Purpose: The goal of this study was to assess the impact of different mulching methods on the survival of soil and epiphytically-associated fecal indicator bacteria on organically grown lettuce during different growing seasons (spring and fall).
Methods: Lettuce was grown under organic practices using various mulches (polyethylene plastic, corn-based biodegradable plastic, paper mulch) and bare ground in a randomized complete block design. At 8 weeks post-transplanting, lettuce was inoculated with manure containing known levels of generic Escherichia coli and Enterococcus spp. Leaves and soil samples were collected at intervals over a 2-week period, and quantitatively analyzed for E. coli and fecal coliforms on Tryptone Bile X-Glucuronide (TBX) agar and Enterococcus spp. on Enterococcosel agar.
Results: Bacterial concentrations decreased under all treatments, with more rapid declines in the fall than in spring (slope = -0.75 in fall and - 0.38 in spring). Within each season, no significant differences in the persistence of E. coli, fecal coliforms and Enterococcus was observed among the three types of mulches and bare ground in the rhizosphere and phyllosphere. However, a significant difference was observed between spring and fall seasons.
Significance: This study demonstrates that mulches used in lettuce production are not likely to impact the fate of enteric pathogens in soil or on lettuce. However, the data suggest that the time between exposure to a source of enteric bacteria and harvesting of the crop is seasonally dependent and has implications for determining best harvest times.