Purple Jellydisc Identification Guide

The moment you spot it

Purple Jellydisc registers as a colour long before it registers as a shape. You are usually walking a damp broadleaved wood in the second half of autumn, the leaves already down and slippery, and a fallen trunk off the path carries what looks like a spill of wine or a smear of amethyst jam along one flank. Come closer and the smear resolves into dozens, sometimes hundreds, of small rubbery bodies packed shoulder to shoulder. They crowd into bark cracks, line the sawn end of a stacked log, and run in ribbons along the shaded underside of a barkless beech trunk where the wood stays wet. Nothing else in a temperate wood produces that particular purplish-mauve gloss in that quantity, so the genus is often settled in the first two seconds and the rest of the work is confirming the species.

Two states of the same fungus

The first thing to sort out is which of two forms you are looking at, because they look quite different and both are Ascocoryne sarcoides. The asexual state, long called Coryne, forms irregular lumpy cushions and contorted brain-like masses with no defined outline at all — squashed, top-shaped, sometimes almost coral-like where individuals fuse. The sexual state forms proper discs: flattened to very shallowly cupped, roughly circular when they have space, 3–15 mm across, occasionally to 20 mm, and squeezed into polygons where neighbours press in. Both states routinely appear on the same log, sometimes in the same clump, and finding the pair together is itself a good sign you have this species rather than a lookalike.

Reading the fruitbody surface

On a disc, separate the two faces with a hand lens. The upper, inward-facing surface is the fertile one: even, matt to slightly glossy, unmarked, with no pores, no gills, no teeth, no veins. The outer face is faintly paler and very slightly roughened or scurfy, and it narrows either to a broad point of attachment or to a stubby, ill-defined stalk-like base a millimetre or two long. There is no true stipe. Colour runs from pinkish-purple through reddish-violet to a deep, almost blackish purple in old or sun-exposed material. Air-dried bodies shrink into dark, hard, resinous-looking scabs on the wood, and a night of rain swells them back to full size — that revival habit is worth knowing, because a log that looks bare in a dry spell can be covered a day later.

The texture test

Press a fruitbody with a thumbnail. Purple Jellydisc is firmly gelatinous and elastic: it dents, springs back, and tears with a slippery, jelly-like break rather than crumbling or snapping. It is not sticky to the touch and it does not shatter like a brittle-fleshed disc fungus. Slice one in half with a blade and the interior is a paler, translucent version of the outside, uniform throughout, with no distinct layers, no milky fluid, and no colour change of any kind on cutting or on bruising. That absence is diagnostic in its own right — several superficially similar jelly fungi darken or blacken where handled.

Taking and reading a spore deposit

This is an ascomycete, so the spores are fired actively from asci packed across the flat upper face. A conventional print is awkward but achievable: pick a fresh, fully expanded disc, rest it fertile-side-down over a strip of black card or a glass slide, cover it with an upturned tumbler to hold humidity, and leave it somewhere cool for several hours. What you get is a thin whitish haze rather than a solid deposit. The real payoff is under the microscope. Spores are smooth, ellipsoid to slightly curved and cylindrical, typically 12–18 µm long by 3–5 µm wide, usually with a small oil droplet at each end. Crucially, many of them bud off tiny secondary spores in the deposit — a scattering of minute rod-shaped conidia around the larger spores is a strong confirmation. Asci are eight-spored with tips that stain blue in iodine.

Substrate and season

The species is a wood decayer of dead broadleaves: beech and birch above all, then oak, ash, alder, poplar, hazel and willow, with only occasional records on conifers. It wants well-rotted, damp, usually barkless wood — fallen trunks resting on the ground, stumps a few years past cutting, cut faces of timber stacks, and the sodden undersides of large branches. It follows earlier colonisers into wood already softened by white rot rather than starting the job itself. Fruiting runs from late summer through autumn and well into mild winters; in an oceanic climate you can find it in January. Consistency of substrate matters for identification: a purple gelatinous disc on soil, on leaf litter, or on a living trunk is almost certainly something else.

Working through the lookalikes on morphology

Ascocoryne cylichnium is the genuine problem. It shares the colour, the substrate and the season. In the field it tends to make deeper, more distinctly cup-shaped fruitbodies, slightly larger on average, and it produces far fewer of the lumpy asexual cushions. The reliable separation is spore size: cylichnium spores are markedly longer, roughly 18–30 µm, and often develop several cross-walls. If you can measure, the question is closed in a minute.

Neobulgaria pura is paler, a translucent pinkish-lilac rather than wine-purple, distinctly top-shaped, and noticeably firmer and more cartilaginous under the thumbnail. It favours fresh beech and often grows in tighter, more regular clusters.

Ascotremella faginea produces convoluted, lobed, brain-like masses that are more translucent and browner-purple, with a looser, more watery jelly and a wrinkled rather than smooth surface.

Bulgaria inquinans is black to very dark brown, tough and rubbery, top-shaped, and grows on the bark of recently fallen oak; its deposit is black.

Exidia species are basidiomycete jellies. Exidia glandulosa and its relatives are brown to black, often carry tiny pimple-like papillae on the fertile face, and collapse to a thin watery film in dry weather rather than hardening into a scab.

Chlorociboria stains its wood blue-green and has small blue-green cups — no overlap once seen.

A quick field checklist

Gelatinous and elastic, not brittle. Purple to reddish-violet, unchanging when cut. Flat discs, lumpy cushions, or both together. Crowded in large troops. Dead, damp, well-rotted broadleaf wood. Autumn into winter. Whitish spore haze with long narrow spores that bud tiny secondary spores. Meet all of those and Ascocoryne sarcoides is the answer, with only a microscope check against A. cylichnium left outstanding.