Cryptic diversity, pathogenicity, and evolutionary species boundaries in Cercospora populations associated with Cercospora leaf spot of Beta vulgaris

Cercospora polygonatum, a New Species Causing Gray Leaf Spot Disease in Polygonatum cyrtonema

This study identified a new species (Cercospora polygonatum) that causes gray leaf spot (GLS) disease in cultivated Polygonatum cyrtonema. This fungal species was isolated from the affected region of GLS on P. cyrtonema leaves. Pathogenicity bioassays were conducted based on Koch's postulates. Morphology was examined based on the features of conidiomata, conidiogenous loci, conidia/conidiophores, and conidiogenous cells. The rDNA internal transcribed spacer region, calmodulin, translation elongation factor 1-alpha, and histone genes were subjected to phylogenetic analysis using the MrBayes tool in Phylosuite. Bootstrap support analysis for phylogenetic placement confirmed the new species, which was significantly different from the closely related species C. senecionis-walkeri and C. zeae-maydis. The morphological characteristics also supported this finding, with the conidiogenous cells of C. polygonatum being considerably shorter than those of C. senecionis-walkeri or C. zeae-maydis. In addition, C. polygonatum was distinguished by its cultural characteristics. As this fungus was isolated from P. cyrtonema, it was named C. polygonatum F.Q. Yin, M. Liu & W.L. Ma, sp. nov. The type specimen (H8-2) was preserved at the China General Microbiological Culture Collection Center. This is the first report of GLS caused by C. polygonatum on P. cyrtonema leaves in China. The current study enriches the knowledge regarding Cercospora sp., contributes to the identification of a species causing GLS in P. cyrtonema, and provides useful information for the effective management of this disease.

Alternative Hosts of Cercospora beticola in Field Surveys and Inoculation Trials

Cercospora beticola, the cause of Cercospora leaf spot (CLS) of sugar beet and table beet, has a broad range of potential alternative hosts. The role of these hosts as inoculum sources in the field is unclear and has had limited investigation since the advent of DNA-based pathogen identification. The presence of C. beticola on alternative hosts associated with table beet fields of New York was assessed in field surveys during 2016. Lesions were collected, and 71 cercosporoid conidia were isolated for phylogenetic comparison. C. beticola was identified from Solanum ptycanthum (n = 4), Chenopodium album (n = 2), and Spinacia oleracea (n = 1), whereas C. chenopodii was identified on Chenopodium album (n = 51). Artificial inoculation of 21 plants species demonstrated that C. beticola was pathogenic to Brassica kaber, Chenopodium album, Carthamus tinctorius, Rumex obtusifolius, and Spinacia oleracea. These results indicate that although C. beticola may be pathogenic to a range of plant species, the role of symptomatic tissue for inoculum production on alternative hosts in the field appears limited. Observations of C. beticola on necrotic and naturally senescent tissue suggest saprophytic survival on plant debris of a range of species, which has implications for CLS epidemics and disease management.

Fungal species boundaries in the genomics era

Genomic data has opened new possibilities to understand how organisms change over time, and could enable the discovery of previously undescribed species. Although taxonomy used to be based on phenotypes, molecular data has frequently revealed that morphological traits are insufficient to describe biodiversity. Genomics holds the promise of revealing even more genetic discontinuities, but the parameters on how to describe species from genomic data remain unclear. Fungi have been a successful case in which the use of molecular markers has uncovered the existence of genetic boundaries where no crosses are possible. In this minireview, we highlight recent advances, propose a set of standards to use genomic sequences to uncover species boundaries, point out potential pitfalls, and present possible future research directions.