Investigating Metrics Proposed to Prevent the Harvest of Leafy Green Crops Contaminated by Flood Water

Introduction:  Produce crops have been associated with foodborne disease outbreaks in the US. Pathogens in manure deposits can be transported by water through macropores in soil to contaminate distant leafy greens. The California Leafy Green Products Handler Marketing Agreement (LGMA) states that leafy green crops within 30 ft (9 m) of the edge of a flooded field should be destroyed due to potential contamination.

Purpose:  This experiment investigated if a 30-ft ‘buffer zone’ after flooding is sufficient to prevent the harvest of contaminated leafy greens.

Methods:  One end of a spinach bed (Beltsville, MD) was flooded with contaminated water containing 6 log CFU/ml Escherichia coli. Surface and sub-surface samples of bulk and rhizosphere soil, along with foliar samples, were taken 0, 1.5, 5, 15, and 30 ft from the flood edge. E. coli prevalence was determined by MPN analysis, and direct comparisons between soil samples, and between spring and fall trials, were made using t-tests (significance level 0.05). 

Results:  No significant differences in E. coli populations were found between bulk, rhizosphere, surface, or subsurface samples. No E. coli were detected on plants outside the flood zone after 14 days. On day 60, E. coli populations in the flood zone soil were significantly higher in the fall (2.77 log MPN/g) than in the spring (0.4 log MPN/g). Populations of E. coli in soil 1.5-ft from the flood zone were greater than at any other distance from the flood in both seasons, but were significantly lower in the fall than in the spring. By day 60, no E. coli were detected beyond 5ft from the flood in the spring, and populations were <0.1 log MPN/g in the fall. 

Significance:  These data suggest the 30-ft buffer zone is appropriate to prevent the harvest of contaminated leafy greens after a flooding event.