Anamorph: conidiomata superficially resembling ascomata, developing singly or in lines parallel to the venation of the host, opening by longitudinal splitting of the epidermis to reveal a black conidial mass. Setae lacking. Conidiogenous cells arising from a reduced layer of pale brown pseudoparenchymatous tissue. Conidiophore mother cells ampulliform, ovoid to subglobose, pale brown, with superficial granular depositions, opening widely at the apex and appearing cupulate, producing a single conidiophore. Conidiophores cylindrical, straight or flexuous, unbranched, hyaline to pale brown, with granular depositions near the apex; septate, the septa not markedly thickened or pigmented. Conidia borne singly, apically and laterally, the oldest at the top of the conidiophore; 10-12 x 9-11.5, angular in face view, the apex ± flat with a pale horizontal germ slit just below the top and a circular abscission scar 2-3 µm diam. at the base, dark brown, thick-walled, smooth, without a gelatinous sheath or appendages.
Teleomorph: stromata reduced, consisting of a basal cushion of dark thick-walled cells on which the ascoma develop. Ascomata forming in groups of 2-5, forming irregular black spots 0.5-1.0 mm diam.; subepidermal, covered with a thin black stroma that opens by a slit in the epidermis at maturity. Ascomata 250-450 µm diam. and 200-400 µm tall, black, subglobose to globose with a somewhat flattened base, minutely papillate. Peridium 10-15 µm thick, slightly thinner basally, composed of thin-walled, pale brown angular elongate cells ca 10-15 x 5 µm in size. Periphyses present, ca 2 µm diam., thick-walled. Interascal tissue of simple rarely branched thin-walled paraphyses 3-4 µm diam. and slightly longer than the asci, without a mucous matrix. Asci 85-110 x 20-24 µm, broadly clavate to cylindric-clavate, thin-walled and not fissitunicate, the basal part fairly short and tapering, the apex rounded with a minute J- apical ring ca 2.5 µm diam. and 1 µm thick sometimes visible, 8-spored. Ascospores biseriately arranged, 24.5-27.5 x 7.5-9 µm overall, ovoid-fusiform, apiosporous, sometimes curved or naviculate, 1-septate, the apex rounded to obtuse; the lower cell 3.5-4 µm long and in diam., not or slightly constricted at the septum, the base rounded; hyaline, thin- and smooth-walled, when immature with a uniform gelatinous sheath 3-4 µm thick.
Not formally assessed. Only known from a single collection, but further work is needed on the limits and relationships of this taxon before a formal assessment is possible.
Most similar to Arthrinium hysterinum (syn. Apiospora bambusae, A. tintinnabula), but that species has noticeably larger conidia and ascospores.
On the dead marginal regions of living leaves of Sasa veitchii.
Known from a single site in VC14 E Sussex (Wakehurst Place).