The genera Didymella and Leptosphaerulina, which produce hyaline dictyospores and 1- septate phragmospores, both are classified in the Didymellaceae (de Gruyter et al. 2009). Five out of the nine Phoma sections are related to the genus Didymella. The genus Didymella is based on the type species Didymella exigua.

The genus Didymella is considered to be the sole sexual genus of Phoma s. str. Didymella contains some very important and serious plant pathogenic species, as well as species that are endophytic and sarobic on a wide range of crops (Aveskamp et al. 2010).

Didymella exigua (Niessl) Sacc., Michelia 2: 57 (1880). Didymosphaeria exigua Niessl, Oesterr. Bot. Z. 25: 165 (1875).

Ascomata subepidermal in the cortex of stems or in bracts of dead inflorescences, erumpent, subglobose to flattened, small, up to 170 μm diam, papillate; wall thin, 10–15 μm diam, outer zone consisting of 2–3 layers of textura angularis. Pseudoparaphyses hyaline, 1.5–2.5 μm diam, septate. Asci bitunicate, clavate to short cylindrical, 45–70 10– 12 μm. Ascospores uni- to biseriate, ellipsoidal, straight to slightly curved, 12–16 4.5–6 μm, hyaline, smooth, apex obtuse, base broadly obtuse to subobtuse, medianly 1-septate, upper cell often wider than lower cell, slightly constricted at the septum.

Neotype: dried culture CBS H-20123, culture ex-neotype CBS 183.55, ex Rumex arifolius (Polygonaceae) France.

The asexual state of D. exigua represents a species of Phoma. Corbaz (1957) observed pycnidia in nature and culture to resemble ascomata in size, and to give rise to conidia that are short cylindrical to bacilliform, 0(–1)-septate, hyaline, 9–13 4–6 μm. Many collections of D. exigua have been obtained from litter on a wide range of herbaceous dicotyledons (Corlett 1981) in temperate and subtropical regions.

Literature

Aveskamp M, Gruyter H de, Woudenberg J, Verkley G, Crous PW (2010). Highlights of the Didymellaceae: A polyphasic approach to characterise Phoma and related pleosporalean genera. Studies in Mycology 65: 1–60.
Corbaz R (1957). Recherches sur le genre Didymella Sacc. Phytopathologische Zeitschrift 28: 375–414.

Corlett M (1981). A taxonomic survey of some species of Didymella and Didymella-like species. Canadian Journal of Botany 59: 2016–2042.
Gruyter J de, Aveskamp MM, Woudenberg JHC, Verkley GJM, Groenewald JZ, Crous PW (2009). Molecular phylogeny of Phoma and allied anamorph genera: towards a reclassification of the Phoma complex. Mycological Research 113: 508–519.