Submitted to: Mycologia
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/24/2012
Publication Date: 9/1/2012
Citation: Aoki, T., Tanaka, F., Suga, H., Hyakumachi, M., Scandiani, M.M., O'Donnell, K. 2012. Fusarium azukicola sp. nov., an exotic azuki bean root-rot pathogen in Hokkaido, Japan. Mycologia. 104(5):1068-1084. Interpretive Summary: A novel Fusarium pathogen isolated from azuki bean (Vigna angularis) plants exhibiting root-rot symptoms in Hokkaido, Japan was characterized in this study. Phenotypic studies demonstrated that this bean root-rot (BRR) pathogen could be distinguished from all other known Phaseolus/Vigna BRR and soybean sudden death syndrome (SDS) pathogens by the production of wider and longer asexual spores when grown in pure culture. Analyses of DNA sequences from six independent genes strongly supported the novel species status of the azuki bean pathogen. Thus, the pathogen was formally described as F. azukicola. We completed Koch’s postulates by demonstrating that isolates of F. azukicola could 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 in a pathogenicity experiment. These research findings will be of interest to plant pathologists and quarantine officials who are charged with controlling the spread of this economically destructive disease and they will alert bean breeder’s that their cultivars need to be tested for resistance to this pathogen.
Technical 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 is formally 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 4-septate conidia cultured on synthetic nutrient agar(SNA). Molecular phylogenetic analyzes of four anonymous intergenic loci, a portion of the translation elongation factor (EF-1a) gene and the nuclear ribosomal intergenic spacer region (IGS rDNA) strongly supported 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, were unresolved by phylogenetic analyses of the individual and combined datasets, with the exception of the IGS rDNA partition which strongly supported it as a sister to the soybean SDS pathogen F. brasiliense. A previously published multilocus genotyping assay was updated to include primer probes that successfully distinguished F. azukicola from the other soybean SDS and BRR pathogens. Results of a pathogenicity experiment revealed that the F. azukicola isolates were 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 working 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.