Unexpected Diversity of <i>Pectobacterium</i> Species Responsible for Causing Bacterial Soft Rot on Chinese Flowering Cabbage in Guangdong Province, China

Chinese flowering cabbage (<i>Brassica parachinensis</i>), commonly known as Choi-sum, is an important bolting stem vegetable 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 <i>Pectobacterium</i> 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 and 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 and confirmed via Koch's postulates. The isolates were identified as <i>Pectobacterium brasiliense</i> (63), <i>P</i>. <i>carotovorum</i> (4), <i>P</i>. <i>aquaticum</i> (2), <i>P</i>. <i>quasiaquaticum</i> (1), <i>P</i>. <i>colocasium</i> (1), and <i>P</i>. <i>aroidearum</i> (1). Host-range tests showed that all strains except for <i>P</i>. <i>aquaticum</i> 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 <i>Pectobacterium</i> 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.