Flat Oysterling Identification Guide

The log and the first glance

Flat Oysterling is a species you meet by crouching. The setting is a well-rotted broadleaf log in damp shade, usually one that has lost its bark and gone soft and pale, in summer or early autumn after a wet spell. You lean down or roll the log over and find rows of small white fans shingled along the flank and the underside, sometimes several dozen overlapping like roof tiles, occasionally more than a hundred on a single trunk.

The first impression is of something thin, pale and stemless: a soft white shelf glued to the wood by one edge. What tells you this is not a miniature oyster mushroom is the colour the gills take on as the fruit bodies mature. Look at a tier and you will see the top fruit bodies still white while the ones lower down have turned buff, then pinkish-tan, then a clear cinnamon-brown, and the shelves beneath will be dusted with fallen brown spores.

Reading the cap

The cap is 1 to 5 cm across, fan-shaped, shell-shaped or kidney-shaped, sometimes nearly circular with a lateral notch where it meets the wood. It is attached directly by a narrow point or a small lateral stub; occasionally there is a rudimentary stem only a few millimetres long, but there is never a proper central stipe.

The surface is smooth, moist and slightly greasy to the touch, white to cream when fresh and drying to buff, ochre or tan. The species is strongly hygrophanous, so its colour depends heavily on how wet it is at that moment, and the same log can carry pale watery fruit bodies and dry cream ones side by side.

The margin is thin and often wavy or shallowly lobed. When the fruit body is saturated the margin becomes translucent-striate, with the gill lines showing through, and older caps are frequently pleated or grooved for a short way in from the edge. Around the attachment point there is a small patch of fine white tomentum, best seen with a lens.

Gills, flesh and the absent stipe

The gills radiate outward in a fan from the point of attachment, which is the structural signature of the whole group. They are close to crowded, narrow, with plenty of short gills interleaved between the full-length ones, and the edges are even.

Gill colour is the most useful field character. They start white, pass through buff, and mature to pinkish-tan and then cinnamon-brown as the spores ripen. Any small white shelf fungus whose gills are turning brown from within, rather than staying white or being stained brown by damage, is worth a serious look.

Check the attachment with a lens. There should be no true stipe, no ring, no veil remnant and no volva: just a white cottony pad where the fruit body joins the wood.

The flesh is very thin, only a millimetre or two, white, soft and watery, and it does not change colour when cut. One physical test matters here. Grip the cap margin between finger and thumb and pull gently: in this species the surface tears almost immediately, because there is no gelatinous elastic layer in the cap skin. Remember how that feels, because the nearest relative behaves quite differently.

Taking a spore print

This is the decisive step, and it is quick because the species prints heavily. Lay three or four mature caps gill-side down on white paper, cover them with a cup and check after two hours; four is ample.

Use white paper. A brown deposit shows up clearly on white and disappears on black, which is the opposite of the usual advice for pale-spored mushrooms. The print is pinkish-brown to cinnamon-brown, and it is never white.

Under the microscope the confirmation is elegant. The spores of this species are globose, genuinely round rather than merely broad, 4 to 6 micrometres across, and finely punctate or minutely spiny. Round ornamented spores separate it from most of its congeners, which have smooth ellipsoid spores. The gill edge carries variable, often flexuous and forked cystidia.

Substrate, season and range

Flat Oysterling is a decomposer of dead broadleaf wood, and it specialises in the softened late stages of decay: bark-free logs, spongy stumps, fallen branches, woodchip mulch and buried sticks under the litter. In some regions it also turns up on conifer wood. It fruits from summer through autumn, and into early winter where the climate is mild, and it is widespread across North America, Europe and Asia.

It is almost always gregarious. Fruit bodies grow in dense overlapping tiers, and finding one usually means finding twenty. Because they are thin they dry and rewet repeatedly, so a single log can show fresh white fans beside limp tan ones that have been through several cycles.

Separating it from its look-alikes

Crepidotus mollis is the closest relative and the most common confusion. It is larger, up to 7 cm, duller and browner, with scurfy brown fibrils near the attachment point, and its cap skin is gelatinous. That last point is a direct field test: take the margin between your fingers and pull, and the cuticle of C. mollis stretches noticeably like a thin rubber sheet before it gives, while Flat Oysterling tears at once. Its spores are ellipsoid rather than round.

Crepidotus crocophyllus has a cap dressed with orange-brown scaly fibrils and gills that are distinctly yellow to orange from the start, and its spores are ellipsoid.

Crepidotus versutus and Crepidotus epibryus are much smaller and densely white-hairy over the whole cap surface, often growing on twigs, herbaceous stems and moss rather than on substantial logs.

Cheimonophyllum candidissimum is a small brilliant white fan that looks almost identical when young, but its gills stay white right through maturity and its spore print is white.

Pleurotus, Panellus, Hohenbuehelia and Resupinatus are all white-spored, so a print settles them in a couple of hours. Beyond that, Pleurotus is much larger and thick-fleshed, Hohenbuehelia is rubbery with a gelatinous layer inside the flesh, and Resupinatus is dark grey and gelatinous throughout.

Schizophyllum commune looks like a small white-grey fan but its surface is densely hairy and its apparent gills are split lengthwise along the edge and curl back in dry weather, which a hand lens shows immediately.

Field checklist

White fan on rotting broadleaf wood; no stipe; gills radiating from the attachment; gills maturing white to buff to cinnamon; thin unchanging flesh; cap skin that tears rather than stretches; pinkish-brown print on white paper; round ornamented spores.