Isolation and identification of Pseudomonas protegens sp. causing soft rot of Chinese cabbage in Shandong Province, China

The potential pathogen causing soft rot of Chinese cabbage were isolated from the infected leaves and contaminated seeds and identified by molecular and biochemical methods. The maximum likelihood phylogenetic tree based on the concentrated sequences with the four housekeeping genes 16S rDNA-gyrB-rpoB-rpoD of the isolates using the best-fit partition model obtained by online IQ-tree plug-in in PhyloSuite v 12.1 showed that the novel isolates clustered into one branch, then formed a unique branch with Pseudomonas protegens DSM 19095 indicating that the isolates were the same isolate and relative to P. protegens DSM 19095. In addition, the emergence rate of the healthy seeds after incubation with the pathogen at 8th day significantly decreased by 27.21% compared with the PBS inoculated seeds. By quantitative PCR, the original pathogen load of JinDi CR breeding was 1310.8 ± 521.7 CFU in 2 μl template, while no pathogen contaminants observed in other breedings, which indicated the low prevalence of the pathogens in the random selected six seedlots. The study enriched the knowledge of P. protengens sp. and highlighted the precautions of the newly emerging infectious disease and the importance of evolutionary study.