In the summer of 2020, Canada and the US experienced a large outbreak of Salmonella Newport that was epidemiologically linked to onions grown in California. An extensive outbreak investigation failed to yield conclusive evidence to the source or circumstances that led to widespread contamination; however, it is likely that water played a role as either source or dissemination vector. Water is not used in post-harvest onion handling; however, water is commonly applied to the crop via drip irrigation and foliar spray. Dry bulb onion growing regions in the western US have a limited number of water sources, including surface waters of poor microbiological quality. The objective of these studies was to provide field evidence of the risks associated with late season application of contaminated water via drip irrigation and foliar application. In the Oregon study, Vaquero (yellow) onions were irrigated via drip using water inoculated with 0, 1, 2, or 3 log CFU/ml of a cocktail of rifampicin-resistant Escherichia coli (TVS 353, TVS 354, TVS 355). In the California study, Ovation (yellow) onions were sprayed with 25 ml of water containing the E. coli cocktail (2-3 log CFU/ml). Onions were finished in the field following regional practices and were sampled over the 28-day curing period. Onions (n = 10-300/treatment) were analyzed for surviving E. coli using a combination of standard plate count, filtration, and enrichment methods. A minor percentage (13.3%; 20/150) of onions were contaminated due to drip irrigation with the most contaminated water. The percentage of contaminated onions decreased over time with no contamination detected on any onions from any treatment after lifting (7+ days). Foliar spray resulted in the consistent contamination of 2-3 log CFU/onion; however, die-off occurred rapidly over the first 4 hours. As field curing progressed, the detection of E. coli in onions decreased; however, in a small percentage of onions, E. coli grew and reached levels as high as 6-7 log CFU/onion on day 4 and remained detectable at these levels after 21 days of field curing. These studies demonstrated that the use of contaminated water via drip irrigation poses very little risk to dry bulb onions; however, the use of contaminated water for foliar sprays leads to significant contamination and can lead to E. coli growth in a significant percentage of the crop.

Linda Harris

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