Bisporella subpallida

Very small (c. 1.8mm)  yellowish, sessile or short-stemmed discs, usually growing in dense crowded clusters on wood. The margins become irregularly lobed with age. 

Several other species form yellow or yellowish discs on wood.  Calycina citrina, the Lemon Disco, is a brighter yellow.  Calycina chloroflava is lemon or sulphur yellow and associated with dead pyrenomycetous fungi.  Bisporella pallescens is associated with the hyphomycete Bispora antennata.  Also see Phaeohelotium, Orbilia, Hymenoscyphus and Hamatocanthoscypha.

There are several other species with yellow or yellowish discs on wood; these are hard to separate from photos, and examination of spores or confirmation of species by an expert is recommended.   Also photograph in habitat and note substrate and any associated plant or fungi species.

Decorticated wood or cut surfaces of trunks and branches of deciduous trees.

Late autumn to winter,

Peter Thompson 'Ascomycetes in Colour'