Location: Sugarbeet and Potato ResearchTitle: Cryptic diversity, pathogenicity, and evolutionary species boundaries in Cercospora populations associated with Cercospora leaf spot of Beta vulgaris
|VAGHEFI, NILOOFAR - Cornell University|
|KIKKERT, JULIE - Cornell University|
|HAY, FRANK - Cornell University|
|CARVER, GAVRIELA - Cornell University|
|KOENICK, LORI - Cornell University|
|SECOR, GARY - North Dakota State University|
|PETHYBRIDGE, SARAH - Cornell University|
Submitted to: Fungal Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/31/2018
Publication Date: 2/9/2018
Citation: Vaghefi, N., Kikkert, J.R., Hay, F.S., Carver, G.D., Koenick, L.B., Bolton, M.D., Hanson, L.E., Secor, G.A., Pethybridge, S.J. 2018. Cryptic diversity, pathogenicity, and evolutionary species boundaries in Cercospora populations associated with Cercospora leaf spot of Beta vulgaris. Fungal Biology. 122:264-282. https://doi.org/10.1016/j.funbio.2018.01.008.
Interpretive Summary: Species in the genus Cercospora cause disease on nearly every major crop worldwide. An extremely damaging disease called Cercospora leaf spot (CLS) occurs on sugar beet, the primary source of sugar in the United States, as well as the related food crops table beet and swiss chard. Despite the economic importance of this disease, it has not been clear which Cercospora species is the primary cause of CLS of these crops. In addition, it is not clear if the multiple Cercospora strains found on these crops should actually be considered a single species. It is critical to establish the correct disease-causing organism so plant breeders and pathologists can develop crops with resistance to disease. To gain a better understanding of this conundrum, six genes were sequenced in a collection of Cercospora strains collected from around the world on each of the three crops. Analysis of the sequenced DNA showed that Cercospora beticola should be maintained as a separate species and considered the primary species causing CLS on all three crops. However, other species such as Cercospora flagellaris were shown to cause CLS in table beet, which is important because this same species causes economically important diseases called Cercospora leaf blight and purple seed stain in soybean. Consequently, farmers must consider crop rotation regimes between soybean and table beet carefully. These results suggest that future research is needed to identify and discriminate all species that are able to cause CLS on each of the three crops.
Technical Abstract: Cercospora is one of the largest genera of hyphomycetes accommodating several important phytopathogenic species associated with foliar diseases of vegetable and field crops. Cercospora leaf spot (CLS), caused by C. beticola, is a destructive disease of Beta vulgaris (sugar beet, table beet and swiss chard) worldwide. Previous genotyping-by-sequencing studies suggested the presence of multiple Cercospora species in association with CLS symptoms on B. vulgaris. This study investigated the taxonomy and evolutionary species boundaries in a global collection of Cercospora isolates (n = 102) from B. vulgaris, based on sequences of six loci; internal transcribed spacer of the nrDNA, calmodulin, actin, histone H3, translation elongation factor 1-alpha, and the gene encoding for cercosporin facilitator protein. Species boundaries were assessed using concatenated multi-locus phylogenies, Generalized Mixed Yule Coalescent, Poisson Tree Processes, and Bayes factor delimitation (BFD) framework. Cercospora beticola was confirmed as the primary cause of CLS on B. vulgaris. Cercospora apii, C. cf. flagellaris, Cercospora sp. G, and C. zebrina were also identified in association with CLS on B. vulgaris. Cercospora apii and C. cf. flagellaris were pathogenic to table beet but Cercospora sp. G and C. zebrina did not cause disease. While Genealogical concordance phylogenetic species recognition, GMYC and PTP methods failed to differentiate C. apii and C. beticola as separate species, multi-species coalescent analysis based on BFD supported separation of C. apii and C. beticola into distinct species, and provided evidence of evolutionary independent lineages within C. beticola. Extensive intra- and intergenic recombination, incongruency of gene genealogies and dominance of clonal reproduction in some species complicate evolutionary species recognition in the genus Cercospora. Host association based on the substrate from which Cercospora species have been isolated was found to provide little value for determining taxonomic position. The results indicated that the number of Cercospora species associated with CLS on B. vulgaris has been underestimated. The implications for these findings suggest morphological and phylogenetic analyses to disentangle cryptic speciation within C. beticola may be warranted.