Bisporella citrina, commonly known as yellow fairy cups or lemon discos, is a species of fungus in the family Helotiaceae.

The fungus produces tiny yellow cups up to 3 mm (1⁄8 in) in diameter, often without stalks, that fruit in groups or dense clusters on decaying deciduous wood that has lost its bark.

Found in late summer and autumn, the fungus is fairly common, but is easily overlooked owing to its small size.

Microscopically, B. citrina can be distinguished from these lookalikes by its elliptical spores, which have a central partition, and an oil drop at each end.

[12] Samuel Frederick Gray called it the "lemon funnel-stool" in his 1821 work A Natural Arrangement of British Plants.

[23] Fruit bodies of Bisporella sulfurina have a coloration similar to B. citrina, but they are smaller and grow in clusters on old, blackened, fungal stroma on wood.

[21] Many other small, yellow discos have fringed or hairy margins to the discs, like Anthracobia melaloma; this latter species grows on or near moss, rather than wood.

Another cup fungus that grows on dead beech wood is Neobulgaria pura, but its fruit bodies are larger, ranging from 2–4 cm (3⁄4–1+5⁄8 in).

Fruit bodies are typically encountered growing in dense clusters on the surface of rotten wood (especially deciduous trees), particularly beech.

[25] In a study of the succession of fungi associated with the decay of a 120-year-old healthy beech tree uprooted by strong winds, B. citrina was found on the wood about three years after the fall.