Irrigation method matters: Contamination and die-off rates of Escherichia coli on dry bulb onions after overhead and drip irrigation in Washington State (2022-2023).

Two U.S. outbreaks of salmonellosis in 2020 and 2021 were epidemiologically linked to red onions. The 2020 outbreak investigation implicated production agricultural water as a likely contamination source. Field trials were designed to investigate prevalence and survival of Escherichia coli (surrogate for Salmonella) on dry bulb onions after application of contaminated irrigation water at the end of the growing period. Irrigation water was inoculated at 3 log most probable number (MPN)/100 mL (2022 and 2023) or 5 log MPN/100 mL (2023, drip only) with a cocktail of rifampin-resistant E. coli and applied with the final irrigation (0.4 acre-inch/0.4 hectare-cm) to onions. Onion bulbs (40 or 80) were sampled immediately after irrigation and throughout field curing (4 weeks) and E. coli was enumerated using a MPN method. For drip irrigation, at 3 log MPN/100 mL E. coli was detected on 13% of onions at 24 h but not detected at 0 h; at 5 log MPN/100 mL for drip irrigation applied to saturated soil, E. coli was detected in 63% of onions at 0 h. Prevalence significantly (P<0.05), decreased after 7 d of curing with cell densities of 1-1,400 MPN/onion. At the end of field curing in 2023, 1/80 onions had detectable E. coli (2.04 MPN/onion). E. coli was detected in a significantly smaller percentage of onions (2022: 13%; 2023: 68%) after a contaminated drip irrigation event compared to overhead irrigation (98-100%; P<0.05). After overhead irrigation E. coli was detected in onions (1-1,000 MPN/onion) on day 0. Prevalence decreased significantly (P <0.05) after 7 d of field curing in both years (2022: 15%; 2023: 7%). E. coli was not detected on Calibra onions (80/year) at the end of field curing in either year but was detected at <12 MPN/onion in 2.5-3.75% of onions (n=80) for other cultivars. These data confirm limited contamination risk associated with drip irrigation water quality and begin to quantify contamination risks associated with overhead irrigation of dry bulb onions.

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Declaration of Competing Interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Joy Waite-Cusic reports financial support was provided by the Center for Produce Safety, U.S. Department of Agriculture’s (USDA) Agricultural Marketing Service, and Washington State Department of Agriculture’s Specialty Crop Block Grant Program. Other authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.