Abstract
Two U.S. outbreaks of salmonellosis in 2020 and 2021 were epidemiologically linked to red onions. The 2020 outbreak investigation implicated the production of agricultural water as a likely contamination source. Field trials were designed to investigate the prevalence and survival of Escherichia coli (surrogate for Salmonella) on dry bulb onions after the 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 ha-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 an 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 of 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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Claims 11
Claim · model-written summary
After 3-log contaminated drip irrigation, E. coli was detected on 13% of onions at 24 h and absent at 0 h.
Verbatim
e0“at 3 log MPN/100 mL E. coli was detected on 13% of onions at 24 h but not detected at 0 h”
t0“Dry Bulb Onions”
Claim · model-written summary
After 5-log contaminated drip irrigation on saturated soil, E. coli was detected on 63% of onions at 0 h.
Verbatim
e0“at 5 log MPN/100 mL for drip irrigation applied to saturated soil, E. coli was detected in 63% of onions at 0 h”
t0“Dry Bulb Onions”
Claim · model-written summary
E. coli prevalence decreased significantly after seven days of curing.
Verbatim
e0“Prevalence significantly (P < 0.05), decreased after 7 d of curing with cell densities of 1-1,400 MPN/onion”
t0“Dry Bulb Onions”
Claim · model-written summary
At the end of 2023 field curing, one of 80 onions had detectable E. coli at 2.04 MPN/onion.
- Taxon
- onions
Allium cepae0 - Outcome
- detectable E. colie0
- Value
- 2.04 MPN/onione0
- Qualifier
- 1/80 of onionse0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“At the end of field curing in 2023, 1/80 of onions had detectable E. coli (2.04 MPN/onion)”
Claim · model-written summary
Contaminated drip irrigation left E. coli on 13% of onions in 2022, fewer than overhead irrigation.
- Taxon
- onionsAllium cepae0
- Treatment
- contaminated drip irrigatione0
- Outcome
- E. coli was detectede0
- Direction
- smallere0
- Value
- 13%e0
- Qualifier
- 2022e0
- Comparator
- overhead irrigatione0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“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)”
Claim · model-written summary
Contaminated drip irrigation left E. coli on 68% of onions in 2023, fewer than overhead irrigation.
- Taxon
- onionsAllium cepae0
- Treatment
- contaminated drip irrigatione0
- Outcome
- E. coli was detectede0
- Direction
- smallere0
- Value
- 68%e0
- Qualifier
- 2023e0
- Comparator
- overhead irrigatione0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“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)”
Claim · model-written summary
Following overhead irrigation, E. coli was detected on onions at 1-1,000 MPN/onion on day zero.
- Taxon
- onionsAllium cepae0
- Outcome
- E. coli was detectede0
- Value
- 1-1,000 MPN/onione0
- Qualifier
- day 0e0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“After overhead irrigation, E. coli was detected in onions (1-1,000 MPN/onion) on day 0”
Claim · model-written summary
Following overhead irrigation, E. coli prevalence fell significantly to 15% after seven days of curing in 2022.
Verbatim
e0“Prevalence decreased significantly (P < 0.05) after 7 d of field curing in both years (2022: 15%; 2023: 7%)”
t0“Dry Bulb Onions”
Claim · model-written summary
Following overhead irrigation, E. coli prevalence fell significantly to 7% after seven days of curing in 2023.
Verbatim
e0“Prevalence decreased significantly (P < 0.05) after 7 d of field curing in both years (2022: 15%; 2023: 7%)”
t0“Dry Bulb Onions”
Claim · model-written summary
E. coli was absent from Calibra onions at the end of curing in both years.
- Taxon
- onionsAllium cepae0
- Material
- Calibrae0
- Outcome
- E. coli was not detectede0
- Qualifier
- at the end of field curing in either yeare0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“E. coli was not detected on Calibra onions (80/year) at the end of field curing in either year”
Claim · model-written summary
Other onion cultivars had E. coli in 2.5-3.75% of bulbs at less than 12 MPN/onion after curing.
- Taxon
- onionsAllium cepae0
- Material
- other cultivarse0
- Outcome
- detectede0
- Value
- 2.5-3.75%e0
- Qualifier
- <12 MPN/onione0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“was detected at <12 MPN/onion in 2.5-3.75% of onions (n = 80) for other cultivars”
The same questions in other papers
irrigation · organism · Allium cepa (onion)Positive and negative results
7 papers in the same direction as this paper · 3 in the opposite direction
- negative
- negative
- negative
7 papers more in this group
See all 11 papers →Claims are grouped by organism, treatment and what was measured or targeted. A paper counts as positive when any of its claims in the group is positive.