Postharvest application of imazalil sulphate using a heated flooder for control of citrus green mould

Abstract

High volume aqueous applications of imazalil (IMZ) are commonly used in major citrus producing industries for control of green mould, caused by Penicillium digitatum. The flooder application is a more novel means of application where the heated fungicide solution is applied in high volume and low pressure through weirs creating a waterfall over a set of rotating brushes. A five-weir flooder is compared to the conventional dip tank application in terms of residue loading, curative and protective control as well as sporulation inhibition. Imazalil sulphate was applied with a flooder (250 mg L-1 at pH ≈ 5 for 8 s) or a dip tank (500 mg L-1 at pH ≈ 3 for 60 s) on various citrus types at solution temperatures of 25, 35 or 52 °C. All treatments deposited IMZ residues in the fruit of ≈ 1.00 mg kg-1, except residues of ≈ 2.00 mg kg-1 after treatment at 52 °C. Flooder application gave similar curative control and better protective control compared to the dip application. Sporulation control by IMZ was significantly improved when applied with the flooder at 52 °C, compared to the dip treatment. Imazalil sulphate application at 250 and 500 mg L-1 was optimised at temperatures from 25° to 60°C. Regression models describing residue loading in a pH 5, 250 mg.L-1 solution applied by flooder at 26.9 or 47.4 °C would deposit residues of 1 and 2 mg.kg-1, respectively. Regression models describing residue loading in a pH 3, 500 mg.L-1 solution applied by flooder at 38.0 or 63.20 °C would deposit residues of 1 and 2 mg.kg-1, respectively. Curative and protective control was generally good (> 80%), but consistently better after treatment with 500 mg L-1. The influence of number of flooder weirs (1−5) and IMZ solution temperature range of 25–55 °C was determined. A single weir loaded an imazalil residue between 1.00 and 1.50 mg kg-1 regardless of solution temperature. Residue loading was markedly increased at 45 °C with three weirs and 2 mg kg-1 was only exceeded at 55 °C with 3–5 weirs. Curative green mould control (≈90%) and sporulation inhibition (>50%) were more consistent and better when imazalil was applied at 45 °C or higher using three or more weirs. The heated flooder application is an effective alternative to the fungicide dip tank.