Both the anamorph and the teleomorph forms occur on the thalli and apothecial margins, or more rarely in the hymenium, of Xanthoria parietina. When growing on the thallus, both perithecia and pycnidia are immersed in the thallus when immature but later cause a raised thalline collar to develop as the spore-bearing structure erupts through the host cortex. Infected areas are often tinged orange-red. Infections in host apothecia produce a red discolouration in the host’s ascospores.
Anamorph: Conidiomata pycnidial, ± globose, 80-120(-150) μm diam., wall brown, c. 10 μm thick. Conidiogenous cells ellipsoid to flask-shaped, hyaline, 8-10(-12) x 5-6 μm. Conidia non-septate, hyaline, ellipsoid to oblong, many slightly bent, (5-)6-8(-9) x 2.5-3.5 μm, with a guttule at each end, length/breadth ratio c. 2.
Teleomorph: Ascostromata perithecial, scattered or in small groups, immersed, globose to broadly oval in longitudinal section, (150-)200-300 μm diam., wall dark red-brown, laterally 15-20 μm thick, wall up to 40 μm thick near ostiole, composed of 4-5 layers of cells. Paraphyses 1.5-2 μm wide. Asci fissitunicate, narrowly cylindrical, (80-)90-110 x 7-10 μm, 8-spored. Ascospores ±uniserate, 1-septate, with one cell often slightly larger than the other, constricted at septum, pale brown, verriculose, (11-)13-15 x (5.5-)6-7 μm.
Didymocyrtis slaptoniensis can be differentiated from Sphaerellothecium parietinarium, which also produces perithecia with brown, two-celled ascospores on X. parietina, by the arrangement of the ascospores into a single line in the ascus and by the warted ascospore walls. In Sphaerellothecium parietinarium the ascospores are biserrately arranged and smooth-walled.
Parasitises Xanthoria parietina.
Southern England with scattered records elsewhere.