First Report of Soft Rot Caused by Pseudomonas fulva on Postharvest Eggplant ( Solanum melongena ) in China

Eggplant (Solanum melongena L.) is a widely cultivated and consumed vegetable in China (Chen et al. 2026). In July 2025, soft rot symptoms were observed in farmland in Chongqing, China (29°40′N, 105°56′E). Infected eggplants initially developed yellow-brown lesions that progressively deepened and enlarged; the skin softened and collapsed, causing complete internal rot with pale-yellow exudate. Approximately 50% of postharvest eggplants exhibited these symptoms. To isolate the pathogenic bacterium, infected eggplant tissues were surface‑sterilized with 75% ethanol and a 5% sodium hypochlorite solution, followed by rinsing with sterile distilled water (SDW). The treated tissues were then ground in 1.5 mL centrifuge tubes using disposable sterile micro pestles, resuspended in SDW, and serially diluted (Wu et al. 2022). The diluted suspensions were spread onto LB agar plates and incubated at 37°C for 24 h, yielding six morphologically similar isolates designated DY-1 to DY-6. Colonies were circular, milky‑white, mucoid, with smooth and regular edges. Cells were Gram-negative rods, 1.5–2.5 μm in length and 0.5–0.8 μm in width, as determined from 50 cells in three microscopic fields. The isolates were catalase- and urease-positive, consistent with Pseudomonas spp. (Yang et al. 2019). Genomic DNA was extracted using the Bacterial Genomic DNA Extraction Kit (Solarbio, China) according to the manufacturer's protocol. The 16S rRNA gene (Winsley et al. 2012) and portions of the housekeeping genes gyrB and rpoB (Meng et al. 2024) were amplified and sequenced. DY-1 sequences were deposited in GenBank as PZ298202, PZ372999, and PZ373000, respectively. DY-2–DY-6 sequences were assigned PZ574201–PZ574205 (16S rRNA), PZ580988–PZ580992 (gyrB), and PZ580993–PZ580997 (rpoB). BLASTn analysis indicated that DY-1 sequences shared 99.37%, 94.22%, and 100.00% sequence identity with P. fulva (MF462953, CP161341, CP161342, respectively). Phylogenetic analyses of the concatenated three-gene dataset, utilizing both Maximum Likelihood and Bayesian inference methods, strongly supported the placement of isolates DY-1 through DY-6 within the P. fulva clade. Based on the combined morphological, biochemical, multilocus sequence, and phylogenetic analyses, these six isolates were identified as P. fulva. To confirm pathogenicity, DY-1 was selected as the representative isolate, and healthy eggplants were surface-disinfected. A sterile needle was used to create a wound approximately 0.45 mm in diameter and 2 mm deep on each fruit. A 10-μL aliquot of a suspension of strain DY-1 at 1.0 × 10⁸ CFU/mL was inoculated into each wound. Control fruits were wounded identically and inoculated with SDW. Five fruits were used for each treatment, and the experiment was independently repeated three times. Fruits were placed in sterilized plastic containers and incubated at 24°C and 90% relative humidity under a 10-h photoperiod for 5 days. Symptoms similar to those observed in the original fruit samples manifested on all inoculated fruits, whereas control fruits remained symptomless. The bacterium was reisolated from symptomatic tissues, and the recovered isolates were consistent with strain DY-1 in colony morphology and gyrB and rpoB sequence analyses. To our knowledge, there are few reports of P. fulva causing plant diseases. This study is the first to report P. fulva causing postharvest soft rot in eggplants in China.