Conidiomata uniloculate, subcuticular. Upper wall comprises a layer 4-6 µm thick of dark material with no obvious cellular structure, lining the host cuticle. Lower wall rests on more or less intact host epidermal cells (epidermal cells and uppermost layer of hypodermal cells beneath conidioma packed with yellow material, but not containing fungal hyphae), 12-20 µm thick, comprising 2-4 layers of angular to isodiametric cells 5-6 µm diam. with very pale brown walls.

Conidiogenous cells 11-12.5 x 5-6.5 µm, short-cylindric, solitary, with a broad, single, apical conidiogenous locus, wall slightly thickened immediately below point where conidia are released. Conidia 19-27 x 5.5-7 µm, cylindric, slightly curved, apex rounded, base with truncate scar, and often with a small frill, wall hyaline, smooth, 0-septate.

The coelomycete genus Monostichella Höhn. includes leaf pathogens which form subcuticular, acervular conidiomata having short-cylindric conidiogenous cells with percurrent proliferation at the broad, single, apical conidiogenous locus, and large, non-septate, hyaline conidia with a truncate base (Sutton 1980). M. nothofagi is the second Monostichella species to have been reported from the Fagaceae, M. falcata (Dearn. & House) Arx having been described from Lindera from North America. Based on the description of Arx (1980), the two species are macroscopically similar, but they are clearly distinguished by conidial shape (Arx 1980, Morgan-Jones 1971).

In many features M. nothofagi is both macroscopically and morphologically similar to the Baeumleria species discussed in this paper. The Baeumleria species differ in having the upper wall of the conidiomata with a distinctly cellular structure.