Brassica juncea var. gemmifera (Er Cai) is a cold-tolerant mustard vegetable widely cultivated in southwestern China for its edible swollen lateral buds. In March 2025, stem rot symptoms were observed in a Er Cai field in Qujing, Yunnan Province, China (25.29°N, 103.93°E), with an estimated disease incidence of approximately 10%. Symptomatic plants exhibited brown, water-soaked lesions at the base of outer leaves and hollow stem rot. To identify the causal agent, four symptomatic stem samples were collected. Tissue pieces (0.5 cm) excised from the margin between healthy and diseased tissues were surface-rinsed with sterile water, macerated in 200 µL sterile water, serially diluted to 10⁻⁶, plated onto LB agar, and incubated at 30°C in darkness for 24 h (Chu et al. 2024). Approximately 10 bacterial colonies were purified by repeated streaking three times on LB agar. Pathogenicity screening using a pinprick inoculation method, in which approximately 20 μL of bacterial suspension (10⁸ CFU/mL) was applied onto the wound site, identified eight pathogenic isolates capable of inducing soft rot symptoms on detached Er Cai stems (Singh et al. 2013). Three representative isolates, designated ECRF1, ECRF2, and ECRF3, were selected for further characterization. Colonies on LB agar were white, circular, convex, and tough in texture. For molecular identification, the 16S rRNA gene and three housekeeping genes, glyceraldehyde-3-phosphate dehydrogenase (gapA), isocitrate dehydrogenase (icdA), and gamma-glutamylphosphate reductase (proA), were respectively amplified using primer pairs 27F/1492R (Weisburg et al. 1991), gapA326F/gapA845R, icdA400F/icdA977R, and proAF1/proAR1 (Meng et al. 2017), respectively. Sequences were deposited in GenBank under accession numbers PZ374378–PZ374380 (16S), PZ381109–PZ381111 (gapA), PZ381112–PZ381114 (icdA), and PZ381115–PZ381117 (proA). BLASTn analysis showed 99.79% identity for 16S rRNA, 99.16% for gapA, 98.71% for icdA, and 97.21% for proA with Pectobacterium brasiliense strain ZRIMU1701SM, all with 100% query coverage. A maximum-likelihood phylogenetic tree based on concatenated 16S-gapA-icdA-proA sequences clustered the three isolates with reference strains of P. brasiliense with 98% bootstrap support. Pathogenicity tests were performed on 18 healthy Er Cai plants, including three plants inoculated with each of the representative isolates ECRF1, ECRF2, and ECRF3 and nine plants inoculated with sterile water as controls, using pinprick inoculation of petioles with bacterial suspensions (10⁸ CFU/mL). Inoculated plants were maintained under high humidity by covering with plastic bags for 12 h. Typical stem rot symptoms developed within 1–2 days after inoculation, whereas nine control plants inoculated with sterile water remained asymptomatic. Bacteria reisolated from symptomatic tissues exhibited colony morphology identical to the original isolates and were confirmed as P. brasiliense by sequencing of gapA, icdA, and proA, thereby fulfilling Koch’s postulates. The experiment was repeated twice with similar results. Pectobacterium brasiliense has been reported causing soft rot diseases on numerous economically important crops worldwide (Oulghazi et al. 2021). In China, P. brasiliense was recently reported causing soft rot on leaf mustard (Brassica juncea var. multiceps) and was also identified among the Pectobacterium species associated with bacterial soft rot of Chinese flowering cabbage (Brassica rapa var. parachinensis) (Li et al. 2025; Yang et al. 2025). To our knowledge, this is the first report of P. brasiliense causing stem rot on B. juncea var. gemmifera in China. This finding expands the known host range of P. brasiliense and provides a foundation for future epidemiological studies and disease management strategies.
Li et al. (Sun,) studied this question.