Influence of pH and temperature in a heated flooder application on imazalil residue loading, citrus green mould infection and sporulation inhibition

Green mould (caused by Penicillium digitatum) is the largest contributor to loss due to postharvest decay of citrus fruit. Imazalil (IMZ) in the IMZ sulphate formulation is the most important fungicide in green mould management in citrus packhouses. Studies have highlighted the importance of pH and temperature of IMZ sulphate solutions, as well as the exposure time of fruit to the solution, but were focussed on dip application as this was the most common IMZ application method. The heated flooder (in-line drench or cascade) has widely replaced the use of a dip or bath for IMZ application in citrus packhouses globally; however, studies on its optimal use and efficacy were needed. Variables that were studied included the effects of pH (3, 4, 5 and 6), the temperature of the solution (45, 55 and 65 °C at pH 6), and concentration (250 or 500 mg L⁻¹) in a time of 8 s. Imazalil residue loading models were developed for the effects of pH and temperature. Imazalil residue levels loaded on Satsuma mandarin, lemon, navel and Valencia orange fruit increased as pH, temperature range and concentration were increased, and were generally in the range between 0.4 and 3.0 mg kg⁻¹. The South African Maximum Residue Limit (MRL) of 5.0 mg kg⁻¹ was often exceeded at a pH of 6 at temperatures of 55 and 65 °C. At residue levels below the MRL, IMZ application using the heated flooder offered excellent curative action on 24-hour-old infections, excellent protective control, as well as sporulation inhibition. Sporulation inhibition of P. digitatum from green mould lesions was modelled on residue levels, and 90 % inhibition was obtained at residue levels of 1.84–2.47 mg kg⁻¹.