Optimising imazalil application in citrus packhouses

CONTACT DETAILS: +27 21 808 3721 / +27 83 290 2048 / phf@cri.co.za
DURATION: Two years
LEAD INSTITUTIONS: Citrus Research International and Stellenbosch University (Department of Plant Pathology)
BENEFICIARY: The citrus fruit industry

The South African citrus industry suffers huge economic losses every year due to post-harvest decay. The situation is caused by post-harvest pathogens, mainly by Penicillium digitatum, which leads to green mould of citrus fruit. Imazalil is an excellent fungicide for the control of green mould – if it is applied correctly and in adequate concentrations. However, suboptimal imazalil residue loading in pack houses leads to inadequate; poor green mould control, and necessitated research to optimise imazalil application.

Investigating commercial imazalil application in citrus pack houses

Dr Paul Fourie of Citrus Research International led the project, and is based at the Department of Plant Pathology of Stellenbosch University as a senior researcher. The study was focused on reducing economic losses due to green mould by optimising imazalil application methods on citrus.

Some of the research objectives were to conduct imazalil residue analyses and biological efficiency trials to develop industry guidelines; to establish the residue levels required to control green mould, and to inhibit sporulation of imazalil resistant and sensitive strains.

A survey of South African commercial citrus pack houses

The project commenced with a countrywide pack house survey to investigate imazalil application methods and the resultant residue levels.

  • The fungicide bath was identified as the most common (78.4%) imazalil application method in South African pack houses – either alone (38%) or in combination with wax (38%), or with drench and wax (3%).
  • In fungicide baths, imazalil residue loading on fruit was influenced by the interaction of factors such as bath temperature, pH, cultivar and the duration of exposure.
  • The most effective imazalil residue levels were obtained when fungicide bath application was followed by wax application.
  • However, insufficient residue loading [below 1 ppm (parts per million)] on citrus was generally found – the maximum residue limit is set at 5 ppm, but 2-3 ppm is required to control green mould or at least inhibit sporulation.
  • The presence of imazalil resistant strains of P. digitatum, coupled with insufficient residue loading, aggravates the problem of poor control of green mould.

Laboratory trials on imazalil residue loading

Export quality Clementine soft citrus, ‘Eureka’ lemons, navel and Valencia oranges were used to study the effects of various factors, such as cultivar, bath temperature, duration of exposure, and pH of the solution on residue levels and green mould control.

  • Imazalil residue levels of 1 ppm provided curative and protective control of fruit inoculated with imazalil sensitive strains, but did not control imazalil resistant strains.
  • Resistant strains required residue levels higher than 2 ppm to provide some level of control, albeit not complete, and did not inhibit pathogen sporulation.
  • Sodium carbonate is a GRAS (generally regarded as safe) chemical. It helped to improve imazalil residue loading considerably when it was used to buffer fungicide baths at higher pH levels, compared with unbuffered baths (pH 3).

Benefits to industry

  • The information generated by this project is used to improve industry guidelines, specifically post-harvest handling of citrus and diligent management of fungicide application.
  • This industry-funded project is ongoing and continues to investigate the optimised application of imazalil and other citrus post-harvest fungicides.