Postharvest dip, drench and wax coating application of pyrimethanil on citrus fruit: Residue loading and green mould control

Residue loading of pyrimethanil (PYR) with application methods typically used in South African packhouses and green mould control was studied. PYR was applied curatively (after inoculation) and protectively (before inoculation) in dip, drench and wax coating treatments and fruit were inoculated with an imazalil (IMZ)-sensitive or an IMZ-resistant isolate of Penicillium digitatum, the causal agent of citrus green mould. The dip treatments consisted of PYR concentrations 0, 50, 100, 250, 500 and 1000 mg L−1; fruit were dipped for 60 s at 18 °C at pH 7. Residues loaded differed between fruit type and batch. At the registered concentration of 1000 mg L−1, an average of 1.96 mg kg−1 PYR (range 1.40-2.95 mg kg−1) was loaded. Increased residue levels generally resulted in improved control, but better curative than protective control was observed with effective residue levels of 0.27 and 0.91 mg kg−1 for 50% and 75% curative control of green mould, respectively. Effective residue levels for 50% protective control were 0.97-1.43 mg kg−1. Drench treatments consisted of exposure times of 30, 60 and 90 s with 1000 mg L−1 PYR. PYR residues on Valencia oranges were 2.46-4.22 mg kg−1 PYR, and on navel oranges were 2.09-7.64 mg kg−1. Green mould control on Valencia orange fruit was moderate (57.3-69.8% for curative treatment after 6 h) to relatively poor (4.9-14.6% control for curative treatments after 24 h and 18.1-26.4% for protective treatments). Control on navel oranges following these treatments was poor (0-7%). Drench applications with different combinations of PYR, IMZ, thiabendazole (TBZ), guazatine (GZT) and didecyl dimethyl ammonium chloride were applied to navel orange fruit with 60 s exposure time. In general, fungicide mixtures performed better than single fungicides (44.4-64.6% for mixtures vs. 24.3, 39.6 and 46.5% for PYR, GZT and TBZ respectively). Drench applications where fruit were inoculated with spore suspensions containing 106, 105, 104 or 103 spores mL−1 showed that infections from lower inoculum loads were better controlled (90.6-98.6% vs. 32.6-62.2% for 103vs. 106 spores mL−1). Wax with 2000 or 4000 mg L−1 PYR was applied at 0.6, 1.2 and 1.8 L wax ton−1 of fruit. Residues loaded on navel oranges were 1.75-6.33 mg kg−1, and on Valencia oranges were 1.05-9.91 mg kg−1. Green mould control on navel oranges was poor and were 0-12.2%. On Valencia oranges, higher levels of protective control (28.8-54.0%) than curative treatments (10.5-27.3%) was observed. The IMZ-resistant isolate was successfully controlled by all PYR treatments, in some cases better than the sensitive isolate. In conclusion, PYR provided very good curative control in dip applications, but gave poor results in drench and wax applications. Green mould control following PYR drench treatment improved on fruit inoculated with smaller spore loads, or when PYR was mixed with other fungicides.