Effects of postharvest treatments on recovery of Xanthomonas citri subsp. citri in infected grapefruit leaves

Abstract

Studies were conducted to evaluate treatments that reduce recovery of Xanthomonas citri subsp. citri (Xcc) in infected grapefruit leaves. To investigate effects of temperature (0, 10, 40, and 50 °C), disinfectant (none or Pro-San), and treatment duration (0, 2, 10, and 20 min) on survival of Xcc in vitro, a split–split plot experimental design was utilized. Recovery of Xcc in vitro in the absence of Pro-San decreased with increasing treatment duration at 50 °C but not at temperatures <50 °C. Xcc in vitro was not detected after any treatment combination involving Pro-San. Decontamination of grapefruit leaves infected with Xcc in relation to disinfectant (none or Pro-San), temperature (0, 10, 40, 45, and 50 °C), treatment duration (0, 2, 5, 10, and 20 min), and assessment time (0, 2, 7, and 14 days post treatment [dpt]) was examined using a split–split–split plot design. Reductions in Xcc recovery generally increased with increasing treatment duration and temperature, and they were greater for treatments involving Pro-San. To examine the general trend of increased Xcc recovery with increasing dpt, nonlinear mixed regression analysis was used to fit a monomolecular model to relative Xcc recovery data. Results indicated that increases in relative Xcc recovery after 14 dpt were insignificant and unsubstantial. Treatment at 45 °C for 20 min or 50 °C for ≥5 min resulted in leaf tissue damage in some instances; in two cases, tissue damage was observed on untreated leaves 14 dpt. Experiments were conducted to investigate the relationships of tissue damage with leaf age and location of tissue damage in relation to point of inoculation. Tissue damage was observed on only the youngest, most supple leaves, and its localization did not appear to be related to naturally occurring citrus canker lesions or artificial inoculation sites. Results from these studies may be useful in formulation of future regulatory policies regarding trade of citrus foliage, especially those used as condiments.