Bryocentria hypothallina

Hosts: Metzgeria furcata

Like several other members of the genus, Bryocentria hypothallina produces orange-yellow perithecia on the undersides of infected host shoots and the ostiole perforates through the host cells to disperse spores when the fruitbodies are mature. B. hypothallina is a necrotrophic pathogen, infecting healthy green shoots and killing them after fruitbody development and spore release. Infected host patches are often obvious in the field, showing radiating necrotic patches (see below). Perithecia are often abundant at the borders of such necroses, never (in my experience) on the dead whitish-grey host tissue. Necroses are particularly conspicuous on beech (Fagus sylvatica) tree trunks where the host is often abundant and can be observed from several meters away. Fruitbodies are not always present on all necroses as previous years’ dead material can persist on the tree.

Conspicuous necrotic patches betray the presence of B. hypothallina. Huge amounts of material were found in late December during a mild (c. 10 Celsius) and rainy winter in south England. Marks Corner, Parkhurst Forest, VC10, Dec 2021
A heavily infected thallus fragment. B. hypothallina is a necrotrophic parasite and fruitbodies are usually found on hosts that are being actively killed. In these hosts, the midrib is often green. In long-dead host patches, this becomes white. Marks Corner, Parkhurst Forest, VC10, Dec 2021

The perithecia of B. hypothallina are visible as yellow-orange spots under a hand lens in the field, measuring up to around 1/5 of a millimetre in diameter. This is around a quarter to a third of the width of the host thallus. Microscopically, the spores are 1-septate, cylindrical with ± rounded ends, and the lack thickened medial bands (stained in lactophenol cotton blue) found in several other species in the genus (E.g., B. brongniartii and B. metzgeriae). Using an oil immersion lens, the exterior walls of the spores are seen to be rough – though this is not always obvious.

Ascospores in water (1000X). Note the cylindrical shape, the rounded ends and the absence of a conspicuous medial band. Marks Corner, Parkhurst Forest, VC10, Dec 2021
Spores showing rough walls in water from material from Leigh Woods NNR. July 2022.

Notes on similar species:

Bryocentria metzgeriae is perhaps the most similar in both ecology, as it is also necrotrophic, and shared host preferences. B. metzgeriae is most frequently found on Radula complanata, though collections have been made on Metzgeria furcata, Frullania dilatata, Porella platyphylla and Lejeunea cavifolia (Döbbeler, 2004b). The two are easily distinguished microscopically by the larger spores of B. hypothallina lacking cyanophilous medial bands and being cylindrical rather than fusiform.

Chromocyphella muscicola, a basidiomycete, forms necroses on epiphytic mosses and is perhaps one of the most common bryophilous fungi in English, Scottish and Welsh woodlands based on personal observations. It is usually seen on pleurocarpous mosses but can spread onto any nearby bryophytes. The clusters of whitish sclerotia and / or tiny white bracket-like basidiocarps distinguish it from other necrotrophic species.

“Ilyonectria sp.“, a necrotrophic mould, has been found on a range of epiphytic bryophytes and may be mistaken for Bryocentria hypothallina / metzgeriae in the field based on the radiating necroses it causes. Closer examination of infections of this fungus always contain protruding conidiophores and an aerial mycelium not seen in Bryocentria. No teleomorph of this putative Ilyonectria has been observed and further studies are required to clarify its systematic position and identification.

Dozens of perithecia on infected host plants. Marks Corner, Parkhurst Forest, VC10, Dec 2021

More information may be found in the protologue of the species as well as a publication by Greiff et al. (2025) where several British specimens were DNA barcoded.