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Serratia is a genus of Gram-negative bacteria classified under the phylum Pseudomonadota, order Enterobacterales, and family Yersiniaceae. As of August 2025, the genus comprises 24 validly published species with correct names, as listed in the List of Prokaryotic names with Standing in Nomenclature and recognized under the rules of the International Code of Nomenclature of Prokaryotes (Parte et al. 2020). Among them, S. marcescens is the most frequently reported plant-pathogenic species, known to infect a wide range of crops, including cucurbits, pepper, and corn (Besler and Little 2017). In onion (Allium cepa L.), S. plymuthica remains the only S. species previously reported to cause disease (Kowalska et al. 2011). While S sarumanii is a newly described species within the genus, it was originally isolated from human clinical samples, including wound and urine specimens (Klages et al. 2024). In 2021, an isolated diseased onion bulb was collected from Las Cruces, New Mexico, USA, to investigate the bacterial communities associated with symptomatic tissues. The outer bulb scales exhibited clear signs of rot. Bacteria were isolated from the margins of symptomatic bulb lesions and screened for pathogenicity using red onion scale and bulb assays, following the procedures and controls described previously (Khanal et al. 2023). All isolates were identified by 16S rRNA gene sequencing. One pathogenic strain, isolated from a bulb exhibiting outer scale rot, was identified as a S. species (GenBank accession no. OP325569). For the scale assay, a 10 µL suspension containing 108 CFU/ml of the bacterium was applied to the center of 3 cm-wide sections of red onion scales that had been punctured with a sterile needle. After incubation at 25 °C for 4 days, necrotic lesions (3–5 mm in diameter) with a creamy light-yellow appearance developed at the inoculation sites, whereas PBS-treated controls remained symptomless. BLAST analysis of the 928-bp 16S rRNA gene sequence revealed 99.78% and 99.68% similarity to the type strains of S. nematodiphila and S. marcescens, respectively. Due to its high relatedness to multiple S. species, whole-genome sequencing was performed for this strain (Genbank assembly GCA_028307185.1). Genome-based taxonomic analysis using TYGS (https://tygs.dsmz.de/) (Meier-Kolthoff and Göker 2019) and JSpeciesWS (https://jspecies.ribohost.com/jspeciesws/) (Richter et al. 2016) identified the strain as S. sarumanii. The strain showed a digital DNA–DNA hybridization (dDDH) value of 90.2% and average nucleotide identity based on BLAST (ANIb) of 98.4% with the type strain K-M0706T (Genbank assembly GCA_029962605.1), exceeding the species delineation threshold (Chun et al. 2018) of 70% (dDDH) and 95% (ANI). To further confirm pathogenicity, a 500 µL suspension containing 108 CFU/mL of the bacterium was inoculated into the upper shoulder of three yellow onion bulbs using a needle-syringe. The bulbs were then incubated at 25 °C for 12 days. After incubation, the onion bulbs were sectioned and exhibited clear signs of rot, including disintegration of scale tissue and brown discoloration. The bulb inoculation assay was repeated twice, consistently producing similar symptoms across all tested bulbs. Five daughter isolates recovered from lesions on inoculated onion scales and bulbs were morphologically uniform and reproduced identical symptoms when re-inoculated onto healthy onion bulbs. Furthermore, 16S rRNA gene sequencing of these daughter isolates (GenBank accessions PX021965–PX021969) revealed complete identity with the original (mother) strain, thereby fulfilling Koch’s postulates. This report constitutes the first documented case of S. sarumanii causing bulb decay and rot in onion, establishing it as a novel plant pathogen. This finding underscores the potential risk of S. sarumanii to onion production and postharvest storage in New Mexico and other onion-growing regions, warranting further monitoring and management considerations.