Purpose: The aim of this study was to evaluate the fate of foliar applied indicator microorganisms on the surface of grove oranges.
Methods: Three orange trees were sprayed with water of low microbial quality (~106 CFU E. coli/ml), and three trees served as the controls. Three replicates of 10 pooled oranges each were harvested from each tree; coliforms and E. coli were enumerated by plate count and MPN. Populations were determined at 0, 2 and 6 hours after spraying and was continued until Escherichia coli could not be detected by enrichment. Field trials were completed with Valencia variety in April, May and June, and with Hamlin variety in October, November and December 2012. Solar radiation, temperature, relative humidity (RH) and rainfall data were obtained.
Results: In April, E. coli populations were detected at day 12, but not beyond day 17. In May, E. coli populations were undetectable on day 8, detectable on days 10 and 13 and undetectable at day 15. In June, there were significant increases in E. coli populations on days 2 and 3, populations were detected on day 16 but became undetectable beyond day 23. In October, E. coli populations increased significantly on day 2 and remained detectable up to day 20. In November and December, populations of E. coli became undetectable on days 12 and 8 respectively. Fluctuations in RH were correlated with declines in E. coli populations. Rainfall and high RH were correlated with the increase in E. coli populations at the initial stages of sampling as well as recovery of E. coli during the late stages of sampling.
Significance: Low microbial quality water used in foliar sprays may introduce microorganisms to the surface of citrus. Wet weather conditions can extend the survival of indicators on surfaces of oranges.