Lemon Disco

Lemon Disco is a tiny inoperculate disc fungus that makes up in numbers what it lacks in size. Individual apothecia are only 1–3 mm across, but they appear in swarms of hundreds or thousands, turning the surface of a rotting log a startling egg-yolk yellow that is visible from several paces away.

Bisporella belongs to the Helotiaceae in the order Helotiales, the group of inoperculate ascomycetes whose asci open by a pore rather than a lid. The genus name refers to the tendency of the spores to become two-celled at maturity, a microscopic character that separates it from several similar yellow disc genera.

Its appeal is visual: few fungi are as photogenic as a rain-darkened beech log studded with hundreds of glowing yellow buttons, and it is one of the first small ascomycetes most people learn to recognise.

Fruiting body

Apothecia are 0.5–3 mm across, at first shallowly cup-shaped or almost spherical and soon flattening to a disc or slightly convex button with a smooth, even margin. They are bright lemon to egg-yolk yellow throughout, sometimes deepening to orange-yellow with age or drying to a dull ochre-brown.

Attachment

There is either no stalk at all or a very short tapering stipe less than 1 mm long, so the discs appear to sit directly on the wood. Growth is densely gregarious, with discs often touching and deforming one another into angular shapes across a continuous patch of substrate.

Flesh and spore print

The flesh is waxy and extremely thin, the same yellow as the surface, and a fruiting body will simply squash rather than break cleanly. There is no colour change on handling. Spores are colourless, elliptical and become two-celled with a central septum when mature; a spore deposit is white. The complete absence of hairs on the margin or outer surface, best confirmed with a lens, is a useful negative character.

The fungus fruits on well-rotted, damp hardwood that has lost its bark, particularly beech, oak, birch, hazel and alder, and occasionally on conifer wood. Fallen logs, cut stump surfaces, large branches and even damp sawn timber left in shade are all colonised, with the densest displays on the underside or shaded flanks where moisture is retained.

It fruits from late summer into early winter, peaking in the damp weeks of mid to late autumn, and lingers after the first frosts. It is common and widespread throughout temperate Europe, North America and Asia, and is often one of the most frequently encountered small ascomycetes in any well-wooded landscape.

Bisporella citrina is a saprotroph of decaying wood, working on cellulose and lignin residues in timber that is already soft and moisture-saturated. It arrives as a later-stage decomposer, colonising wood that pioneer white-rot fungi have already opened up, and often shares a log with mosses, slime moulds and other small disc fungi.

Spore release is by inoperculate asci that discharge through an apical pore. Because each apothecium is minute, the strategy is to produce vast numbers of them across a wide surface, and the vivid pigmentation may serve to protect the shallow fertile layer from ultraviolet light in the open positions the fungus often occupies.

Bisporella sulfurina is smaller and paler sulphur-yellow and typically grows on wood already occupied by a black pyrenomycete stroma. Calycina (formerly Calycella) citrina is essentially the same fungus under an alternative name in some treatments, reflecting ongoing reclassification.

Hymenoscyphus species such as H. calyculus are pale cream to yellowish but are clearly stalked, with the disc raised on an obvious stipe. Dacrymyces stillatus, the Common Jellyspot, is a superficially similar orange-yellow blob on wood, but it is gelatinous and cushion-like rather than a thin flat waxy disc, and squashes to a slimy smear. Orbilia species are smaller, translucent and more watery orange, with a distinctly glassy look.