Rain splash-mediated dispersal of 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 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 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 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 (<0.001), fecal age (<0.001) and mulch (<0.01) were factors for transfer from point source to lettuce. The highest and lowest estimates were measured from lettuce grown on biodegradable plastic and straw, respectively, with a 2-log MPN difference (<0.001). Mulch and distance were also significant factors in recovery 3 days post-rain (both <0.001), where plastic mulches differed from bare ground and straw (<0.01). For all treatments, fewer were retrieved from lettuce at 0.3 m, 3 days post-rain compared to 1 day (0.001). Fitting the data to a Weibull Model predicated that a 7-log reduction in 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 (<0.05) and 0.43 m on straw (0.01).

DISCUSSION

Straw and bare ground limited rain-mediated dispersal from feces to lettuce compared to plastic mulches. Fecal age was negatively associated with dispersal. These findings can inform harvesting recommendations for measures related to animal intrusion in vegetable production areas.