Occurrence, characteristics and PCR-based detection of Pectobacterium polaris causing soft rot of Chinese cabbage in China.

Bacterial soft rot is an important disease of Chinese cabbage (Brassica rapa L. ssp. pekinensis) in China and many other countries. Four pectinolytic bacterial strains (WBC1, WBC6, WBC9 and WBC11) were isolated from soft-rotted Chinese cabbage in Beijing, China. Based on 16S rDNA and pmrA gene sequence analyses, multi-locus sequence analysis (MLSA), and genomic average nucleotide identity (ANI) analysis, these four strains were identified as Pectobacterium polaris. This species, previously reported from potato in countries not including China, is a new soft-rot pathogen of Chinese cabbage in China. Biochemical characteristics of these P. polaris strains tested by Biolog were mostly consistent with those of P. polaris NIBIO1006T. Their pathogenicity on Chinese cabbage is temperature-dependent, with all four strains as well as the type strain exhibiting high pathogenicity at 23°C and 28°C. These four strains infected Lactuca sativa, Daucus carota, Solanum tuberosum and Capsicum annuum by artificial inoculation. Specific PCR/qPCR primers for P. polaris were developed based on its specific gene sequences (determined using genome comparison methods). Both PCR and qPCR detected not only genomic DNA of P. polaris but also the pathogen from diseased plant tissues even before external symptoms appeared. Their detection sensitivities were as low as 1 pg and 100 pg genomic DNA of P. polaris, respectively. This study is the first to both report the emergence of P. polaris on Chinese cabbage in China, and provide rapid and accurate PCR/qPCR-based detection systems specific for P. polaris for the first time.