Diplodia Fr., in Montagne, Annls Sci. Nat., Bot., sér. 2 1: 302 (1834).
Index Fungorum number: IF 8047; Facesoffungi number: FoF 00147, 397 morphological species (Species Fungorum 2022), 30 species with molecular data.
Saprobic on host. Sexual morph: Ascomata unilocular, solitary or clustered, immersed, partially erumpent when mature, dark brown to black, thick-walled, wall composed of outer layers of thick-walled, dark brown textura angularis, inner layers of thin-walled, hyaline textura angularis. Ostiole central, circular, papillate, periphysate. Pseudoparaphyses hyaline, branched, septate. Asci clavate, stipitate, bitunicate, containing eight, biseriate ascospores. Ascospores fusiform, hyaline, thin-walled, smooth, aseptate, rarely becoming light brown and 1–2-septate with age. Asexual morph: Mycelium immersed or superficial, branched, septate, melanised, dark brown. Conidiomata pycnidial, ostiolate, formed in uni- or multiloculate stromata, immersed, becoming erumpent at maturity. Ostiole central, circular, papillate. Paraphyses lacking. Conidiophores (when present) hyaline, simple, occasionally septate, rarely branched, cylindrical, arising from the cells lining the pycnidial cavity. Conidiogenous cells holoblastic, hyaline, cylindrical, determinate or proliferating at the same level giving rise to periclinal thickenings, or proliferating percurrently and forming two or three annellations. Conidia initially hyaline, aseptate, thick-walled, becoming 1–2-septate and pale transluscent brown after discharge from the pycnidia, but the colouration is often delayed or never occurs, in some species the conidia become pigmented while still enclosed in the conidioma and in these species the conidia rarely become septate (Adapted from Phillips et al. 2013).
Type species: Diplodia mutila (Fr.: Fr.) Fr.
Notes: Diplodia was introduced by Montagne (1834) with D. mutila as type species. Diplodia is characterized by solitary or clustered, immersed ascomata, clavate, stipitate, bitunicate asci and fusiform, hyaline, thin-walled, smooth, aseptate ascospores. Diplodia exhibit two different conidial morphologies. In one type, conidia are hyaline and aseptate and turns pale to dark brown and 1-septate on maturity. Pigmentation often occurs late and some species never form dark conidia. In the second type, the conidia turn pigmented at an early stage of development even when they are still inside the pycnidia and rarely become septate. This character is phylogenetically significant as these two morphological groups form two separate phylogenetic lineages. de Wet et al. (2003) differentiated Diplodia scrobiculata from D. sapinea based on multiple gene genealogies using six protein coding genes and six microsatellite loci. Alves et al. (2004) re-studied the type species D. mutila and mentioned that the latter has a “Botryosphaeria stevensii” sexual state. The taxonomy and phylogeny of Diplodia have been problematic due to the lack of ex-type culture linked to the holotype of D. mutila. Diplodia species have been largely defined based on host association (Phillips et al. 2008). Alves et al. (2014) studied numerous collections of Diplodia obtained from V-shaped cankers and dieback of woody hosts and provided a phylogeny from ITS and TEF-1 sequence data. Alves et al. (2014) designated an epitype for Diplodia mutila with related ex-epitype cultures and a neotype for D. fraxini. Several authors added new species to Diplodia (Damm and Crous 2008; Ariyawansa et al. 2015; Osorio et al. 2016; Yang et al. 2016; González-Domínguez et al. 2017; Tennakoon et al. 2021) Hyde et al. (2014) provided a backbone tree for 20 Diplodia species using holotype and ex-type sequence data available in GenBank. Diplodia is a distinct genus in Botryosphaeriaceae. Reliable molecular markers to delineate species of Diplodia are LSU, SSU for generic level and ITS, TEF and BTUB for species level. A combination of ITS and TEF sequence data provide higher phylogenetic resolution to delineate Diplodia species.