Postharvest 'Bai' Pear Rot Caused by <i>Exiguobacterium acetylicum</i> in Hebei, China

'Bai' pear (Pyrus bretschneideri Rehd.) is a major cultivated pear in Hebei Province, China. Postharvest diseases cause severe economic losses. In October–November 2024, postharvest rot occurred on stored ‘Bai’ pear in Baoding, Hebei, with 18.5% incidence across 3 warehouses(860 fruits total). Symptoms included water-soaked lesions, skin cracking, and dark brown soft rot. Lesions were excised from diseased margins, surface-sterilized in 75% ethanol (20 s) and 0.1% mercuric chloride (30 s), rinsed three times with sterile water, then incubated on LB agar at 28°C for 48 h. Three purified isolates (Sph 1–3) showed identical morphology. All were Gram‑positive, short, nonmotile bacilli. Colonies on LB agar (28°C, 24 h) were round, milky white, smooth, convex, and moist. The 16S rRNA and gyrB genes were sequenced for all three isolates (Sph 1–3), which showed 100% sequence identity. Isolate Sph 1 was thus used as the representative strain. Pathogenicity was tested on surface‑sterilized ‘Bai’ pear by wounded and non‑wounded inoculation; sterile water served as control. Wounded fruits were punctured (1 mm × 1 mm) and inoculated with 20 μL suspension (10⁸ CFU/mL); non‑wounded fruits were inoculated directly. Fruits were incubated at 28°C, 90–95% RH, 12 h light/dark. Each treatment had three replicates (three fruits per replicate), and the experiment was repeated three times. Typical soft rot appeared at 48 hpi in wounded fruits and 72 hpi in non‑wounded fruits, while controls remained symptomless. Re‑isolated strains were identical to Sph 1 by morphology, Gram stain, and 16S rRNA/gyrB sequencing, satisfying Koch’s postulates. Genomic DNA was extracted using a commercial kit (Biomed BMamp Rapid Bacteria DNA Kit, Beijing, China). The 16S rRNA and gyrB genes were amplified with primers 27F/1492R (Lane 1991) and gyrB-F/gyrB-R (Brady et al. 2013), respectively. PCR products were purified and sequenced. The 16S rRNA (884 bp; PZ212927) shared &gt;99% identity and 100% coverage with Exiguobacterium acetylicum (OQ619145.1), and the gyrB sequence (945 bp; PV476105.1) shared &gt;99% identity and 100% coverage with E. acetylicum (DQ019163.1). Maximum-likelihood phylogenetic analysis (MEGA11.0, 1000 bootstraps, rooted with Exiguobacterium chiriqhucha) confirmed Sph 1 clustered with E. acetylicum. To our knowledge, this is the first report of E. acetylicum causing postharvest ‘Bai’ pear rot in Hebei, China. This bacterium is traditionally a biocontrol agent (Zhang et al. 2013). Our findings provide new insights into E. acetylicum, demonstrating that this biocontrol bacterium can act as a weak pathogen during the postharvest storage of ‘Bai’ pear.