Assessing the potential for production practices to impact dry bulb onion safety

In 2020, the dry bulb onion industry faced their first significant outbreak of foodborne illness when red onions grown in California were epidemiologically linked to >1,000 cases of salmonellosis. Since then, industry and food safety experts have been scratching their heads to figure out how this could have happened. After review of outbreak data and consultation with various stakeholders, we have identified a small number of practices with the potential to contribute to a large-scale contamination event, through water or agricultural input contamination. We have designed field trials to determine the risks of using contaminated water source or other agricultural input when 1) applying crop protection sprays (pesticides and/or clay) and 2) during irrigation (overhead vs. drip). Our primary goal is to collect evidence demonstrating risks of these practices and to communicate our findings to relevant stakeholders to reduce the potential for outbreak like this from recurring in the future. We will share our findings via a broad outreach strategy that communicates with industry throughout the two-year study. Outreach activities culminate with the development and delivery of a workshop and best practices guide that enables growers to better understand risks and implement changes to minimize the likelihood of crop contamination.

The 2020 Salmonella Newport outbreak linked to onions demonstrated the potential for a significant contamination event of foodborne pathogens during dry bulb onion production activities. However, the intensive investigation did not reveal the cause or mitigating factors that led to the contamination event. Previous research by our group has demonstrated that drip irrigation with poor quality water does not constitute a significant contamination risk; however, we have demonstrated that foliar application may pose a significant contamination risk that might lead to growth of pathogens in a portion of the onions. The proposed research will focus on the risk of dry bulb onion contamination when poor quality water or contaminated input is used for crop protection sprays (pesticides or clay) or for overhead irrigation applied at the end of the growing season. We propose to conduct a total of four field trials (two in Oregon and two in Washington) to characterize the risks associated with these practices. A well-characterized surrogate cocktail of rifampicin-resistant Escherichia coli strains will serve as a surrogate for Salmonella behavior in the field setting. Onions will be sampled throughout a 30-day curing period and E. coli will be enumerated using a combination of standard plating, filtration, and enrichment techniques for improved range of quantification and detection. Results from these studies will provide the onion industry with clear evidence of the risk of contamination due to the foliar application of crop protection sprays and overhead irrigation late in the season. We will maximize the opportunities to communicate about our research and findings to stakeholders throughout the project using long standing interactions between PI, co-PIs, and stakeholders. Near the end of the grant, we will create and deliver educational materials (workshop and best practices guide) to actively engage growers and support changing practices that improve food safety. Effective communication of our research findings will guide production practices that will minimize the likelihood of future outbreaks associated with dry bulb onions.

1. Quantify Escherichia coli contamination rates, die-off, and potential growth in onions due to application of crop protection sprays, including clay application to prevent sunburn. 

2. Quantify E. coli contamination rates, die-off, and potential growth in onions due to overhead irrigation with contaminated water.

Four field trials investigating three unique direct water application methods (overhead irrigation, drip irrigation, and crop protection sprays) provided further evidence of risk reduction during standard field curing practices used by onion growers in the western US. However, risk reduction does not equal risk elimination, as results also demonstrated that a small percentage of onions (<3%) could host a small number of viable E. coli (and by inference Salmonella) at the end of field curing. It is unknown whether this finding represents a reasonable risk, but this data does provide key information to support evidence-based decisionmaking for the industry. Data from these field trials clearly demonstrated differences in contamination due to different types of water applications at moderately high contamination levels (3 log CFU/100 mL). Overhead irrigation with contaminated water led to the immediate contamination of nearly the entire crop (>97%). Crop protection spray applications resulted in immediate contamination of a majority of the crop (72% on average). Drip irrigation resulted in the lowest percentage of contaminated onions (13%). E. coli was seemingly more stable on the onions during the 2023 field trials compared with 2022. The weather in these two growing regions tends to be consistently very dry; however, both locations experienced rare and significant precipitation at the end of the growing season and during field curing. The impact of these precipitation events cannot be confirmed as the reason for E. coli’s increased stability on the onions; however, this data prompts a consideration of these types of weather events on microbial survival.