First Report of Bacterial Soft Rot of Lettuce Caused by <i>Pectobacterium brasiliense</i> in Guangdong, China

Lettuce (Lactuca sativa L.), one of the world's most important leafy vegetables, is widely cultivated globally primarily for fresh consumption. Lettuce faces threats from numerous diseases during cultivation, harvest, and distribution. Among these, bacterial soft rot is a widespread and highly destructive disease. In November 2024, bacterial soft rot was observed in lettuce fields (23.31°N, 113.59°E) in Huangpu District, Guangzhou City, China, with a field incidence reaching 5%. The disease initiates at the crown region. Initial symptoms appeared as translucent lesions inside the crown, which expanded into irregular and water-soaked areas. The diseased areas subsequently became soft rot and emitted a foul odor. The disease rapidly progressed upward from the base, causing head rot and, in severe cases, leading to wilting and death of the entire plant. Four diseased plants were collected from affected fields for pathogen isolation. Pieces (approximately 5×5 mm) were excised from the junction of diseased and healthy tissues. Surface sterilization was performed using 75% ethanol for 30 s, followed by treatment with 2% NaClO for 1 min, and rinsed three times with sterile water. The tissues were then plated on Nutrient Agar (NA) and incubated at 30°C for 48 h, the bacterial colonies were tiny and streaked on NA plate for purification. Bacteria eliciting a hypersensitive response in tobacco were selected, and single colonies were purified through three successive generations. A representative strain, designated HP-11, was selected. PCR amplification and sequencing were performed using primers targeting the dnaX and leuS genes (Portier et al., 2019). The gene sequences (GenBank accession numbers PV991059 and PV991060) exhibited 96.59% and 98.68% similarity with 100% query coverage to the corresponding sequences of the Pectobacterium brasiliense type strain 1692 (CP047495.1). Phylogenetic analysis based on the concatenated sequences of these two genes placed HP-11 within a clade shared with representative Pectobacterium brasiliense strain (Supplementary Figure A). Pathogenicity tests were conducted on 21 disease-free lettuce seedlings. Root wounds were inflicted using a sterile blade, followed by inoculation with 10 mL of an HP-11 bacterial suspension (108CFU/mL) per plant. Another 21 plants were treated with sterile NA medium and served as control. Severe soft rot symptoms developed in 15 out of 21 inoculated seedlings 7 d post-inoculation, while control plants remained asymptomatic (Supplementary Figure B). Bacteria re-isolated from symptomatic root tissues yielded single colonies. Subsequent PCR amplification and sequencing of the dnaX gene identified the re-isolated bacterium as P. brasiliense (Supplementary Figure C), fulfilling Koch's postulates. Bacterial soft rot pathogens, primarily species within the genera Pectobacterium and Dickeya (Charkowski, 2018), are the main causal agents of lettuce soft rot. For instance, Pectobacterium carotovorum causes bacterial soft rot on lettuce in Malaysia (Nazerian et al., 2011). In 2022, Pectobacterium polaris was found infecting lettuce and causing severe bacterial soft rot in Yunnan Province, China (Lin et al., 2025). However, to our knowledge, this is the first report of bacterial soft rot of lettuce caused by Pectobacterium brasiliense in Guangdong Province, China.