Submitted to: Postharvest Biology and Technology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/23/2011
Publication Date: 11/1/2011
Citation: Erasmus, A., Lennox, C., Hennie, J., Smilanick, J.L., Keith, L., Fourie, P. 2011. Imazil residue loading and green mold control in south african citrus pack-houses. Postharvest Biology and Technology. 62(2):193-203. Interpretive Summary: A large portion of citrus fruit will rot after harvest unless actions are taken to retard this process, such as the application of fungicides to the fruit after harvest. We examined the commercial use of one of these fungicides in order to know the amount of the chemical left on the fruit after treatment, then evaluated the impact of pH, temperature, the duration of fungicide application, and the effect of other additives on the final fungicide content of the fruit. Increased retention of the fungicide in the fruit was associated with increasing pH, temperature, and duration of treatment; the inclusion of sodium bicarbonate (baking soda) in the fungicide solution improved its effectiveness but did not alter fungicide retention in the fruit. The practical messages of this study are that with precise control of these parameters the proper and minimal amounts of the fungicide can be applied and the residue predicted with good accuracy, so the fruit will retain their quality during prolonged transport, extended storage, and in the possession of consumers.
Technical Abstract: Imazalil (IMZ) is commonly applied in South African citrus pack-houses for the control of green mould, caused by Penicillium digitatum, yet the disease still causes significant postharvest losses. Maximum residue limit (MRL) for IMZ on citrus fruit is 5 µg.g-1, whereas 2-3 µg.g-1 is regarded as a biologically effective residue level that should at least inhibit green mould sporulation. Standard compliance auditing of residue levels of citrus fruit, however, indicated that fruit from the majority of pack-houses have residues below 1 µg.g-1. Poor disease control from insufficient residue loading might further be compounded by the presence of IMZ-resistant isolates of P. digitatum in pack-houses. This study was conducted to assess the current status of IMZ application in South African pack-houses, to determine the adequate residue levels needed to control green mould and inhibit its sporulation using both IMZ sensitive and resistant isolates, to investigate IMZ application methods and resultant residue levels in commercial citrus pack-houses, and to study optimization of modes of IMZ application in citrus pack-houses. Factors studied were IMZ concentration, application type (spray vs. dip and drench), exposure time, solution temperature and pH, as well as curative and protective control of Penicillium digitatum. The pack-house survey showed that the majority of pack-houses applied IMZ in a sulphate salt formulation through a fungicide dip tank, and loaded an IMZ residue of ˜1 µg.g-1. In dip applications that simulated commercial practices, IMZ had excellent curative and protective activity against Penicillium isolates sensitive to IMZ. However, curative control of IMZ resistant isolates was substantially reduced and protective control was lost, even at twice the recommended concentration, nor was sporulation inhibited. The use of sodium bicarbonate (2%) buffered imazalil sulphate solutions at pH ±8, compared with pH ±3 of the unbuffered solutions, markedly increased IMZ residue loading on Navel and Valencia oranges and improved curative and protective control of IMZ resistant isolates. Exposure time did not affect IMZ residue loading in IMZ sulphate solutions at pH 3, although the MRL was exceeded after 45 s exposure in pH 8 solutions. Imazalil applied through spray or drench application improved residue loading, but green mould control was less effective than dip application.