Conidiomata initially subcuticular (subepidermal in NZFRI 2334), epidermal cells often becoming invaded (at least in part) as conidiomata mature, host cells otherwise remaining more or less intact; intercellular spaces in the leaf immediately below and around conidiomata containing scattered hyphae 2.5-3 µm diam. with hyaline and thin walls. Upper wall 10-25 µm thick, comprising several rows of angular cells 4-6 µm diam. with very thick and very dark walls, the cells paler and thinner-walled along inside edge of wall. Lower wall 10-15 µm thick, comprising a compact layer of angular cells 4-8 µm diam. with thin and hyaline walls, lined with the conidiogenous layer.

Conidiogenous cells 14-19.5 x 7-9.5 µm, cylindric, with a series of prominent annellidic rings in the upper part (the annellidic rings poorly developed in PDD 992, if present then restricted to 2-3 rings), with a broad, single, apical conidiogenous locus; solitary, forming palisade-like layer across lower wall of conidioma. Conidia 39-46 x 13.5-14.5 µm, broad-cylindric, straight, apex broadly rounded, base with small truncate scar, 0-septate, wall hyaline, smooth, about 1 µm thick.

Two of the Baeumleria nothofagi collections examined (the holotype and NZFRI 2334) are macroscopically very similar. The third collection (PDD 992) differs in having larger conidiomata, which develop on both the upper and lower surface of the leaves. PDD 992 also has less prominent annellations on the conidiogenous cells. The conidial locules on the type collection of Trabutia nothofagi, reportedly the teleomorph of B. nothofagi, has conidiogenous cells in which annellations were never observed. NZFRI 2334 has conidiomata which develop below the epidermal cells, while the other two collections are subcuticular. This may reflect anatomical differences between the leaves of the hosts of these collections, N. truncata and N. solandri var. cliffortioides respectively. Although the significance of these differences remains uncertain, for now they are considered to represent natural variation within the species.

See also notes under Trabutia nothofagi.

Fruiting bodies develop on upper surface of leaves, scattered randomly over whole of the surface, not obviously associated with necrosis of leaf tissue; 0.5-1.5 mm diam., irregular in outline, pustulate, surface irregularly lumpy, wall black, shiny.

Fruiting bodies multiloculate, with several ascomatal locules, and sometimes with one or more conidiomatal locules at the margin; subcuticular, host epidermal cells remaining intact with little or no hyphae in the leaf tissue. Upper wall with the outer 40-50 µm comprising angular cells 4-7 µm diam. with very dark and very thick walls, inside this are columns of long-cylindric cells 3-6 µm diam. with brown and thin walls; locules develop amongst the long-cylindric cells, these cells being broken-down and pushed aside as ascomatal locules develop, but remaining more or less intact as separating tissue between locules. Lower wall more or less lacking, with subhymenium of ascomata, and conidiogenous cells of conidiomata, developing on the host epidermal cells.

Paraphyses 1.5 µm diam., branched occasionally, persistent. Asci 145-175 x 27-37 µm, saccate with a small basal foot; wall very thin when immature, about 2 µm thick when mature, appears to have 2 layers but these not separating with ascospore discharge, undifferentiated at apex; 8-spored. Ascospores 35-46 x 12.5-14.5, oblong, each end broadly rounded, straight or slightly curved, 0-septate, wall hyaline, 0.5-1 µm thick.

Conidiogenous cells 10-17 x 7-10 µm, more or less ampulliform to cylindric, wall thickened immediately below point at which conidium delimited, with a single, apical conidiogenous locus; solitary, forming a palisade-like layer across base of the conidioma. Conidia 38-47 x 12.5-16.5, oblong or sometimes irregularly tapering toward base, more or less straight, apex rounded, base with truncate scar, sometimes with small frill around scar, 0-septate, wall hyaline, smooth, about 1 µm thick.

Specimens of T. nothofagi and its putative anamorph, Baeumleria nothofagi, are known from only one collection on the leaves of Nothofagus fusca (Hook.f.) Oerst., two on N. solandri var. cliffortioides (Hook.f.) Poole, and one on N. truncata (Colenso) Cockayne. The teleomorph is known from only a single collection, on N. fusca. There is considerable morphological variation between collections, especially in the size of the fruiting bodies, whether they develop on the upper or lower leaf surface, and the development of annellations on the conidiogenous cells. The differences cannot clearly be correlated with host species, and as these features may be affected by maturity of the fungus, or by the environment in which it is growing; many more collections are required before the significance of the observed differences can properly be assessed. See also notes under Baeumleria nothofagi.

T. nothofagi was reported by Hood (1985) and Dingley (1969) to be common throughout New Zealand. However, examination of all herbarium material putatively of this species in the two largest fungal herbaria in New Zealand (PDD and NZFRI), has shown that several macroscopically similar tar-spot like fungi occur on Nothofagus leaves in New Zealand, and that the only collections that can unequivocally be attributed to T. nothofagi or its anamorph are the three cited by Sydow (1924) and Hennings (1903) in their original descriptions, plus one more recent collection.

The collection on N. solandri var. cliffortioides discussed under "cf. Trabutia sp." appears to represent a second Trabutia species on Nothofagus in New Zealand. Although the Baeumleria-like anamorph is well represented in the collection, the sexual state is immature, and the species is not formally described here. This fungus differs from T. nothofagi in the shape of the conidia. See notes under cf. Trabutia sp.

Based on the descriptions and notes published by Barr (1987) and Cannon (1991), there are some differences between T. nothofagi and the Quercus-inhabiting species of this genus from the Northern Hemisphere, including the type species of the genus, T. quercina (F.Rudolphi) Sacc. & Roum. These authors described the immature asci of Trabutia as thick-walled, and the paraphyses as Phyllachora-like, broad, thin-walled and collapsing. Based on these kinds of characters Arx (1987), Barr (1987) and Cannon (1991) discussed an appropriate family placement for these ascomycetes. If such features truly are significant at the family level, then consideration needs to be given to the possibility that the Nothofagus-inhabiting species (with its thin-walled asci and persistent, narrow paraphyses) should be in a genus distinct from the Northern Hemisphere species on Quercus.