Purpose: To evaluate the survival, prevalence and post-harvest persistence of EcO157 under organic and conventional spinach production.
Methods: Spinach was cultivated under organic and conventional conditions in San Benito and Monterey County, California. Standard management practices were followed with the subsequent modifications: nitrogen management, field location and spinach cultivar. Pre-harvest inoculum doses were 1.45 and 3.4 CFU/m2 of planted bed. Three plant developmental stages were evaluated. Harvested product was stored for up to 14 days at 5 °C. Survival of EcO157 in soil under both cultivation systems was also monitored over 45 days after incorporation of inoculated plants (at varying C/N in crop residue) sprayed with log 4 CFU/ml. Post-harvest inoculations were done with attenuated EcO157 and four different pathogenic EcO157 strains at log 2 CFU/ml and stored for 6 days at 15 °C.
Results: High N grown leaves supported greater survival of EcO157 irrespective of farm practice (P < 0.05). Greater survival of EcO157 was observed in older leaves irrespective of cropping system (P < 0.05). Prevalence of EcO157, applied pre-harvest, at 5 °C was observed during 14 days of storage with greater number of positive samples for high inoculum doses (P < 0.05). Greater survival was observed for pathogenic than non-homologous attenuated strains (60 of 80 vs. 34 of 80, respectively; P < 0.05) applied post-harvest. Pathogenic strains with greater attachment traits were recovered in higher numbers than other test isolates (52 of 80 vs. 25 of 80, respectively; P < 0.05). Survival of EcO157 in both systems was prolonged in the presence of spinach residues (31 vs. 45 days, P < 0.05) irrespective of C/N ratio.
Significance: Risk assessment of farm practices under organic and conventional spinach should focus prevention strategies on optimal N fertilization and minimizing contamination potential close to harvest.