Investigating species boundaries in Colletotrichum

Colletotrichum is one of the most important plant pathogenic genera that is responsible for numerous diseases which can have a profound impact on the agricultural sector. Species delineation is difficult due to a lack of distinctive phenotypic variation. Therefore, in this study three different genomic approaches based on phylogenetic, evolutionary and coalescent-based methods are applied to establish robust species boundaries. The reliability of five different DNA barcodes was also assessed to provide further insights into species delineation. The ITS region can resolve the placement of taxa up to the species complex level. The GAPDH and TUB2 markers are determined to be the most informative for most complexes. However, no single marker could discriminate between species in all complexes, therefore different molecular approaches based on multi-locus datasets are recommended. This is the first study to provide an estimated divergence time for all species complexes in Colletotrichum. The estimated divergence time for species complexes ranged between 4.8 to 32.2 MYA. Based on the high level of congruent results obtained from the different molecular approaches, a new species complex, the Colletotrichum agaves complex is introduced. This complex consists of five taxa which are characterised by the presence of straight or slightly curved conidia with obtuse apices. This study shows that coalescent approaches and multi-locus phylogeny are crucial to establish species boundaries in Colletotrichum. The taxonomic placement of three singleton taxa Colletotrichum axonopodi, C. cariniferi and C. parallelophorum is revised. We accept 248 species and provide recommendations regarding species boundaries in the graminicola–caudatum complex.