Does Salmonella move through the irrigation systems of mixed produce farms of the southeastern United States?

Our goal is to develop knowledge which will allow vegetable producers who rely on untreated surface sources of irrigation water to effectively address proposed FDA rules. These rules may require that all agricultural water, including irrigation water, be safe for its intended use. In a small pilot study we conducted in the southeastern United States, we found that Salmonella is present in irrigation systems using water from ponds to irrigate vegetables. These findings are intriguing but preliminary and a more comprehensive study is needed to draw defensible conclusions. With that in mind, our objective is to understand if Salmonella moves through irrigation systems of mixed produce farms of the southeastern United States; if it persists on the crop until harvest; and if chlorine dioxide treatment of the water prevents produce contamination. We will measure Salmonella concentrations in ponds and wells, irrigation systems (pivot, sprinkler, drip), and produce (cantaloupe, tomatoes, peppers, leafy greens) irrigated by these systems on five farms in southern Georgia to answer these questions. Knowledge resulting from this project will allow vegetable producers that rely on untreated surface sources of irrigation water to effectively address new rules the FDA may implement on safe agricultural water.

The overall goal of this proposal is to develop knowledge which will allow vegetable producers who rely on untreated surface sources of irrigation water to effectively address recently proposed FDA rules. In January 2013, the FDA proposed that all agricultural water must be safe for its intended use (proposed § 112.41.) FDA’s definition of agricultural water is water that comes into direct contact with produce and includes irrigation water that is applied using direct water application methods such as overhead sprinkler irrigation. FDA also proposed that agricultural water be considered safe if test results do not exceeded 235 CFU per 100 mL generic E. coli. Companion CPS-funded studies conducted in the southeastern United States by members of our project team have consistently found measurable concentrations of Salmonella and other pathogens in ponds used to irrigate fruits and vegetables. Members of the project team also conducted a pilot study to assess the presence of Salmonella in irrigation water in four different irrigation systems on one farm. Salmonella and generic E. coli were found in samples collected from all three irrigation systems fed by pond water. Generic E. coli concentrations in all samples from the irrigation systems containing Salmonella were below the proposed threshold of 235 CFU/100 mL. In contrast, no Salmonella or generic E. coli were found in samples collected from an irrigation system fed by a deep groundwater well. These preliminary findings are intriguing but the sample sizes were small and a larger and more comprehensive study is needed to draw defensible conclusions. With that in mind, our objective is to understand if pathogenic organisms, and specifically Salmonella, move through the irrigation systems of mixed produce farms of the southeastern United States and if so, do they persist on the crop until harvest. We have developed a research plan which will measure Salmonella and generic E. coli concentrations in source water (ponds and wells), irrigation systems (pivot, sprinkler, drip), and produce (cantaloupe, tomatoes, peppers, leafy greens, and others) irrigated by these systems on five farms in southern Georgia. We will also evaluate if chlorine dioxide treatment of irrigation water drawn from ponds effectively removes pathogens. Finally, we will assess the validity of measuring generic E. coli as an indicator for Salmonella serovars. The expected outcome of our proposed project is information on whether Salmonella moves through the irrigation systems of mixed produce farms of the southeastern United States and if so, does it persist on the crop until harvest. We will also provide information on whether treatment of the irrigation water after it is withdrawn from the pond eliminates the pathogen threat. This knowledge will allow vegetable producers that rely on untreated surface sources of irrigation water to effectively address new rules the FDA may implement on safe agricultural water. If our assessment of the validity of measuring generic E. coli as an indicator for Salmonella serovars produces highly significant results, then these results could potentially be applied to the entire country and multiple fresh produce commodities.

The overall goal of our project team is to develop the knowledge which will allow vegetable producers that rely on untreated surface sources of irrigation water to effectively address new rules the FDA may implement on safe agricultural water. With that in mind, our proposal’s direct goal is to understand if Salmonella moves through the irrigation systems of mixed produce farms of the southeastern United States and if so, if it persists on the crop until harvest and can be mitigated by treating the irrigation water with chlorine dioxide. To address this goal, we have developed the following objectives: 

1. Sample irrigation systems on 5 farms to determine if the pilot project findings are representative of the overall condition and to determine which combinations of water source and irrigation system type are more prone to pathogen transport. 

2. Determine if Salmonella strains in irrigation water contaminate produce and if the Salmonella persists on the crop until harvest. 

3. Determine if chlorine dioxide treatment of irrigation water drawn from ponds effectively removes pathogens and prevents contamination of produce following irrigation. 

4. Assess the validity of measuring generic E. coli as an indicator for Salmonella serovars under southeastern Coastal Plain conditions.

• Approximately 32% of the water samples collected from the irrigation ponds used in this study were positive for Salmonella. Concentrations were low and consistently below 1 MPN/100 mL. Generic E. coli were not found in all pond samples and all pond concentrations were below 70 MPN/100 mL, with the exception of one sample. These data are consistent with findings from previous studies conducted by our team in the same ecoregion (Antaki et al., 2016; Li et al., 2014, 2015; Luo et al., 2014, 2015; Gu et al., 2013a, 2013b). 

• The bacteria pass through the irrigation systems at about the same rates as found in the ponds regardless of whether irrigation is by overhead sprinkler or drip systems. 

• Of 65 produce samples from untreated systems, two were found positive for Salmonella: one cucumber sample and one cantaloupe sample. PFGE and serotyping did not detect any matches between Salmonella isolated from produce and Salmonella isolated from irrigation water. Other environmental factors may be responsible for contaminating the two produce samples. One potential contamination pathway may be soil splashed on the produce by rain. 

• Irrigation water disinfection, in this case a calcium hypochlorite tablet chlorination system calibrated for a <2 mg/L free chlorine residual, resulted in greater than 50% reductions in the percentage of samples positive for Salmonella and an even greater reduction in the percentage of samples positive for generic E. coli. No produce samples were found positive for Salmonella when irrigation water was chlorinated. 

• A 4-page extension booklet titled “Pre-harvest irrigation water: Methods for disinfection” was developed and will provide fruit and vegetable growers with valuable and non-biased information about irrigation water disinfection systems. 

• FSMA guidelines based on generic E. coli are not a good indicator for the presence of Salmonella in the ponds and irrigation systems of the southeastern Coastal Plain, as they result in an approximately 42% false negative rate.