Teleomorph: not known.
Anamorph: conidiomata pycnidial. Pycnidia initially immersed in the host, becoming superficial, ellipsoidal, 80-125 µm high and 50-90 µm diam., with setae near the ostiole. Setae dark brown, thick- and smooth-walled, 30-70 µm long and 3.5-5 µm diam., 3- to 5-septate, the terminal cell being paler and finely verrucose. Pycnidial wall composed of several layers of polyhedral to subspherical cells, 4-7 µm thick. Conidiophores elongated, filiform, with several septa, hyaline, lining the pycnidial cavity, 4-18 × 1-1.8 µm. Conidiogenous cells enteroblastic, integrated, appearing as very short lateral branches just below each septum of the conidiophores, hyaline, smooth-walled. Conidia hyaline, simple, smooth-walled, ellipsoid, 3-3.5 (-4) × 1.4-1.8 (-2) µm.
This lichenicolous fungus has pycnidia with setae, the pycnidia are initially immersed and later becoming sessile on the thallus and apothecia of the host (Xanthoria parietina). The simple, hyaline ellipsoidal conidia are produced by conidiogenous cells borne laterally below septa on elongate conidiophores.
Several lichenicolous fungi produce small immersed to emergent black perithecia and pycnidia on the thallus and apothecia of Xanthoria parietina. Candidates for Pyrenochaeta xanthoriae might be recognised in the field by the dark setae giving the pycnidia a bristly appearance but microscopic examination is required for confirmation.
Lichenicolous on the thallus and apothecia of Xanthoria parietina.
In GB&I known from a single collection from Ringstead Cemetery, Northamptonshire, May 2016.