Risk Management for Field-Grown Tomatoes: Comparing Source of Spray Water On the Potential for Food-Bourne Human Pathogens

A relationship has been shown to exist between water quality and the bacterial loads on the surface of produce. Water can serve as a vector for pathogenic bacteria by introducing potentially pathogenic bacteria to the phyllosphere environment of the crop. We proposed to test the water quality of two agricultural water sources: surface water and groundwater, to characterize the impacts water source has on microflora on tomatoes. Enterobacteriaceae-specific petrifilms, total coliform petrifilms, and RapidChek Salmonella assays were used to test for potentially pathogenic bacteria in water samples and “washes” taken from tomato fruit surfaces. Libraries of 16S rRNA gene fragments were also assembled for each sampling point to provide more taxonomic information about the microflora present in water and tomato phyllosphere samples.

The experiment was conducted in field plots of tomatoes at the Wye and Salisbury Research and Education Centers (WREC and LESREC) on Maryland’s Eastern Shore. Ten plots were sprayed on a ten-day pesticide schedule with fungicides and insecticides mixed into groundwater or surface water. Groundwater was used for trickle irrigation as needed. Enumeration of Enterobacteriaceae and total coliforms was also conducted on ripe tomatoes at twice during the 2008 growing season. Five replicate fruit samples of each treatment were taken at each date.

No samples tested positive for Salmonella and levels of coliform bacteria in groundwater samples were undetectable. In contrast, the surface water samples maintained a higher bacterial load with an upward seasonal trend in total coliforms. An increase in bacterial load appeared to be due to increasing surface water temperature during the growing season. Bacterial counts of Enterobacteriaceae and total coliforms on tomato fruit phyllosphere paralleled the results seen in direct water enumerations. Tomatoes treated with surface water contained the highest bacterial load whereas groundwater treated fruit had lower bacterial counts. In both cases, counts increased between the two sampling dates demonstrating that water source and harvest date are both important when considering water used for vegetable production. The results of the gene sequencing are currently being interpreted for significant differences in bacterial phyla between the groundwater and surface water treatments.