First Report of Bacterial Soft Rot of Potato Caused by Pectobacterium carotovorum subsp. brasiliense in Guangdong Province, China

Abstract

China is one of the world’s largest potato (Solanum tuberosum L.) producing countries, with a production of 5.68 million ha. Among them, winter-grown potato has been developed rapidly in Guangdong Province in recent years, with an area of ​​nearly 66,000 ha and an output value of about 4 billion yuan. In February 2019, soft rot symptoms were observed occurring on potato (cv. Xisen) in a commercial field (15 ha) in Kaipin, Guangdong, China, with a disease incidence of about 10%. Early symptoms included darkened and water-soaked necrotic lesions at the exterior of the infected tubers. The lesions spread quickly, and the inside of the tubers was slimy and emitted a foul smell. Bacterial exudate from water-soaked areas was also observed. Twenty diseased samples were collected, and the margins between the diseased and healthy areas were cut into pieces (5 × 5 mm), which were surface sterilized using 75% ethanol for 30 s and 2% NaOCl for 1 min, followed by three rinses with sterile water. The sterilized sections were macerated in drops of sterile water, and the extract was streaked onto nutrient agar medium and incubated for 24 h at 30°C. Single colonies were chosen after three times of subculturing (Zhang et al. 2014). Fifteen bacterial isolates were obtained, and a representative isolate KP1 was chosen for further analyses. Its 16S rDNA gene sequence (GenBank accession MK674243) amplified by primer pair 27F/1492R showed 99% identity with Pectobacterium carotovorum subsp. brasiliense (Pcb) strains (nos. CP024780 and CP020350) using BLASTn. Phylogenetic analysis (neighbor-joining method and 1,000 bootstrap values) with strains from different Pectobacterium species showed KP1 was grouped into a clade with Pcb strains, based on the multilocus sequences of gapA (GenBank accession no. MK674244), icdA (MK674245), mdh (MK674246), mtlD (MK674247), proA (MK674248), and pel genes (MK674249) that were amplified using the primer pairs provided in previous studies (Darrasse et al. 1994; Ma et al. 2007). Pathogenicity tests were conducted by injecting 100 µl of bacterial suspensions (10⁸ CFU/ml) into the potato tubers (cv. Xisen). For noninoculated controls, another 10 healthy potato tubers were injected with 100 µl of sterile water. Tubers were placed in an incubator at 30°C in darkness at 95% humidity. After 3 days, inoculated tubers showed diseased symptoms similar to those observed in the field. No symptom was observed on the controls. We reisolated the pathogen from the symptomatic tissues and confirmed it to be Pcb using PCR with 16S rDNA primers. This pathogen was previously reported to infect potato in Inner Mongolia Autonomous Region, Northern China (Zhao et al. 2018). To our knowledge, this is the first report of Pcb causing bacterial soft rot on potato in Guangdong Province, Southern China. This study expanded the geographic location of Pcb as the causal agent on potato in China and helped raise attention on controlling the spread of this pathogen to minimize the losses.