Molecular phylogeny and taxonomy of plant pathogenic complex of bipolaris, cochliobolus, curvularia and endophytic colletotrichum

Abstract

The plant pathogenic generic complex Bipolaris - Cochliobolus - Curvularia comprises many destructive plant pathogens that cause severe cereal crop losses worldwide. The taxonomy of this complex is confused as frequent nomenclatural changes occurred over the past 50 years. The Bipolaris - Cochliobolus - Curvularia complex is investigated based on a set of ex-type cultures and collections from northern Thailand and elsewhere. Combined gene analysis of ITS (nuclear ribosomal internal transcribed spacer), GPDH (glyceraldehyde 3-phosphate dehydrogenase), LSU (large subunit) and TEF (translation elongation factor 1-α) revealed the segregation of this generic complex into major two groups. Bipolaris and Cochliobolus species clustered in Group 1 along with their type species, whereas Curvularia species (including species named as Bipolaris, Cochliobolus and Curvularia) clustered in Group 2, with its generic type of Curvularia. The initial generic resolution was fundamental to the comprehensive assessments of taxonomy and phylogeny. The nomenclatural conflict in this complex is resolved giving priority to the more commonly used established generic names (6) Bipolaris and Curvularia, therefore the Cochliobolus names were recognised as synonyms. The genus Bipolaris is revisited with all available type materials and most of the additional materials selected from worldwide herbaria. A phylogenetic analysis of the genus provided with fresh collections and including the additional authenticated cultures. The generic type Bipolaris maydis is neo-typified with other ex, neo-types of B. chloridis, B. cynodontis, B.maydis, B. salviniae, B. victoriae, and B. yamadae. Lectotypes were designated for B. hadrotrichoides and B. stenospila. Modern descriptions and illustrations are provided for 46 species of Bipolaris, with a revision of synonyms, and notes on nomenclature, biology and ecology of the species. A taxonomic key is provided for the morphological identification of the species within the genus. The generic wide phylogenetic tree is provided for future studies within the genus. The genus Curvularia is revised in two consecutive phylogenetic assessments resulting 3 new species including C. asianensis, C. alcornii and C. boedjinii. Additionally the generic type C. lunats and C. kusanoi is neotypified, and the generic concepts were reassessed. Nineteen species previously thought to be Bipolaris are transferred to the genus Curvularia, broadening the species numbers within the genus based on phylogenetic and morphological deference. The endophytic Colletotrichum from several grass species were studied with ITS, ACT (actin), CAL (calmodulin) and GPDH phylogeny and morphology. A new species, C. endophytica is described with full descriptions and illustrations. This study revealed an unknown diversity of Colletotrichum species associated with tropical grass with perspectives for future studies. (7) In overall, this thesis serves as a complete account of graminicolous species complex Bipolaris-Curvularia, as well as endophytic Colletotrichum providing grounds for further study in plant pathology and fungal systematics.