Transfer and Survival of Microorganisms to Produce from Surface Irrigation Water

Surface water irrigation sources are widely used for fruit and vegetable crop production in the United States. Surface water is inherently prone to direct and indirect contamination with animal fecal material. Hence, the microbial quality of surface water sources can be highly variable. Water used for irrigation is considered a common source for produce contamination. In addition to this, fruits and vegetables are commonly consumed fresh or minimally processed, thus emphasizing the need for preventative measures in production of fresh produce. This study will examine transfer from naturally contaminated irrigation water to cantaloupes using drip and overhead spray irrigation methods. Additionally, the use of plots with bare ground or plastic mulch will be evaluated for contamination risk.

Water from a pond naturally contaminated with STEC was passed through a sand filter and used to irrigate cantaloupes. Cantaloupe plots contained cross-classified combinations of overhead or surface drip irrigation in addition to bare ground or plastic mulch raised bed preparation. Surface water was sampled from the source pond and from overhead spray emitters weekly across six consecutive weeks for enumeration of STEC, generic E. coli, and coliforms using routine enumeration methods. Cantaloupes were harvested and processed using a rinse technique across four consecutive harvest weeks. Cantaloupe rinsates were enriched and DNA was extracted. Microbial DNA from each cantaloupe was tested for the presence/absence of stx and eae genes using multiplex PCR.

No significant correlations were observed between STEC and any indicator organism in the irrigation water source. Cantaloupes were contaminated regardless of irrigation method and seedbed preparation with no significant differences between treatments. Contamination rates for bare ground plots with drip irrigation and plastic mulch plots with overhead spray irrigation were 20.4% and 19.7%, respectively. Positive samples were also found for bare ground plots with overhead irrigation (14%) and plastic mulch plots with drip irrigation (12%). Transfer was shown to occur in treatments using drip irrigation. In this study, generic E. coli was not found to be a suitable predictor of STEC levels in the pond water used for irrigation.