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

Abstract

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 0–2000μgmL−1; fruit were dipped for 60s at 22°C at a pH of 7. Residues differed between fruit batches and ranged from 0.5 to 1.7μgg−1 at 1000μgmL−1 TBZ. Curative dip treatments almost completely controlled green mould (>96% at 1000μgmL−1 TBZ). The residue level needed for 75% curative control ranged from 0.06 to 0.22μgg−1, depending on citrus type. Protective treatments were unreliable and control varied from 17% to 97.9% at 1000μgmL−1 TBZ between fruit batches. Drench treatments consisted of exposure times of 30, 60 and 90s with 1000 or 2000μgmL−1 TBZ. Average TBZ residues were 2.14μgg−1 for Clementine mandarin fruit and 3.50μgg−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 24h and protective treatments, respectively, depending on fruit batch. Wax with 4000μgmL−1 TBZ was applied at 0.6, 1.2 and 1.8Lwaxton−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 2.7μgg−1 at the recommended 1.2Lton−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 5μgg−1 at higher wax loads (1.2 and 1.8Lton−1). Wax treatments showed a significant reduction in chilling injury; TBZ had an additive effect. TBZ resistant isolates could not be controlled.