Oyster mushroom

Description

The oyster mushroom grows in clusters resembling beds of oysters: large and small shell-shaped or fan-shaped caps 5-25 cm across, often fused together, attached sideways to wood. The colour is variable, from grey-brown and ash-grey to steel-bluish and violet. The gills are crowded, whitish to grey and run deeply down onto a short, thick, white stem.

It grows on dead wood, less often on weakened living broadleaved trees: beeches, hornbeams, willows, poplars and birches. Fruiting bodies appear in late autumn, from the end of October into winter, and in a mild winter also during it. It is common throughout Poland.

Young fruiting bodies are best; older ones become tough, especially the stem. If you are not sure of the species, do not pick or eat it.

Identification features

Cap

Diameter 5-25 cm (some descriptions give 2-30 cm). At first arched with a long-inrolled margin, later shell-shaped, fan-shaped or funnel-shaped; attached to the substrate sideways. Soft and springy, surface smooth and bare, covered with white felt only near the stem. Colour variable: various shades of brown, ash-grey, grey-brown, steel-blue and violet (according to some descriptions from white through grey to brown). Margin smooth, often wavy or lobed.

Hymenophore (underside of the cap)

Gills crowded, running deeply down the stem, from whitish to grey, even violet in older specimens (according to some descriptions whitish, sometimes yellowing).

Stem

Short and thick, 1-4 cm, white or grey, lateral or attached to the wood at the side (practically absent in some fruiting bodies); bare, covered with bristles only at the base.

Flesh

White; in the cap soft when young, springy and tough in older ones, in the stem corky and woody in older ones. Taste slightly sour; smell bitterish-sweet, reminiscent of benzaldehyde or bitter almonds, according to other descriptions slightly of anise. No data on colour change when cut.

Spore print and spores

Spore print white (according to some descriptions white to lilac-grey, best visible on a dark background). Spores ellipsoid, smooth, 8-11 × 3-4 µm.

Habitat and distribution

Ecology. Saprotroph, white rot of wood, a primary decomposer of broadleaved wood, especially beech; according to some descriptions it is also sometimes a parasite of living, old, weakened or damaged trees (others state that it appears to be exclusively a saprotroph). Grows on hornbeam, beeches, willows, poplars, black locust, birch and walnut, usually in clusters. The mycelium is carnivorous: it paralyses and digests nematodes (which allows it to obtain nitrogen). Bioaccumulates lithium.

Distribution. Outside Antarctica it occurs on all continents and many islands; common in Poland. Not legally protected. In north-western North America it is replaced by P. pulmonarius and P. populinus.

Fruiting. In Poland from the end of October into winter (in a mild winter also during winter), sometimes in early spring. In Great Britain fruiting bodies may appear throughout the year. Frost-resistant; it needs plenty of light to produce fruiting bodies.

What can it be confused with?

• Pleurotus pulmonarius (pale oyster) — Pale cap, anise smell, fruits from early summer to autumn.
• Pleurotus dryinus (veiled oyster) — Paler, whitish cap with a somewhat woolly surface.
• Pleurotus cornucopiae (branched oyster) — Trumpet-shaped cap and grooved stem.
• Sarcomyxa serotina (late fall oyster) — Smaller, often with greenish tones; stem yellow, gills run down less and form a clear boundary between cap and stem.
• Omphalotus olivascens, Omphalotus nidiformis, Clitocybe dealbata, some Lentinellus — Poisonous similar species (mainly outside Europe); species of the genus Lentinellus have gills with ragged edges and a finely hairy cap. No further details.

Never rely on a single feature. If you are not sure — do not pick it; show the mushroom to a qualified mushroom expert.

Interesting facts

• The genus name Pleurotus ("side ear") refers to the lateral attachment of the stem, and ostreatus to the shape of the cap resembling an oyster.
• Commercial cultivation of the oyster mushroom was started in Germany during the First World War as a remedy for food shortages.
• The mycelium is carnivorous: it paralyses and digests nematodes living in wood.
• Experiments in Mexico showed that the oyster mushroom can grow on used disposable nappies, reducing the mass of their degradable components by as much as 90%.