Ascomata subcuticular. In vertical section the upper wall of unopened ascomata is up to 70 µm thick, comprising mostly brown to dark brown, thick-walled, angular cells. The outer part of the upper wall near the centre of the ascomata is composed of dense, black tissue with no obvious cellular structure, and the dense, black material often extends into what appear to be cracks in the host cuticle. The inside of this part of the upper wall is lined with a layer of hyaline, somewhat disorganised tissue. A narrow extension to this hyaline layer extends vertically through part of the dense, black layer in the outer part of the wall. In opened ascomata the upper wall is up to 85 µm thick near the ascomatal opening, becoming abruptly thinner, to 10-25 µm thick in lower half. Wall comprising brown to dark brown, thick-walled 5-7 µm diam. cells, with dense, black tissue with no obvious cellular structure in the outer part of the wall, near opening slit. Exposed face of the broken upper wall is lined with a layer of 1-4 µm diam., branching, hyaline, thin-walled, cylindric cells. Lower wall 20-40 µm thick, of angular cells, cells dark brown and thick-walled in outer part of wall, paler and thinner-walled in inner part.

Paraphyses 0.8-1.0 µm diam., apices undifferentiated or loosely circinate, extending 20-30 µm beyond asci. Asci 140-160 x 12-17 µm, clavate-stipitate, tapering to small, truncate apex, wall thinner at apex, 8-spored, spores confined to upper half of ascus. Ascospores 21-36 x 4.5-5.5 µm (average 24.8 x 4.8 µm), in face view oblong-elliptic, tapering slightly to rounded ends, 0-septate, surrounded by narrow, gelatinous sheath. Conidiomata subcuticular. In vertical section the upper wall is absent. The lower wall is 5-10 µm thick, of 1-2 rows of thick-walled, brown, angular to cylindric cells. Conidiogenous cells solitary or on short conidiophores, developing on the lower wall, 10-18 x 2-3 µm, cylindric, with percurrent proliferation, sometimes with several distinct annelations near apex. Conidia 5-10 x 1 µm, cylindric, straight, ends rounded, 0-septate, hyaline.

ETYMOLOGY: named after host substrate.

Hypoderma bidwillii can be distinguished by macroscopic appearance. The immature ascomata have a distinctive paler zone along the future line of opening, with this paler zone interrupted near the centre of the ascomata by a small, circular patch of dark tissue. In addition, the distribution of dense, black tissue in the ascomatal upper wall differs from that seen in all other New Zealand Hypoderma spp.