Lemon Disco Identification Guide

First impression: a yellow haze on bare wood

The Lemon Disco is one of the few ascomycetes that beginners spot without being told to look. Walk a damp deciduous wood in autumn and a fallen branch will suddenly read as yellow from three or four metres away, as though someone had flicked bright paint along it. Kneel down and the haze resolves into hundreds of separate discs, each between about 0.5 and 3 mm across, packed shoulder to shoulder over the bare surface of the wood.

The colour is the hook. It is a clean, saturated chrome to lemon-yellow, deeper and more even than the dull ochres and buffs that dominate rotting wood, and it holds its intensity even in flat winter light.

The setting and substrate

Almost every find is on decorticated hardwood: branches, logs and stumps whose bark has already fallen away, exposing pale grey, damp, well-rotted wood. Beech, oak, birch, hazel, ash, sycamore and alder are all regular. The fungus favours humid microsites, so search the shaded undersides of fallen limbs, the sawn faces of stacked logs, the parts of a branch resting in leaf litter, and anywhere the wood is soft enough to dent with a thumbnail.

It is a wood saprotroph, arriving late in the decay sequence once other fungi have already softened the timber. Colonies commonly follow the grain in drifts, and a single branch may carry several distinct patches with bare wood between them.

Reading a two-millimetre fungus

A hand lens is not optional here; ten times magnification turns a yellow speck into a structure you can actually describe.

Shape. Each fruit body is an apothecium: a shallow cup when very young, flattening with age into an even disc, and finally becoming slightly convex and cushion-like in old crowded colonies. Isolated discs are neat circles. Where a colony is dense, neighbouring discs press against one another and deform into polygons, which is why a swarm can look like tightly packed yellow tiles.

Upper surface. This is the hymenium, the spore-producing face. It is smooth, even, matte to faintly waxy, and a uniform bright yellow with no zoning, no darker centre and no bloom.

Lower and outer surface. The same yellow but very slightly paler and duller, and finely granular under a good lens rather than hairy.

Margin. Even and entire. There is no fringe, no rim of white hairs, no fine teeth. That negative character does real work, because several of the look-alikes are defined by a hairy or toothed edge.

Stalk, attachment and flesh

Most discs are effectively sessile, sitting flat on the wood. A minority taper to a very short, narrow stalk under half a millimetre long, which is easiest to see at the edge of a colony where discs are less crowded. There is no basal disc of hairs, no subiculum of coloured mycelium and no dark stroma beneath them; the discs arise directly from pale wood.

The flesh is thin, waxy-gelatinous, translucent and pale. Press a disc with a fingernail and it flattens rather than crumbling. In dry weather the entire colony contracts to hard, dull orange dots that look like flecks of rust; rain rehydrates them, and a branch that looked bare on Monday can be brilliantly yellow again by Wednesday.

Getting a spore deposit

Ascomycetes shoot their spores actively, so the routine differs from an agaric. Two approaches work.

In the field, breathe gently on a fresh, well-hydrated colony or nudge it after a moisture change. A mature swarm often puffs visibly, releasing a fine mist of spores that catches side light. That is a good confirmation that the discs are mature and fertile.

At home, cut a chip of wood carrying a colony, invert it a few millimetres above a clean glass slide or dark card inside a closed humid box, and leave it overnight. The deposit is white and inconspicuous, which is expected, and is best judged on dark glass.

Under a microscope the genus character appears: eight spores per ascus, hyaline, ellipsoid to narrowly spindle-shaped, each with a prominent oil drop near either end, and each becoming divided by a single central septum into two cells at maturity. That two-celled spore is what Bisporella is named for and settles the identification.

Season

The main flush is autumn, but this species fruits opportunistically through any mild damp period, and colonies can be found in winter and early spring on sheltered wood. Because the discs revive after rain, the same colony may be recorded repeatedly across several months.

Separating it from the look-alikes

Bisporella sulfurina is smaller and a paler, more sulphurous yellow, and critically it grows on the old blackened stromata of pyrenomycete fungi rather than on bare wood. Check the substrate under the discs with a lens: if there is a dark crust beneath them, think sulfurina.

Hymenoscyphus and related stalked cups have a clearly differentiated cup and stalk, tend to whitish, cream or dull pale yellow rather than bright chrome, and are usually found on leaf petioles, twigs, catkins and fallen litter instead of bare rotten logs.

Orbilia species are translucent, watery-gelatinous and pale yellow to peach or orange, generally flatter and more scattered, and often have a slightly notched or crenulate margin. Held up to light they look glassy rather than opaque.

Dacrymyces stillatus shares the size and the substrate but is a jelly fungus: orange, gelatinous, cushion- or blob-shaped, with no flat hymenial disc and no defined margin. Squash one and it deforms as jelly rather than as a waxy plate.

Lachnellula and Dasyscyphella species produce yellow to orange discs but carry a conspicuous rim of white hairs, and most are found on conifer wood.

Chlorociboria aeruginascens is the same growth form on the same rotten hardwood but is blue-green, and it stains the wood itself green.

Ascocoryne and Bulgaria species are purple to blackish and far larger and more gelatinous.

A working field sequence

1. Confirm the substrate: bare, barkless, well-rotted hardwood.
2. Check the colour is a clean bright yellow, not orange and not greenish.
3. Use a lens: flat discs with a smooth even hymenium, not gelatinous blobs.
4. Inspect the margin: even and entire, with no white hairy fringe.
5. Look under the discs for a black stroma; its absence supports B. citrina.
6. Confirm maturity by puffing, then take a humid-chamber deposit and expect white.
7. If a microscope is available, look for eight hyaline, two-celled, bi-guttulate spores per ascus.