Graduate Thesis Or Dissertation
 

The Examination of Escherichia coli Contamination via Agricultural Water Applications and Survival During Pre-Harvest Field Curing of Dry Bulb Onions

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/qf85nm05h

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  • 2020 and 2021 brought the first reported outbreaks of salmonellosis epidemiologically linked to dry bulb onions in the United States. Outbreak investigations were not able to implicate a specific source of the contamination but speculated that contaminated water was a significant contributor. The Produce Safety Rule established minimum standards for the safe handling, growing, and harvesting of produce for human consumption, including standards for microbial water quality. This regulation requires growers to assess their water sources to manage food safety. One method to mitigate contamination risks when using agricultural water of poor microbial quality for pre-harvest use (e.g., irrigation or crop protection sprays) is to allow for a time-to-harvest window. Dry bulb onion production can leverage this mitigation strategy by following their standard field curing practices. Onions are often field cured for 3-4 weeks between the last irrigation and when they are harvested. This research project was designed to quantify the risks of using contaminated water for different irrigation methods as well as crop protection sprays on onions and how these contaminants would behave during field curing. The first objective of this project sought to quantify contamination risk associated with the use of drip or overhead irrigation methods. This field trial took place in the Columbia Basin (Eastern Washington). Onions (yellow ‘Calibra’ cultivar) were inoculated at the final irrigation event of the season when bulbs had reached maturity. In the first year (2022), a rifampicin-resistant E. coli cocktail was prepared and used to inoculate drip or overhead irrigation water on the final day of irrigation (0.40 acre-inch). Water samples collected from field plots determined the initial contamination to be ~3 log CFU/100 ml water. E. coli was recovered from 95% (38/40) overhead irrigated onions at an average cell density of ~9 MPN/onion. No E. coli was recovered from drip irrigated onions immediately after the completion of irrigation. Twenty-four hours later, 13% (5/40) drip irritated onions were contaminated at a cell density of ~7 MPN/onion, but beyond 2 days of field curing no E. coli was recovered from drip irrigated onions. Overhead irrigated onions demonstrated prolonged survival of E. coli through 15 days of field curing when 3% (1/40) onions confirmed contamination of 88 MPN. No E. coli was recovered from drip or overhead irrigated onions on day 28 at the end of field curing (0/40). Based on 2022 results, the experimental design was adjusted the following season. Drip irrigated plots were inoculated at a much higher concentration of 5 log MPN/100 ml to evaluate the potential risk of a single extreme contamination event. Overhead irrigation was expanded to test four onion cultivars (yellow ‘Calibra’, yellow ‘Ovation’, white ‘Cometa’, and red ‘Red Wing’) using the same level of water contamination as the previous year. Prior to inoculation in 2023, two days of precipitation resulted in saturated soils before contaminated irrigation began. Initial contamination levels on onions were much higher for drip irrigation. Sixty-three percent (25/40) of onions were contaminated at an average level of ~256 MPN/onion. Overhead irrigated plots saw a similar contamination as the previous year. Nearly all onions (98%; 157/160) were contaminated at an average level of ~20 MPN/onion. Prevalence of E. coli diminished until the end of curing when E. coli was recovered from 1% (1/80) of drip-irrigated onions and 2% (7/320) of overhead-irrigated onions (range: 2-24 MPN/onion). There was no meaningful difference in E. coli contamination nor survival in the different cultivars. The second objective was to quantify E. coli contamination resulting from pesticide or clay applications prepared with contaminated water. These field trials (2022 and 2023) occurred in the Treasure Valley (Eastern Oregon). Inoculated pesticide (Pristine®) and clay (Kaolin Surround®) solutions were delivered using a backpack sprayer. The 2022 inoculum delivered ~3-4 log MPN/100 ml water mixed with pesticide or clay. The initial prevalence of E. coli recovered from onions (red ‘Red Wing’, and white ‘Cometa’) post-inoculation was 80% (64/80) for pesticide and 56% (45/80) for clay inoculations, respectively. E. coli was not recovered from onions between 2 and 16 days of field curing, but on day 28, 1 onion confirmed the survival of E. coli from pesticide spray application (1/160 onions). The 2023 trial was designed to be identical to the 2022 trial. Pesticide and clay solutions were inoculated at a similar level of ~3-4 log MPN/100 ml. The prevalence of E. coli immediately after spray applications was consistent between years, but E. coli cell density was higher, ranging from 1-275 MPN/onion and 1-196 MPN/onion, respectively, compared to 1-48 and 1-46 MPN/onion in 2022. A heavy precipitation event occurred on day 7 of curing. This sample point also saw a prolonged prevalence of E. coli survival with 18% (14/80) of pesticide-sprayed onions testing positive with levels as high as 110 MPN/bulb. At the end of field curing, 3/320 onions were confirmed positive for E. coli. In both field trials, onions were sampled after 4 months of post-harvest storage. E. coli was recovered from 15% (3/20) of onions inoculated via pesticide spray during the 2023 study. These onions had an external contamination level of 2-83 MPN/onion. Contamination risks due to irrigation timing and potential internalization of bacteria during bulb development of onions was examined in a greenhouse setting. Onions were sprayed at several timepoints during the growing season beginning at bulb set. Initial leaf or bulb contamination levels were <100 MPN/sample. At the end of growing, 6% (5/90) of onions from previous inoculations (4-10 weeks) remained contaminated. Sampling at the end of simulated field curing (3 weeks later), resulted in 5/103 onions testing positive for E. coli. One of the positive onions had been inoculated by an earlier season spray and four were contaminated at the final irrigation event. Three of these onions had elevated levels of E. coli within the bulb, including one at 8 log CFU/onion. This onion was visibly infected, likely Botrytis. The other two onions with elevated E. coli counts showed signs of scale deformation, potentially due to physiological stresses during bulb formation. These onions were the only onions demonstrating a potential for E. coli growth in the greenhouse study. Overall, this research confirmed that field curing does mitigate risks associated with the use of poor-quality water that could be used in various production applications. This research also demonstrated that E. coli, and by inference Salmonella, can survive at low levels on a small percentage of dry bulb onions after 4 weeks of field curing. Growth of E. coli in onions was observed rarely and only in bulbs with internal infections. This is a unique observation and warrants further research to confirm the potential for onions to be co-infected with plant and human pathogens. This research has demonstrated potential risk factors such as rare late season precipitation events. Growers and government officials may use this research to support evidence-based decision-making for food safety management.
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  • Funding for field trials was made possible, in part, by The Center for Produce Safety and the U.S. Department of Agriculture’s (USDA) Agricultural Marketing Service through grant 21SCBPCA1002. This project was also funded in part by Washington State Department of Agriculture’s Specialty Crop Block Grant Program. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of The Center for Produce Safety or the USDA. Post-harvest onion storage and analysis was supported by a grant from the Washington State Specialty Crop Block Grant Program.
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