Thiabendazole residue loading in dip, drench and wax coating applications to control green mould and chilling injury on citrus fruit

• TBZ was most effective for green mould control as a curative dip treatment.
• TBZ in the dip application has poor or unreliable protective action.
• TBZ in wax application gave better protective than curative control.
• Drench application of TBZ gave better curative than protective control.
• Chilling injury was less after TBZ amended wax treatment.

Green mould (caused by Penicillium digitatum) is a major cause of postharvest losses in citrus. Residue loading of thiabendazole (TBZ) with application methods typically used in South African packhouses and green mould control was studied. TBZ was applied curatively and protectively in dip, drench and wax coating treatments and fruit were inoculated with a TBZ-sensitive or a TBZ-resistant isolate of P. digitatum. The dip treatments consisted of TBZ concentrations of 0–2000 μg mL−1; fruit were dipped for 60 s at 22 °C at a pH of 7. Residues differed between fruit batches and ranged from 0.5 to 1.7 μg g−1 at 1000 μg mL−1 TBZ. Curative dip treatments almost completely controlled green mould (>96% at 1000 μg mL−1 TBZ). The residue level needed for 75% curative control ranged from 0.06 to 0.22 μg g−1, depending on citrus type. Protective treatments were unreliable and control varied from 17% to 97.9% at 1000 μg mL−1 TBZ between fruit batches. Drench treatments consisted of exposure times of 30, 60 and 90 s with 1000 or 2000 μg mL−1 TBZ. Average TBZ residues were 2.14 μg g−1 for Clementine mandarin fruit and 3.50 μg g−1 for navel orange fruit. Green mould control on navel orange fruit resulted in 66–92%, 34–90% and 9–38% control for curative treatments after 6 and 24 h and protective treatments, respectively, depending on fruit batch. Wax with 4000 μg mL−1 TBZ was applied at 0.6, 1.2 and 1.8 L wax ton−1 fruit. Chilling injury was evaluated after fruit storage at −0.5 °C for 40 days. Average TBZ residues loaded was 1.3, 1.3 and 2.7 μg g−1 at the recommended 1.2 L ton−1 for Satsuma mandarin, Clementine mandarin and Valencia orange fruit, respectively. Protective treatments showed lower infection levels (14–20%) than curative treatments (27–40%) for Valencia orange fruit. The same trend was observed with Satsuma (92–95% curative; 87–90% protective) and Clementine mandarin fruit (82–90% curative; 59–88% protective), but control was relatively poor. TBZ application in wax exceeded 5 μg g−1 at higher wax loads (1.2 and 1.8 L ton−1). Wax treatments showed a significant reduction in chilling injury; TBZ had an additive effect. TBZ resistant isolates could not be controlled.