Pseudomonas grimontii, causal agent of turnip bacterial rot disease in Japan

From April to May 2012, a rot disease on turnips grown in greenhouses in southern Hokkaido, Japan caused severe leaf blight, brown discoloration of crown surface, and browning and rot of the interior of main roots. From diseased tissues, bacteria that formed pale beige- to cream-colored colonies were isolated and demonstrated to be pathogenic on turnip after inoculation and reisolation of the isolates. They were gram-negative, aerobic rods with one to three polar flagella, and identified as Pseudomonas grimontii based on biochemical and physiological characterizations, multilocus sequence analysis using concatenated sequences of 16S rRNA, rpoD, gyrB, and rpoB genes, and MALDI-TOF MS analysis using the Bruker MALDI Biotyper system. To the best of our knowledge, P. grimontii has not been isolated previously in Japan, so this study might be the first report of the species in Japan. Although the significance of P. grimontii as a plant pathogenic bacterium has not yet been fully evaluated, we proved in this study that the bacterium is the causal agent of the disease and that its virulence is strong. These findings indicate that it is necessary to record P. grimontii as a novel pathogen posing a threat to plants and requiring vigilance.