Purpose: This study investigated the effect of weeds level on the survival of microorganisms on watermelon surface in an on-farm experiment.
Methods: Replicated watermelon plots (18 plots, 3 plots per treatment, plot size 12’x30’, 30 plants per plot) were treated with herbicides (Strategy, Command 3ME, Strategy + Sinbar, Valor and Sinbar and an untreated check). Watermelon samples (n=80 total) were examined for the presence E. coli O157 and Salmonella spp. using immunomagnetic separation technique and quantified for total coliforms and generic E. coli using Quanti-Tray and petrifilm. Additionally, survival of generic E. coli inoculated on watermelon discs (20 cm2 surface area) (6.24 log CFU/cm2) under different weed levels was examined by placing them in each plot up to five days.
Results: Pathogens were not detected from the watermelon samples. Total coliform levels were higher (3.91 log CFU/cm2) on watermelon harvested from high weed plots than from other plots (3.67 log CFU/cm2). Watermelons harvested from medium weed areas had higher generic E.coli levels (1.46 log MPN/watermelon) than high weed (1.23 log MPN/watermelon) and low weed (1 log MNP/watermelon) areas. A significant (P<0.05) die-off of generic E. coli was observed within 12 h on watermelon disc from all weed level plots (>5 log CFU/cm2 in no weed plots and, 0.95-1.14 log CFU/cm2 from other plots); however the level was increased after 60 h. After 108 h, generic E. coli levels on watermelon discs from low weed plots (3.28 log CFU/cm2) were significantly (P<0.05) lower as compare to medium and high weed plots (6.12 – 6.40 log CFU/cm2).
Significance: Our results suggest higher microbial die-off rate on produce surfaces exposed to sunlight radiation.