Piptoporus betulinus

Piptoporus betulinus
Scientific classification
Kingdom:	Fungi
Phylum:	Basidiomycota
Class:	Agaricomycetes
Order:	Polyporales
Family:	Fomitopsidaceae
Genus:	Piptoporus
Species:	P. betulinus
Binomial name
Piptoporus betulinus (Bull. ex Fr.) P. Karst.

Piptoporus betulinus
Mycological characteristics
	pores on hymenium
	no distinct cap
	hymenium is decurrent
	lacks a stipe
	spore print is white
	ecology is parasitic
	edibility: inedible

Piptoporus betulinus, commonly known as the birch polypore, birch bracket, or 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" odor but a bitter taste. The velvety cut surface of the fruiting body was traditionally used as a strop for finishing the finest of edges on razors. It is also said to have medicinal properties.

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. The spores are cylindrical to ellipsoid in shape, and measure 3–6 by 1.5–2 μm.

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.

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.

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Wikipedia