Brown Cubical Rot of Birch (Piptoporus betulinus)
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Phylum: Basidiomycota
Order: Polyporaceae
Family: Fomitopsidaceae
Scientific name: Piptoporus betulinus (Bull.: Fr.) Karst
Common names: Birch polypore.
Derivation of name: Piptoporus means "a polypore that
falls off"; betulinus means "inhabiting birch (Betula) trees."
Piptoporus betulinus is restricted to birch as a host. This fungus is found throughout the range of birch. Saprobic and possibly parasitic; growing alone or gregariously on dead birch trees, logs, and occasionally on living trees; found year-round; occurring wherever birch trees occur naturally. Piptoporus betulinus causes a yellowish to brown cubical rot, and is a powerful decomposer of dead wood. Whether or not it attacks living trees as a parasite is contended.
Identification: The fruiting bodies are annual, leathery, with a short, stout stipe, and a cap from 5 up to 15 cm deep, x 25 cm wide x 6 cm high. Kidney-shaped in outline, broadly convex to more or less flat; growing shelf-like or hoof-like, dry with a smooth or somewhat roughened "skin" that often peels away. The upper surface is smooth and glabrous when young, whitish to
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pale brown with darker brown streaks, becoming darker brown and scaly, with a margin that extends below the pore surface. Stipes (if present) are lateral, thick and up to 6 cm long. The pore surface is white, becoming light brown and slightly tooth-like with age, pores circular, 3-5 per mm. Tubes to 1 cm long. Context white, easily separating from tube layer when fresh.
Stem: Absent or rudimentary and stubby.
Flesh: White; thick; corky.
Taste: Slightly bitter; odor strong and pleasant.
Spore Print: White.
Microscopic Features: Spores: 3-6 x 1.5-2 µ; smooth; cylindrical to long-elliptical. Cystidia absent.
Microscopic Characteristics: Hyphae in the context of the fruiting body of two types: thin-walled, hyaline with clamp connections, and thick-walled, aseptate Basidiospores cylindric, allantoid, hyaline, smooth, IKI-, 5-6 x 1.5-1.7 µm. Growth in culture moderately rapid, mat colourless to white, forming compact balls of mycelium on surface of medium, chlamydospore-like swellings, laccase negative.
Piptoporus betulinus is often present in dead branches of dying trees. After trees die, rot develops in the bark and sapwood, and generally spreads to the centre of the trunk. Infected wood decays rapidly; laboratory studies have shown reduction of wood density of 30-70% in four months. Decayed wood is yellowish-brown and cracks into cubes with thin white mycelial mats forming in the cracks. Wood in advanced stages of decay is very light in weight and easily crumbles to powder. Piptoporus betulinus is one of the few brown rotting fungi that only attacks hardwoods. Although it is restricted to birch hosts in nature, other tree species such as pine, spruce, and poplar have been successfully inoculated in lab and field experiments.
Anyone who has spent time in birch woods has seen Piptoporus betulinus on dead birch trees and logs, or occasionally on living trees. The species is an attractive polypore, easily recognized by its habitat on birch wood and the fact that the cap folds over to make a distinctive, smooth rim around the pore surface. The caps are whitish to brownish, and the pore surface is whitish or grayish brown. Although Piptoporus betulinus is annual and does not actually live for more than one season, its fruiting bodies are somewhat tough and are sometimes found in the next year (usually somewhat blackened).
Piptoporus betulinus is apparently edible when young, but it is quite a tough and corky mushroom. However, the fungus does have antibiotic and styptic properties and the Tyrolean Ice Man was carrying it around for some reason.
Sometimes referred to as Razor strop fungus which reflects the use in Victorian times of this fungus to sharpen cut-throat razors in the absence of leather strops.
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