Birch bracket

Birch Bracket (Piptoporus betulinus - also known as Razor Strop) is one of the most common polyporous bracket fungi and, as the name suggests, grows almost exclusively on Birch trees. The brackets burst out from the bark of the tree, and these fruiting bodies can last for more than a year. Technically, it is an edible mushroom, with a strong, pleasant "mushroomy" odour but a bitter taste. It is said to have medicinal properties, and the velvety cut surface of the fruiting body were used as a strop for finishing the finest of edges on razors. Dried specimens have also been used as tinder, and this fungus was carried by "Ötzi the Iceman" - the 5,000 year old mummy found in the Tyrol.

It is a necrotrophic parasite on weakened Birches, and will cause brown rot and eventually death, being one of the most common fungi visible on dead Birches. It is likely that the birch bracket fungus becomes established in small wounds and broken branches and may lie dormant for years, compartmentalised into a small area by the tree's own defence mechanisms, until something occurs to weaken the tree. Fire, drought and suppression by other trees are common causes of such stress.

In most infections there is only one fungal individual present, but occasionally several individuals may be isolated from a single tree, and in these cases it is possible that the birch bracket fungus entered after something else killed the tree. These fungal "individuals" can sometimes be seen if a slice of brown-rotted birch wood is incubated in a plastic bag for several days. This allows the white mycelium of the fungus to grow out of the surface of the wood. If more than one individual dikaryon is present, lines of intraspecific antagonism form as the two individual mycelia interact and repel each other.

The fruiting bodies (basidiocarps) are pale, with a smooth greyish-brown top surface, with the underside a creamy white and with hundreds of pores that contain the spores. The fruiting body has a rubbery texture, becoming corky with age. Wood decayed by the fungus, and cultures of its mycelium, often smell distinctly of green apples.

P. betulinus has a bipolar mating system where monokaryons or germinating spores can only mate and form a fertile dikaryon with an individual that possesses a different mating-type factor. There are at least 33 different mating-type factors within the British population of this fungus. These factors are all variants or alleles of a single gene, as opposed to the tetrapolar mating system of some other basidiomycete species, which involves two genes.

The geographic distribution of Piptoporus betulinus appears to be restricted to the Northern hemisphere. There is some doubt about the ability of isolates from the European continent, North America and the British Isles to interbreed.