Survival of Escherichia coli on onion during field curing and packout.

The Food and Drug administration has expressed concern that Onions (Allium cepa) irrigated with water contaminated with high rates of Escherichia coli could harbor E. coli on their surface or interior. On the other hand, since onions contain antimicrobial compounds and field conditions may not be conducive to E. coli survival, the E. coli population on the surface of onions might become negligible through the course of field curing. Further, the relationship between the E. coli in the irrigation water to the E. coli on onion bulbs after field curing, harvest, and packout has not been studied. To determine if E. coli should be of concern in onion production, we sought to measure the die-off of E. coli on onions between the last irrigation and harvest and the presence of E. coli on onions after packout. Well water was tested and had no E. coli; ditch water intentionally run across a pasture prior to use had 218 to > 2400 MPN of E. coli/100ml. Onions were sampled from those furrow irrigated (ditch water) and those drip irrigated (well water) starting at lifting 3 September 2013 for four consecutive weeks. At 0 and 28 days after lifting, both interior and exterior of the onions were tested for E. coli. At 7, 14, and 21 days after lifting, only the exterior of the onions was tested. None of the onions contained E. coli internally at 0 or 28 days after lifting. At lifting E. coli was present on the exterior of both the drip and furrow irrigated onions and seemed to be largely unrelated to the irrigation water. The exterior E. coli contamination decreased rapidly after lifting. After harvest and packout on 14 October 2013, no E. coli was detected on the onion bulb exteriors from either irrigation treatment. E. coli introduced into the onion field through furrow irrigation was not present on or in the packed out onion bulbs

Movement of Escherichia coli in soil as applied in irrigation water.

The US Food and Drug Administration (FDA) has proposed that If irrigation water exceeds 235 colony-forming units (CFU) of E. coli /100 ml in any one sample or 126 CFU/100 ml in the average of any five consecutive samples, growers would have to cease using that water in any way that directly contacts the surface of fresh produce (FDA 2013). The FDA has proposed that these E. coli levels are an indication of high risk of bacterial contamination of fresh onion (Allium cepa L.) bulbs regardless of the irrigation system. If onion irrigation exceeds 235 CFU, it is not known whether the contaminated water applied by furrow or drip irrigation actually reaches the onion bulb. Soil could filter E. coli and other bacteria before irrigation water reaches onion bulbs. ?Vaquero? onions were grown on Owyhee silt loam. In our preliminary studies reported here, well water free of E. coli was applied to onions through drip irrigation or through furrow irrigation. A second water source was intentionally enriched with E. coli by being run across a pasture and recaptured prior to use. Furrow and drip irrigation were used to apply this water containing 218 to >2400 MPN/100ml for 11+ hours per irrigation. E. coli was monitored in the soil water at the end of irrigation cycles through direct sampling of the soil. Soil water was also sampled using sterile soil solution capsules (SSSC) to sample E. coli in the soil water that moved into place, to differentiate the movement of soil water from the soil water already in place. Soil water measurements were made adjacent to the water source, half way to the bulbs, and immediately adjacent to the onion bulbs. For furrow irrigation with ditch water the E. coli counts in the soil next to the onion bulbs was only 0% and 21% of the counts in the irrigation water following the first and second irrigations, respectively. During subsequent furrow irrigations, the E. coli counts in the soil water next to the onion bulbs exceeded the counts in the irrigation water. For drip irrigation with ditch water, the E. coli counts in the soil solution next to the onion bulbs remained very low. The soil water sampled by the SSSC adjacent to the onion bulbs drip-irrigated with ditch water also had very low E. coli counts