Oyster Mushroom Identification Guide

First Impressions and the Setting

Oyster Mushroom announces itself from a distance, which is unusual. You are walking a floodplain wood or a hedgerow in cool weather, and part way up a standing dead beech, poplar, willow or aspen you see a tier of pale shelves stacked one above another like roof tiles, each one a smooth fan the colour of old ivory, dove grey or milky coffee. The clusters are often large - a good tree may carry several kilograms of overlapping caps - and they are almost always on wood, usually well above the ground on a trunk, a snag or a large fallen limb. The silhouette does the initial work: a fan or tongue with the attachment at one side rather than a mushroom sitting on a central stalk.

Reading the Cap

Get a single cap in your hand. It measures anywhere from 5 to 25 cm across and is fan-shaped, shell-shaped or broadly tongue-shaped, sometimes nearly circular in specimens that grew on the top of a horizontal log. The surface is smooth and bald - no scales, no fibrils, no velvet - and it feels slightly greasy or moist rather than sticky. Colour varies more than beginners expect, running from near white through pale grey, ash grey, blue-grey, tan and grey-brown; cold weather often produces darker, bluer caps and warm weather paler ones. The margin is inrolled when young and expands to become wavy, lobed and finally split in old specimens. Fresh caps are pliable; they bend rather than crack.

The Gills

Turn the cap over. The gills are the second decisive character. They are white to cream, close, fairly narrow, and unmistakably decurrent: they run without interruption down the underside and converge on the point of attachment, continuing onto the stub of a stalk if one is present. Near that convergence they typically fork and cross-connect, forming a small netted or veined area. The gill edges are smooth and even, never saw-toothed or ragged. In very old fruitbodies the gills yellow slightly, and the whole cap softens.

Stalk and Base

There is often no stalk at all, the cap attaching directly and broadly to the wood. When a stalk is present it is short, thick, lateral or strongly off-centre, white, and set at an angle so the cap hangs outward from the trunk. Feel the extreme base where it meets the bark: it is usually finely woolly or hairy, a small but consistent detail. Pull a cluster free and you should see the individual stalks fused into a common base, which is why they come away as a single mass.

Flesh and Colour Change

Slice a cap through the middle from margin to attachment. The flesh is white and stays white; there is no bruising reaction, no latex, no colour change on cutting. It is thick and firm near the attachment and thins rapidly towards the margin, giving the wedge-shaped section typical of the genus. The texture is soft-fibrous and slightly rubbery in cold weather, and it tears in strings when pulled apart along the grain.

Taking a Spore Print

This step is worth doing because the print carries a subtle diagnostic detail. Cut a medium-sized, freshly opened cap free of the cluster and lay it gill-side down on a sheet of white paper, or better, on paper with one half black so you can judge the colour against both backgrounds. Cover with a bowl and leave four to twelve hours in a still, cool place. Oyster Mushroom drops spores heavily - so heavily that on a productive log the lower caps are frequently dusted white as if with flour. The print is white to pale cream, and in a thick deposit it very often shows a faint lilac, greyish-lilac or pale mauve cast when viewed against black. That lilac tinge is a helpful confirmation, although it is not always evident in thin prints.

Substrate, Season and Ecology

Oyster Mushroom is a white-rot decomposer of broadleaf wood. It attacks the lignin in dead and dying trunks, fallen logs, stumps and large branches, and it will also fruit from wounds on living but stressed trees. Beech, poplar, aspen, willow, elm, maple and birch are all regular hosts; conifers are unusual. Fruiting is triggered by cold, so the main season runs from autumn through winter and into early spring, with flushes commonly appearing after the first frosts and again during mild spells. A colonised trunk fruits repeatedly over several seasons, so it is worth noting locations. Look for the white mycelial felt under loose bark near the fruiting shelves as supporting evidence.

Separating the Look-alikes on Morphology Alone

• Pleurotus pulmonarius: paler, thinner-fleshed, smaller caps with a more distinct off-centre stalk, and it fruits in warm weather rather than cold. The differences are of degree, and the two are close relatives.
• Pleurocybella porrigens: much thinner, pure white, entirely stalkless, very flexible and translucent-looking, growing on conifer wood in dense masses of small ear-shaped caps.
• Hypsizygus ulmarius: superficially similar on elm and box elder, but the stalk is central to slightly off-centre and the gills are notched or merely attached, not running down; caps are thicker, whiter and more rounded.
• Crepidotus species: tiny to small, thin, stalkless, and the gills turn brown as they mature because the spore print is brown, not white.
• Phyllotopsis nidulans: bright orange to buff-orange, densely hairy cap surface, orange gills, and a strong distinctive smell.
• Lentinellus species: gill edges are conspicuously saw-toothed or ragged under a lens, and the flesh is tougher.
• Sarcomyxa serotina: cap surface is olive to green-tinged and distinctly viscid, with a short scurfy lateral stalk; a late-autumn species.
• Omphalotus species: bright orange throughout, growing in tufts at the base of stumps or from buried roots, with thin, sharp-edged decurrent gills and a well-developed tapering stalk, and the whole cluster is orange rather than grey.

Working Checklist

Overlapping fans on hardwood; smooth grey to ivory cap with an inrolled then wavy margin; white decurrent gills forking at the attachment; little or no stalk, woolly at the very base; white unchanging flesh; heavy white to faintly lilac spore print; cold-season fruiting.