Impact of mulches and growing season on indicator bacteria survival during lettuce cultivation

• E. coli populations declined rapidly on lettuce in both seasons.
• Fecal bacterial on lettuce persisted longer in spring compared to fall.
• In spring, E. coli declined faster on bare ground-grown than mulch-grown lettuce.
• Fecal bacteria persisted in soil; coliform levels were higher under plastic mulch.
• Enterococcus spp. became established and persisted in the lettuce phyllosphere.

In fresh produce production, the use of mulches as ground cover to retain moisture and control weeds is a common agricultural practice, but the influence that various mulches have on enteric pathogen survival and dispersal is unknown. The goal of this study was to assess the impact of different mulching methods on the survival of soil and epiphytic fecal indicator bacteria on organically grown lettuce during different growing seasons. Organically managed lettuce, cultivated with various ground covers — polyethylene plastic, corn-based biodegradable plastic, paper and straw mulch — and bare ground as a no-mulch control, was overhead inoculated with manure-contaminated water containing known levels of generic Escherichia coli and Enterococcus spp. Leaves and soil samples were collected at intervals over a two week period on days 0, 1, 3, 5, 7, 10 and 14, and quantitatively assessed for E. coli, fecal coliforms and Enterococcus spp. Data were analyzed using mixed models with repeated measures and an exponential decline with asymptote survival model. Indicator bacterial concentrations in the lettuce phyllosphere decreased over time under all treatments, with more rapid E. coli declines in the fall than in the spring (p < 0.01). Persistence of E. coli in spring was correlated with higher maximum and minimum temperatures in this season, and more regular rainfall. The survival model gave very good fits for the progression of E. coli concentrations in the phyllosphere over time (R2 = 0.88 ± 0.12). In the spring season, decline rates of E. coli counts were faster (2013 p = 0.18; 2014 p < 0.005) for the bare ground-cultivated lettuce compared to mulches. In fall 2014, the E. coli decline rate on paper mulch-grown lettuce was higher (p < 0.005). Bacteria fluctuated more, and persisted longer, in soil compared to lettuce phyllosphere, and mulch type was a factor for fecal coliform levels (p < 0.05), with higher counts retrieved under plastic mulches in all trials, and higher enterococci levels under straw in fall 2014 (p < 0.05). This study demonstrates that mulches used in lettuce production may impact the fate of enteric bacteria in soil or on lettuce, most likely in relation to soil moisture retention, and other weather-related factors, such as temperature and rainfall. The data suggest that the time between exposure to a source of enteric bacteria and harvesting of the crop is season dependent, which has implications for determining best harvest times.