Rain splash-mediated dispersal of Escherichia coli from fecal deposits to field-grown lettuce in the mid- and south Atlantic U.S. regions is affected by mulch type.

Introduction: Wildlife feces can contaminate vegetables when enteric bacteria are released by rain and splashed onto crops. Regulations require growers to identify and not harvest produce that is likely contaminated, but U.S. federal standards do not define dimensions for no-harvest zones. Moreover, mulching, used to retain soil moisture and maximize crop yield may impact rain-mediated bacterial dispersal from feces. Methods: To assess Escherichia coli dissemination from a fecal point source to lettuce grown on various mulches, lettuce cv. 'Magenta' was transplanted into raised beds with plastic, biodegradable plastic, straw, or left uncovered at field sites in Maryland and Georgia. Eleven days post-transplant, 10 g of rabbit manure spiked with ~8 log CFU g-1 E. coli were deposited in each bed. One day following natural or simulated rain events, lettuce was sampled along 1.5 m transects on either side of fecal deposits. Lettuce-associated E. coli was semi-quantified with an MPN assay and dependence on fecal age (stale or fresh), lettuce age (baby leaf or mature head), distance from point source, mulch and post-rain days were statistically evaluated. Results: Distance (p<0.001), fecal age (p<0.001) and mulch (p<0.01) were factors for E. coli transfer from point source to lettuce. The highest and lowest E. coli estimates were measured from lettuce grown on biodegradable plastic and straw, respectively, with a 2-log MPN difference (p<0.001). Mulch and distance were also significant factors in E. coli recovery 3 days post-rain (both p<0.001), where plastic mulches differed from bare ground and straw (p<0.01). For all treatments, fewer E. coli were retrieved from lettuce at 0.3 m, 3 days post-rain compared to 1 day (p<0.001). Fitting the data to a Weibull Model predicated that a 7-log reduction in E. coli from fecal levels would be achieved at 1.2-1.4 m from the point source on plastic mulches, 0.75 m on bare soil (p<0.05) and 0.43 m on straw (p<0.01). Discussion: Straw and bare ground limited rain-mediated E. coli dispersal from feces to lettuce compared to plastic mulches. Fecal age was negatively associated with E. coli dispersal. These findings can inform harvesting recommendations for measures related to animal intrusion in vegetable production areas.

The effect of a first flush rainwater harvesting and subsurface irrigation system on E. coli and pathogen concentrations in irrigation water, soil, and produce.

Climate change is stressing irrigation water sources, necessitating the evaluation of alternative waters such as harvested rainwater to determine if they meet water quality and food safety standards. We collected water, soil, and produce samples between June and August 2019 from two vegetable rain garden (VRG) sites in Frederick, Maryland that harvest rainwater using a first flush system, and deliver this water to produce through subsurface irrigation. The raised VRG beds include layers of gravel, sand, and soil that act as filters. We recorded the average surface soil moisture in each bed as well as antecedent precipitation. All water (n = 29), soil (n = 55), and produce (n = 57) samples were tested for generic E. coli using standard membrane filtration, and water samples were also tested for Salmonella spp. and Listeria monocytogenes by selective enrichment. No Salmonella spp. or L. monocytogenes isolates were detected in any water samples throughout the study period. Average E. coli levels from all harvested rainwater samples at both sites (1.2 and 24.4 CFU/100 mL) were well below the Good Agricultural Practices food safety guidelines. Only 7 % (3/44) of produce samples from beds irrigated with harvested rainwater were positive for E. coli. E. coli levels in soil samples were positively associated with average surface soil moisture (r2 = 0.13, p = 0.007). Additionally, E. coli presence in water samples was marginally associated with a shorter length of antecedent dry period (fewer days since preceding rainfall) (p = 0.058). Our results suggest that harvested rainwater collected through a first flush system and applied to produce through subsurface irrigation meets current food safety standards. Soil moisture monitoring could further reduce E. coli contamination risks from harvested rainwater-irrigated produce. First flush and subsurface irrigation systems could help mitigate climate change-related water challenges while protecting food safety and security.