Search

Because the genus Colletotrichumis among the most important groups of plant pathogenic fungi worldwide, the ability to accurately diagnose species is vital for the implementation of effective disease control and quarantine measures. Although the long-standing, unresolved taxonomic issues in the genus have recently begun to be addressed through multi-locus phylogenetic research, the tools most commonly used for Colletotrichumspecies identification are either insufficiently variable (e.g. morphology), or homoplasic (e.g. morphology and host range criteria). In this study, using the systematically well-defined falcate-spored, grass-associated group (FG) of Colletotrichumas a model, we test the utility of ITS sequence data to diagnose species affiliations through similarity-based searches of the NCBI GenBank database or by means of gene trees constructed using phylogenetic methods. 43% of all Colletotrichumsequences accessioned by GenBank are from the ITS region, making it the single most common sequence curated by the community; however, 34% of the ITS accessions existed only as sequence data in the database, with no associatedPUBLICation. Using ColletotrichumITS sequence data from 53 FG defined isolates and 16 falcate-spored, non-graminicolous isolates to perform GenBank BLASTN searches, we found that erroneous identifications occurred for 86% of the 14 species tested. In contrast, the phylogenetic tree generated by the ITS sequence data, although poorly supported by bootstrap values, correctly grouped most of the species, but 10% of the individual isolates were incorrectly placed. From this study, we conclude that the currently available infrastructure of ColletotrichumITS sequence data may yield unreliable species diagnoses, particularly if sequence similarity alone is the only criterion applied.

Species limits in the fungal genus Colletotrichumare traditionally distinguished by appressorial and/or conidial morphology or through host plant association, but both criteria are criticized for their inability to resolve distinct taxa. In previous research eight novel falcate-spored Colletotrichumspecies were identified from graminicolous hosts using multilocus molecular phylogenetic analysis. In the present work formal descriptions and illustrations are provided for six of the new taxa: C. hanauisp. nov., C. nicholsoniisp. nov., C. paspalisp. nov., C. jacksoniisp. nov., C. miscanthisp. nov. and C. axonopodisp. nov.; and an emended description with epitypification is provided for C. eleusines. Comparison of hyphopodial appressoria and host association against phylogenetic species boundaries and evolutionary relationships in the graminicolous Colletotrichumgroup demonstrate that, while these characters can be useful in combination for the purpose of species diagnosis, erroneous identification is possible and species boundaries might be underestimated if these characters are used independently, as exemplified by the polyphyletic taxa C. falcatum. Appressoria have been subject to convergent evolution and were not predictive of phylogenetic relationships. Despite these limitations, the results of this work establish that in combination appressorial and host range characters could be used to generate informative dichotomous identification keys for Colletotrichumspecies groups when an underlying framework of evolutionary relationships, taxonomic criteria and nomenclature have been satisfactorily derived from molecular systematic treatments.