Imazalil residue loading and green mould control on citrus fruit as affected by formulation, solution pH and exposure time in aqueous dip treatments

Abstract

Green mould, caused by Penicillium digitatum, is responsible for major postharvest fruit losses on the South African fresh citrus export market. Some of these losses as well as fungicide resistance development can be attributed to sub-optimal imazalil (IMZ) residue loading on citrus fruit (<2μgg−1), which is commonly the case in South African packhouses. This will result in loss of control and sporulation inhibition on decayed fruit. IMZ formulation [IMZ sulphate and emulsifiable concentrate (EC)], solution pH (IMZ sulphate at 500μgmL−1 buffered with NaHCO3 or NaOH to pH 6 and 8) and exposure time (15–540s) were investigated in order to improve IMZ residue loading and the green mould control on Clementine mandarin, ‘Eureka’ lemon, and navel and Valencia orange fruit. Exposure time had no significant effect on residue loading in the unbuffered IMZ sulphate solution (pH 3). No differences were observed between the pH buffers used, but residue loading improved with increase in pH. The maximum residue limit (MRL) of 5.0μgg−1 was exceeded following dip treatment in the IMZ EC (after 75s exposure time), and IMZ sulphate at pH 8 using NaHCO3 (77s) or NaOH (89s) as buffer. The MRL was exceeded after 161s in IMZ sulphate solutions buffered at pH 6 with either NaHCO3 or NaOH. An IMZ residue-loading curve was prepared from which residue levels can be predicted for the control of IMZ-sensitive and IMZ-resistant isolates of P. digitatum. From this model the benchmark residue level for 95% control of an IMZ-sensitive isolate and of an IMZ-resistant isolate were predicted to be 0.81 and 2.64μgg−1, respectively. Residue loading can be improved by adjusting the pH level of an IMZ sulphate solution to 6 or by using the IMZ EC formulation, but exposure time should be restricted to 45s so as not to exceed the MRL. Conversely, sufficient exposure time of ≈90s in an unbuffered IMZ sulphate solution (pH 3) will result to improved green mould control, but with residue loading below 2μgg−1. The resistant isolate could not be controlled adequately with residue levels below the MRL, therewith indicating the practical relevance of IMZ resistance.