Submitted to: Postharvest Biology and Technology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/4/2012
Publication Date: 1/1/2013
Citation: Erasmus, A., Lennox, C.L., Smilanick, J.L., Lesar, K., Fourie, P.H. 2013. Imazalil residue loading on citrus fruit as affected by formulation, solution pH and exposure time in aqueous dip treatments. Postharvest Biology and Technology. 77(1):43-49. Interpretive Summary: Post harvest rot of citrus results in significant economic losses to the industry. To control these losses the industry relys on the application of fungicides. We evaluated the use of one of these fungicides (imazalil) in order to maximize its effectiveness while minimizing the amount of the chemical remaining in the fruit. The benefit of this work is to extend the storage life of the fruit, which improves financial returns to growers and extends the period fresh citrus fruit are available to consumers and the life of the fruit after purchase.
Technical 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, i.e. <2 µg.g-1, which is commonly the case in South African packhouses. This will result in loss of control and sporulation inhibition of green mould. IMZ formulation [IMZ sulphate and emulsifiable concentrate (EC)], solution pH (IMZ sulphate at 500 µg.mL-1 buffered with NaHCO3 or NaOH to pH 6 and 8) and exposure time (15 to 540 s) were investigated in order to improve IMZ residue loading and the concomitant green mould control on Clementine mandarin, ‘Eureka’ lemon, 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 5.0 µg.g-1 was exceeded following dip treatment in the IMZ EC (after 75 s exposure time), and IMZ sulphate at pH 8 using NaHCO3 (77 s) or NaOH (89 s) as buffer. The MRL was exceeded after 161 s in IMZ sulphate solutions buffered at pH 6 with either NaHCO3 or NaOH. This study attempted to model an IMZ residue loading curve from which bench mark residue levels can be derived for the control of sensitive and resistant isolates of Penicillium digitatum. From these curves the benchmark residue level for 95% control of a sensitive and resistant isolate was predicted to be 0.81 and 2.64 µg.g-1, respectively. Residue loading can be improved by adjusting the pH of an IMZ sulphate solution or by using the IMZ EC formulation, but exposure time should be restricted so as not to exceed the MRL. The practical relevance of IMZ resistance was shown in this work and benchmark IMZ residue levels for control of green mould were validated.