Bryosphaeria epibrya

Host bryophytes: Amblystegium serpens, Tortula muralis and Didymodon cf rigidulus [England], along with Platygyrium repens, Brachythecium sp., Pylaisia polyantha and Anomodon rugelii in mainland Europe. However, the English collection was strongly associated with the bryophilous fungus Acrospermum adeanum, raising the possibility that this fungus may be mycoparasitic on that rather than the mosses.

My draft description, also featured on a post on AscoFrance:

Teleomorph: Ascomata up to ~100um diam, setose, solitary. Hairs brown below, occasionally branched below, paler above, walls verrucose, length seems variable, up to and in excess of 40um. Peridium outer cells textura angularis. Asci 8-spored, clavate-cylindrical, around 110 x 10, bitunicate. Mature ascospores brown, 3-septate, fragmenting in asci and after dispersal = pseudopolysporous. Spore length and width increase somewhat during maturation, apparently along with the strength of the constrictions at the septa. Spores 11.5-13.6 x 3.4-3.9.

Anamorph: Conidiophores seem to be unbranched with verrucose walls. Some may form on top of the fruitbody from older hairs but I am not sure. Others were definitely beside the fruitbodies on the substrate. I think they were >100um long, 5 wide. Forming conidia apically in short chains but possibly also laterally. Conidia brown, verrucose, (0-)1-septate, 14.0-19.2 x 5.1-6.5(-7.2).

I am uncertain about whether the anamorph is the same fungus as it has not been described previously, but will currently treat it so due to the strong similarities in colour, wall ornamentation and co-localisation.

A perithecium of B. epibrya under the dissecting microscope (45X) surrounded by the arachnoid mycelium of Acrospermum adeanum. Moss = Amblystegium serpens on stone wall. Bristol, VC33, March 2022.
Perithecium on A. serpens leaf.
Ruptured perithecium in water (400X)
Ruptured perithecium in water (400X)
Ruptured perithecium in water, showing peridial cells of textura angularis.
Septate hair with coarsely ornamented cell walls. Reddish brown below, pale brown above.
Asci at various stages of development. Pigmentation increases with age, along with spore fragmentation.
Almost-mature ascospores in asci.
Ascus bases with croziers.
Mature ascus with spores starting to fragment.
Liberated spores in fragments. Initial 8 spores break up into up to 32 spore-lets and are released en masse. Stained in lactophenol cotton blue.
Conidiophores and a septate conidium.
Germinating conidium showing rough cell walls.
Photograph of infected host patch in situ, showing white mycelium and necrosis of Amblystegium serpens by the associated fungus Acrospermum adeanum.
Habitat photo showing the wall where this collection was found.