Submitted to: IMA Fungus
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/25/2014
Publication Date: 3/11/2014
Citation: Crouch, J. 2014. Colletotrichum caudatum s.l. is a species complex. IMA Fungus. 5(1):17-30. Interpretive Summary: Fungi cause leaf spot diseases of warm season grasses including those used for the production of bioenergy. One of these fungi occurs on a wide range of hosts throughout the world. In this research isolates of this disease-causing fungus from four continents were examined to determine if they were correctly identified. Both morphological and molecular characteristics were determined for each isolate. It was discovered that, although these fungi appear to be slightly different on each continent, they are all considered to be a single species. This fungal species is well characterized using DNA markers. Plant pathologists will use this research to determine the accurate identification of the fungi causing leaf spot diseases on grasses grown as bioenergy crops.
Technical Abstract: Colletotrichum caudatum sensu lato is a widespread fungal pathogen of warm-season grasses. The fungus is easily differentiated from other Colletotrichum species through the presence of a unique filiform appendage at the apex of the conidium. Multi-locus phylogenetic analysis of four DNA sequence markers from 21 isolates of C. caudatum s.l. from six grass hosts recovered the morphospecies as a well-supported monophyletic group. Although closely related to other Colletotrichum species pathogenic to warm-season grasses (e.g. C. sublineola, C. falcatum, C. navitas, C. graminicola), the sister taxon placement of C. caudatum remained unclear. Four major subgroups and three monotypic lineages were identified from the C. caudatum s.l. isolates. Despite the presence of localized, taxon-specific incongruence between gene trees and evidence for recombination in the dataset, application of genealogical concordance species recognition criteria diagnosed the four subgroups as phylogenetic species. Traditional morphology-based species concept defines C. caudatum as one species with a broad host range; however, multi-locus phylogenetic analyses refuted this model. Instead, isolates from different hosts were mainly segregated into different lineages. In particular, isolates from the type locale and host (USA, Sorghastrum nutans) collected within a 400 km radius were divided into three distinct species that corresponded with the three sampling sites. These data established that traditional morphological and ecological features are not informative for recognition of taxa within C. caudatum s.l., although there is some evidence that some species may be host specific. To stabilize the application of the name C. caudatum, DNA sequence data from the lectotype was generated, an epitype strain consistent with the type was designated and illustrated, and an emended description of C. caudatum sensu stricto is provided. Colletotrichum alcornii, C. baltimorense, C. somersetense, and C. zoysiae are described as new morphologically cryptic species related to C. caudatum s.s.