Purpose: To compare die-off rates of E. coli and surrogates of Salmonella enterica, Listeria monocytogenes and E. coli O157:H7 inoculated via sprinkler irrigation or spray inoculation to the surface of strawberries during frost protection events.
Methods: Strawberry plants were inoculated via sprinkler irrigation (E. coli) or by spray inoculation (Avirulent Salmonella Typhimurium, E. coli O157:H7 and Listeria innocua) during frost protection events with river, pond and well water. Average inoculum concentration was log 4.5 CFU/ml. Leaves/stems, berries and crown samples (150 – 300 g) were collected at 1, 3, 4, 6, 9 and 36-days post inoculation (DPI). Quantitative and qualitative recovery of each strain at harvest or during storage was achieved via selective/differential culturing methods and presumptive positives were confirmed via probe-based PCR.
Results: Populations of E. coli after 3-DPI using river, pond or surface water were below the limit of detection (LOD). E. coli persistence did not differ (P<0.05) between water sources. However, greater persistence was detected in the crown 36-DPI compared to leaves/stems and berries (100%, 36% and 7% respectively). After 12 days of storage at 8°C, 90% of fruit samples were positive for the inoculated E. coli. Die-off rate of E. coli was 0.12 log CFU/day after 36-DPI. After 4-DPI, all surrogate populations were at/below the LOD. After 9-DPI only the strawberry crowns were positive after enrichment for all inoculated strains.
Significance: Our findings suggest that current alternative provisions adopted by the Produce Rule to meet the necessary pathogen die-off rates after irrigation/frost protection events may need to be revised.