Bacidia rubella
Thallus grey- to yellow-green, thinly to richly granular-isidiate, the granules 60-120 μm diam. Photobiont cells 5-17 μm diam.
Anamorph: conidomata pycnidia, 85-125 μm diam., pale pink to red-brown; conidia 16-24 × 0.5 μm, curved or sigmoid.
Teleomorph: ascomata apothecia, (0.4-)0.7-1(-1.3) mm diam., usually distinctly constricted below, flat, sometimes convex, pale to dark red-brown, the margin not well differentiated when ascomata are mature, but sometimes white pruinose. True exciple colourless but upper part pale yellow-orange or yellow-straw, sometimes (pruinose morphs) with radiating streaks of minute crystals, hyphae with lumina 1-2 μm wide or to 5 μm wide towards outer edge. Hymenium 70-105 μm high, colourless or faintly orange-red or yellow in the upper part; hypothecium colourless, or upper part pale yellow or orange-straw, K± yellow intensifying. Interascal tissue of paraphyses 1-1.5 μm thick, simple or forked above, the apices often slightly swollen to ca 2.5 μm. Ascospores (35-)40-70(-75) × 2.5-3(-4) μm, 3- to 7(-13)-septate, acicular, without a gelatinous sheath or appendages.
Assessed by Woods & Coppins (2012) as of Least Concern.
Easily recognized when fertile, but sterile morphs can be confused with Bacidia biatorina.
The ascomata are sometimes infected by the minute black perithecia of Muellerella hospitans.
Throughout the British Isles, but rather rare in western Scotland.
On trunks of mature trees with nutrient-rich bark (especially Acer, Fraxinus and Ulmus) in parklands, wayside situations and woodlands on rich soils, very rarely on sheltered gravestones or walls; absent from polluted areas.