Phylogenetic placements and phenotypic traits of soft rot bacteria isolated from potato (Solanum tuberosum) in Taiwan

Soft rot bacteria are important potato pathogens. Most of these bacteria were originally classified under the Erwinia genus, but are now identified as species of genera Pectobacterium and Dickeya. Studies have shown that genetic and phenotypic diversities of these pathogens vary across regions, and exploring such aspects in different potato-producing areas could provide insights into these bacteria’ ecology. In this study, 84 Taiwanese strains of potato-isolated soft rot bacteria were subjected to phylogenetic and phenotypic assays using the latest classification schemes. Preliminary testing with genus-specific primers suggested that all strains belonged to Pectobacterium. Multilocus sequence analysis divided them into four clades, each represented respectively by the type strain of P. brasiliense, P. carotovorum, P. versatile, and P. aroidearum. Among the species identified, P. brasiliense was found to be the most genetically diverse and prevalent. Phenotypic tests on representative strains of these species showed that all tested strains were able to grow at pH 5.5–10.0 and 16–37 °C. The ability to grow at pH 4.0–41 °C varied across strains, but the differences were unable to discriminate among species. Additional physiological assays were conducted, and while most assays did not allow reliable distinction among species, the indole production test was able to differentiate P. brasiliense strains from the other species. Inoculation of potato tubers revealed great intra-specific heterogeneity among maceration potentials of the tested strains. Interestingly, the maceration potentials of different Pectobacterium species responded differently to elevated temperature. The findings from this work contribute to a broader understanding of Pectobacterium strains affecting potatoes in Taiwan.