Fusarium azukicola sp. nov., an exotic azuki bean root-rot pathogen in Hokkaido, Japan

Abstract

We report on the phenotypic, molecular phylogenetic and pathogenic characterization of a novel azuki bean (Vigna angularis) root-rot (BRR) pathogen from Hokkaido, Japan, which formally is described herein as Fusarium azukicola. This species can be distinguished phenotypically from the other Phaseolus/Vigna BRR and soybean sudden-death syndrome (SDS) pathogens by the production of wider and longer four-septate conidia cultured on SNA. Molecular phylogenetic analyses of four anonymous intergenic loci, a portion of the translation elongation factor (EF-1α) gene and the nuclear ribosomal intergenic spacer region (IGS rDNA) strongly support the genealogical exclusivity of F. azukicola with respect to the other soybean SDS and BRR pathogens within Clade 2 of the F. solani species complex (FSSC). Evolutionary relationships of F. azukicola to other members of the SDS–BRR clade, however, are unresolved by phylogenetic analyses of the individual and combined datasets, with the exception of the IGS rDNA partition, which strongly supports it as a sister of the soybean SDS pathogen F. brasiliense. A multilocus genotyping assay is updated to include primer probes that successfully distinguish F. azukicola from the other soybean SDS and BRR pathogens. Results of a pathogenicity experiment reveal that the F. azukicola isolates are able to induce root-rot symptoms on azuki bean, mung bean (Vigna radiata), kidney bean (Phaseolus vulgaris) and soybean (Glycine max), as well as typical SDS foliar symptoms on soybean. Our hypothesis is that F. azukicola evolved in South America and was introduced to Hokkaido, Japan, on azuki bean but its possible route of introduction remains unknown.

Key words:

• Glycine max
• pathogenicity
• Phaseolus vulgaris
• phylogeny
• SDS
• taxonomy
• Vigna angularis

Acknowledgments

Special thanks to Stacy Sink for generating the DNA sequence data reported in this study and for designing and validating the Fusarium azukicola-specific probes used in the MLGT assay. We also thank Nathane Orwig for running the DNA sequences in NCAUR's core facility. The mention of firm names or trade products does not imply that they are endorsed or recommended by the U.S. Department of Agriculture over other firms or similar products not mentioned. The USDA is an equal opportunity provider and employer.