Ascomata develop beneath the epidermal cells and the fibre bundles on the outside of the leaf. In vertical section the upper wall is about 10 µm thick near the base, up to 60 µm thick near the top, and comprises brown to pale brown, thin-walled, cylindric, 2-3 µm diam. cells, intermixed with scattered clumps of crystals. The part of the wall adjacent to the hymenium is lined with a layer of inward-projecting, cylindric, hyaline, 10-15 x 1-2 µm, periphysis-like cells. Fungal hyphae ramify amongst the partially broken down host tissue covering the ascomata. The lower wall is 10-15 µm thick, of thin-walled, brown to pale brown, cylindric cells.

Paraphyses 1-1.5 µm diam., often with short, irregular, propoloid branches near the apex, intermixed with crystals, not extending beyond asci. Asci 80-110 x 7.5-9 µm, more or less cylindric, tapering gradually to more or less rounded apex, wall undifferentiated at apex, 8-spored. Ascospores 60-75 x 2-3 µm, filiform-subspathulate, the widest point near the apex but tapering rapidly distally, with the rest of the spore uniform in diam., 1-septate, gelatinous cap at both ends.

CHARACTERISTICS IN CULTURE: Ascospores of PDD 48570 germinated on agar plates, on oatmeal agar colonies 15-20 mm diam. after 5 weeks, aerial mycelium, low, felted, white toward edge of colony, pale pink toward centre, pinkish in reverse.

ETYMOLOGY: from host genus.

NOTES: P. desmoschoeni is the only species of Rhytismataceae known from Desmoschoenus, a sand dune inhabiting sedge. It is very similar to P. dendrobii, described from New Zealand on Dendrobium (Johnston 1986). Both species have ascomata with dark walls, asci and ascospores about the same size, and ascospores with a distinctive swelling in the upper half. P. dendrobii differs in having ascospores with the swelling adjacent to the septum, close to the centre of the spore, rather than near the apex. In addition P. dendrobii has ascomata with radiate opening slits, rather than a single longitudinal slit. This difference may simply reflect the different textures of the host substrates, the leaves of Desmoschoenus having a stronger linear structure than Dendrobium.