Mycelium penetrating the whole thickness of the host leaf as hyaline, intercellular hyphae, without haustoria in the cells, aggregated in places into irregular, loose columns of tissue between the upper and lower epidermis, which darken at one end to form a solid mass of vertical cells 10-18 x 6-8 µ. This dark mass, when at the upper end of the column extends over the palisade tissue, with small extensions down into this. The dark internal stroma enlarges vertically to burst through the epidermis to extend further as an external stroma up to 50 µ thick. Viewed from above the stroma consists of a tightly packed, irregular mass of rounded to subglobose, perithecioid lobes 120-150 µ diam., black, slightly rough on the surface.

Stromata scattered, amphigenous, often more or less confluent, along the leaf margin, surrounded by the broken epidermis, ¼-¾ mm. diam., or to 1 x ¼ mm. In section each lobe contains a single globose loculus, without a distinct perithecial wall, completely immersed in the stroma tissue, subglobose with conoid apical extension, at maturity 70-90 µ diam. and to 110 µ high, opening by an irregular apical aperture, and surrounded by a dark wall of stroma tissue in 2-5 layers, 20-28 µ thick around the sides, thinner towards the apex, the surface slightly rough by projecting, rounded cells. At the sides these lobes are more or less connate, and with the whole basal half continuous with the external stroma, free above. At first the loculus is about three-quarters filled with a solid mass of delicate, hyaline parenchyma, the upper part with looser short, tortuous, hyaline hyphae, some of which remain in the mature, opened loculus at the apex as short, septate "periphyses". The numerous asci develop at the base of the loculus, and replace almost the entire original contents; erect, narrow ellipsoid to subcylindric, subsessile, rounded at apex and there thickened to 3-4 µ, bi-tunicate, 8-spored, aparaphysate, 38-48 x 9-11 µ. Spores obliquely more or less 2-seriate, hyaline, subfusoid with rounded ends, slightly bent, smooth, 1-septate in middle, not or very slightly constricted, 15-19 x 3-4 µ, cells equal.