Roundspored Oysterling Identification Guide

The Setting and the Search

Roundspored Oysterling is a fungus you find on your knees. The productive habit is to walk a damp, shaded stretch of broadleaf woodland or an old hedge in autumn, pick up fallen twigs and small dead branches, and turn them over. The fungus grows on the shaded, humid underside, so it is almost never visible until you rotate the wood.

What you see is a scatter of tiny white shells, usually only a few millimetres to a centimetre or so across, clinging sideways to the bark. On a good branch there may be dozens along its length. Hazel, ash, elder, willow, oak and blackthorn all support it, and so do bramble canes, dead ivy stems and occasionally sturdy herbaceous stems.

The Cap

The cap is 0.3 to 1.5 cm across, occasionally reaching 2 cm, and shell-, kidney- or fan-shaped. Some fruit bodies are almost circular with the attachment point pulled to one side, and very young ones can look like little white cushions before they expand.

Colour is white to ivory when fresh, drying to pale cream or a faint buff. The surface is dry and minutely downy over the whole cap when young, with a noticeably denser tuft of white hairs at and around the point of attachment. That woolly attachment patch is worth checking with a lens. The margin is inrolled at first and may become faintly striate when the fungus is thoroughly wet, but the cap is never slimy and never has a peelable gelatinous skin.

Attachment

There is essentially no stipe. The fruit body is sessile, fixed to the wood by a small lateral point of white mycelium, sometimes with the merest rudiment of a stubby lateral stalk. Because most fruit bodies develop on the undersides of twigs, they usually hang with the gills facing downward, and their orientation is dictated by gravity rather than by the substrate surface.

The Gills

The gills radiate outwards from the attachment point like the ribs of a fan. They are moderately spaced, fairly broad relative to the size of the cap, and their edges are minutely fringed when viewed with a lens.

Colour progression is the most useful field character in the whole genus. Young gills are whitish and match the cap. As spores mature the gills shift through pale ochre to clay-brown and finally to a warm cinnamon-brown, while the cap stays white. A small white pleurotoid fungus with browning gills is very likely a Crepidotus, and that observation alone eliminates the white-spored pleurotoid genera before you have taken a print.

Flesh and Odour

The flesh is extremely thin, soft and white, with no colour change when cut or bruised and no distinctive odour. There is nothing here to work with, which is itself informative: any pronounced smell, any pink or grey flesh, or any bruising reaction points elsewhere.

Taking a Spore Print

A print is small but entirely achievable. Carefully detach several caps, or better, cut off a short section of twig bearing a group of them, and position them gill-side down over white paper or a glass slide inside a closed container with a scrap of damp tissue. A few hours in a cool place is usually enough.

The deposit is a pale ochre to clay-brown. That single observation removes Cheimonophyllum, Resupinatus, small Pleurotus and Panellus species from the list, since all of them print white, and it places you firmly in Crepidotus.

The Microscopic Step

Genus is a field call; species in this group is not. Mount a small scrap of gill in water, squash it gently, and look at spores and gill-edge cells.

Crepidotus cesatii has spores that are globose to subglobose, with a length-to-width ratio close to 1, measuring roughly 6.5 to 9 micrometres. Their surface is distinctly and finely warted or minutely spiny, and they are pale brown. The gill edge carries abundant thin-walled cystidia that are irregular in outline: flexuous and wavy, flask-shaped, sometimes forked or ending in a small knob. Clamp connections are present in the hyphae, and the cap surface is built of loosely interwoven cylindrical hyphae with projecting hairs.

The practical upshot is simple: round, warted spores under 9 micrometres, on a small white fungus on hardwood twigs, is the species.

Substrate, Season and Ecology

The substrate is small-diameter dead wood of broadleaved trees and shrubs, both fallen and still attached, plus bramble canes and other woody stems. Shaded, humid situations are strongly preferred, which is why the undersides of twigs lying on damp leaf litter are the best hunting ground.

Fruiting runs from late summer through late autumn, continuing into mild winters. Ecologically it is an early to mid-stage decomposer of twig litter, working through the fine woody material that makes up a surprisingly large share of forest floor debris. It commonly shares a single branch with corticioid crusts and black flask-fungus stromata, and its fruit bodies are ephemeral, collapsing within days of a dry spell and reappearing after rain.

Separating the Look-alikes

Crepidotus variabilis is the real difficulty, because in the field the two are effectively identical: same size, same white downy cap, same twiggy substrate, same browning gills. The separation is microscopic and clean. Crepidotus variabilis has ellipsoid spores with a length-to-width ratio around 1.6 to 2.0, measuring about 5 to 7 by 3 to 4 micrometres and only minutely roughened. Round spores mean cesatii; oblong spores mean variabilis. Do not attempt this split on macroscopic characters.

Crepidotus applanatus grows larger, up to 3 or 4 cm, on rotten stumps and logs rather than twigs. Its cap is smoother, hygrophanous and translucent-striate when wet, and its spores, though globose, are essentially smooth to very finely punctate.

Crepidotus luteolus carries pale lemon-yellow tints, especially in the flesh and towards the margin, is more translucent, and has ellipsoid spores.

Crepidotus epibryus, sometimes met as Crepidotus versutus, is very small and cushion-like, strongly downy, and grows on grass stems, fern debris and herbaceous litter rather than woody twigs. Its spores are ellipsoid.

Crepidotus mollis is much larger, 2 to 7 cm, brownish when wet, and has a distinctly gelatinous, elastic cuticle that visibly stretches when you pull the cap margin. Nothing about it is delicate.

Cheimonophyllum candidissimum is a convincing white mimic at the same scale, but its gills stay white and it prints white.

Resupinatus applicatus is grey to blackish and cup- or bracket-shaped with a central to rear attachment, and it also prints white.

Schizophyllum commune is tougher, greyish and hairy, with folded gill-like plates that are split lengthwise along their edges, unmistakable once seen with a lens.

Field Checklist

1. Underside of a dead broadleaf twig, branch or bramble cane.
2. Cap under 2 cm, white, shell- or kidney-shaped, downy, with a hairy attachment point.
3. No stipe, or only a rudimentary lateral one.
4. Gills radiating from the attachment and darkening to clay- or cinnamon-brown.
5. Spore print pale ochre-brown.
6. Spores globose to subglobose, finely warted, 6.5 to 9 micrometres.