Ascomata subcuticular. In vertical section upper wall of unopened ascomata up to 70 µm thick, narrower toward edges, comprising mostly brown to dark brown, angular cells. Cells darker and thicker walled along outer edge of the wall, and along most of the inside edge. Along the future line of opening, toward the inside of the upper wall, is a group of paler, thinner-walled cells. In opened ascomata the upper wall is up to 70 µm thick near the ascomatal opening, becoming gradually thinner toward the base of the wall. Upper wall comprising brown to dark brown, thick-walled, angular, 4-8 µm diam. cells. Exposed face of the broken upper wall is lined with cylindric, hyaline, thin-walled, 30-45 x 4-5 µm cells. Lower wall 5-10 µm thick, of 2-3 layers of brown, thick-walled, angular, 4-7 µm diam. cells.

Paraphyses 1-2 µm diam., circinate at apex, extending 5-10 µm beyond asci. Asci 115-145 x 9.5-13.0 µm, clavate-stipitate, tapering to rounded apex, wall undifferentiated at apex, 8-spored, spores confined to upper half of ascus. Ascospores 22-38 x 3-4 µm (average 30.6 x 3.3 µm), sigmoid, tapering to base, 1-septate, surrounded by a narrow gelatinous sheath.

Conidiomata subcuticular. In vertical section upper wall 5 µm thick, comprising dense, dark brown material with no obvious cellular structure. Lower wall 10 µm thick, of 2-3 rows of brown, slightly thick-walled, angular to cylindric, 4-6 µm diam. cells. Lower wall is lined with 2-3 lavers of hyaline, thin-walled, angular, 3-4 µm diam. cells on which the conidiogenous cells develop. Conidiogenous cells 10-17 x 1.5-2.5 µm, cylindric or tapering to apex, percurrent proliferation, wall slightly thickened and sometimes flaring at the single, apical conidiogenous locus. Amongst conidiogenous cells near centre of the conidiomata is a group of hyaline, thin-walled, filiform, 2.5-3.5 µm diam. elements. Conidia 2.5-3 x 1 µm, elliptic-fusiform, pointed to both ends, hyaline, 0-septate.

CHARACTERISTICS IN CULTURE: Ascospores germinated on agar plates after 48 hours. Colonies on OA 45 mm after seven weeks, aerial mycelium sparse, white, cottony to tufted, agar surface with yellow tinge. After 13 weeks agar brownish in colour, black, globose, conidiomata-like bodies present, sterile.

ETYMOLOGY: sigmoideus = sigmoid; refers to shape of released ascospores.

NOTES: Hypoderma sigmoideum can be distinguished from. H. rubi, also found on Nothofagus solandri var. cliffortioides, both macroscopically and microscopically. The ascomata and conidiomata of both species are similarly shaped and sized, but the differentiated zone of cells lining the ascomatal opening in H. sigmoideum is reddish in colour, or if yellowish then turns red when treated with 3% KOH. The cells lining the ascomatal opening in H. rubi are yellow or brownish in colour and do not change colour in KOH. H. rubi has shorter, wider, 0-septate ascospores, and narrower asci which are truncate rather than rounded at the apex. The anamorphs of the two species differ in morphology of the conidiogenous cells, and in shape of the conidia.