Unexpected diversity of Pectobacterium species responsible for causing bacterial soft rot on Chinese flowering cabbage in Guangdong province, China.

Chinese flowering cabbage (Brassica parachinensis), commonly known as Choi-sum, is an important bolting stem vegetable and mainly cultivated in the Guangdong province of China. Bacterial soft rot of Choi-sum is a major disease that is capable of causing widespread outbreaks under conducive conditions. Historically, the disease was attributed to the Pectobacterium genus, but the specific species were not identified clearly. To investigate the species and diversity of the species associated with soft rot of Choi-sum, 72 bacterial isolates were collected from symptomatic plants in three cities (seven counties or districts) of Guangdong province between 2021 to 2022. Field incidence ranged from 1.5% to 45%. These isolates were identified via physiological and biochemical characteristics, molecular analyses including 16S rRNA sequencing, and multilocus sequence analysis (MLSA) and confirmed via Koch's postulates. The isolates were identified as Pectobacterium brasiliense (63), P. carotovorum (4), P. aquaticum (2), P. quasiaquaticum (1), P. colocasium (1) and P. aroidearum (1). Host-range tests showed that all strains except for P. aquaticum GDPAq2, which was not pathogenic to wax gourd, were pathogenic to 'Zengcheng Choi-sum', 'Sijiu Choi-sum', potato plants, potato tubers, tomato, and wax gourd, displaying typical symptoms of bacterial soft rot. These results revealed an unexpected diversity of Pectobacterium species associated with soft rot of Choi-sum in China. To our knowledge, this is the first report providing detailed identification and diversity of the soft rot pathogens on Choi-sum which may affect future management strategies.