologist Petter Adolf Karsten (1834 - 1917) moved the Birch Polypore to a new genus, Piptoporus, which he had created and where it resides with just two other species, both rare, that are known to occur in Britain.

Piptoporus betulinus - a young fruitbody of which is pictured on the left - is the type species of the genus Piptoporus.)

In Britain this is the most common of all the large bracket fungi, and you have to go a long way further north before birch trees with other bracket fungi become more common. (Inside the Arctic Circle where birches grow in the tundra, the Hoof Fungus or Tinder Fungus, Fomes fomentarius, becomes the most common birch bracket.)

Identification Guide

	Cap This very common polypore is grey-brown at first and almost spherical, flattening and turning browner on top and white underneath as it matures. 10 to 25cm in diameter and 2 to 6cm thick when fully mature, the fruiting bodies arise singly but there are often several on the same host tree so that from a distance they look like a series of steps.
	Tubes and Pores The small white tubes are packed together at a density of 3 or 4 per mm; they are between 1.5 and 5mm deep and terminate in white pores that turn buff as they age.
	Spores Cylindrical to ellipsoidal, smooth; 4-6 x 1.3-2μm. Spore print White.
Odour/taste	Bitter tasting, when cut these polypores have a faint but not unpleasant 'mushroomy' odour.
Habitat	Piptoporus betulinus is almost exclusively restricted to dead or dying birch trees. The brackets are annual but may persist thyrough one winter. The Birch Polypore is parasitic on living trees, but it can also live as a saprobe once the tree has died and so is able to fruit in subsequent years until the trunk rots away.
Season	Although you may see these bracket fungi persisting throughout the year, they are annuals and release spores in late summer and autumn.
Occurrence	Very common in Britain and Ireland, the Birch Polypore bracket fungus is found throughout the northern hemisphere, although as with many apparently identical fungi it is as yet unclear whether the form that occurs in North America is truly the same species as (and capable of mating with) the one found in Europe.
Similar species	Although mature specimens are shaped very much like the Dryad's Saddle fungus, Polyporus squamosus, it is difficult to confuse this polypore with any other species because of its distinctive colouring and specific restriction to birch trunks.

Reference sources

Mattheck, C., and Weber, K. Manual of Wood Decays in Trees. Arboricultural Association 2003.

Pat O'Reilly, Fascinated by Fungi, 2011.

Dictionary of the Fungi; Paul M. Kirk, Paul F. Cannon, David W. Minter and J. A. Stalpers; CABI, 2008

Taxonomic history and synonym information on these pages is drawn from many sources but in particular from the British Mycological Society's GB Checklist of Fungi and (for basidiomycetes) on Kew's Checklist of the British & Irish Basidiomycota.