Investigating metrics proposed to prevent the harvest of leafy green crops contaminated by floodwater

Authors: CALLAHAN, MARY THERESA - University Of Maryland; MICALLEF, SHIRLEY - University Of Maryland; Sharma, Manan; Millner, Patricia; BUCHANAN, ROBERT - University Of Maryland

Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/23/2016
Publication Date: 4/15/2016
Citation: Callahan, M., Micallef, S.A., Sharma, M., Millner, P.D., Buchanan, R.L. 2016. Investigating metrics proposed to prevent the harvest of leafy green crops contaminated by floodwater. Applied and Environmental Microbiology. 82:3746-3753.

Interpretive Summary: The California Leafy Green Products Handler Marketing Agreement (LGMA) requires that leafy green crops within 9 meters (30 ft) of the edge of a flooded field not be harvested due to potential contamination. LGMA guidelines also state that flooded fields cannot be replanted with leafy crops until 60 days after the flooding event. The LGMA is a consortium of growers and handlers of leafy green commodities who subscribed to voluntary guidelines intended to decrease risks that may increase foodborne pathogens from contaminating these commodities. The research performed here was to model a “worst case scenario” flood to determine if the LGMA guidelines are scientifically valid. The upper end of a spinach bed (Beltsville, MD) established on a -5% grade was flooded with water containing 6 log CFU/mL Escherichia coli to model a worst-case scenario of bacterial movement through soil. Spinach plants and soil within the 9 meter “no harvest” buffer zone from the edge of the flood were tested for the presence of E. coli. E. coli in soil was detected 9 meters away from the flood zone after 1 day in the spring trial and after 3 days in the fall trial. No E. coli was recovered from spinach plants outside of the flood zone. Overall, E. coli populations in the fall trial were higher than those in the spring trial. E. coli populations declined more quickly in the spring trial than in the fall trial. These results suggest LGMA-proposed metrics should be revised to include considerations of soil and ambient temperatures, possibly incident solar radiation, additional rainfall events, and field slope in determining intervals between flooding and planting, and determining buffer zones to prevent the harvest of contaminated leafy greens after a flooding event. This information should be useful to other scientists, the produce industry and to regulatory agencies.

Technical Abstract: The California Leafy Green Products Handler Marketing Agreement (LGMA) requires leafy green crops within 9 m of the edge of a flooded field not be harvested due to potential contamination. Further, previously flooded soils should not be replanted for 60 days. In this study, the suitability of the LGMA metrics for farms in the Mid-Atlantic region of the United States was evaluated. The upper end of a spinach bed (Beltsville, MD) established on a -5% grade was flooded with water containing 6 log CFU/mL Escherichia coli to model a worst-case scenario of bacterial movement through soil. E. coli prevalence in soil and on foliar tissue was determined by Most Probable Number (MPN) analysis at distances up to 9 m from the edge of the flood for 63 days. While E. coli was quickly detected at the 9 m distance within 1 day in the spring trial and 3 days in the fall trial, no E. coli were detected on plants outside the flood zone after 14 days. On day 63, E. coli populations in the flood zone soil were higher in the fall than in the spring. Regression analysis predicted the time required for a 3-log decrease in E. coli populations inside the flood zone was within the 60-day LGMA guideline in the spring, but would require 90 days in the fall. Overall, data suggest that the current guidelines should be revised to include considerations of field and weather conditions that may promote bacterial movement and survival.