Optimisation of postharvest fungicide application in citrus packhouses: low-tech but high impact

Postharvest decay leads to significant economic losses, considering that these losses typically occur at the end of the value chain. In most citrus fresh fruit producing industries, postharvest decay is managed through postharvest application of fungicides and through cold storage and transit conditions. In South Africa, fungicides are applied by means of various systems, including pre-packhouse drench, inline dip, spray or flooder applications or through incorporation into wax coatings. However, specification of these application systems vary significantly and treatment protocols were not specified. Since 2008, our research group studied the optimisation of postharvest fungicide application, specifically focused on citrus green mold control. The project was initiated by a comprehensive survey of application systems, which indicated that pre-packhouse drench, in-line fungicide dip and wax coating application were most commonly applied. Drench application was generally inferior to dip application, especially when using fungicides alone; mixtures of fungicides demonstrated improved and synergistic action and was recommended to also manage fungicide resistance development. Dip application showed excellent curative control, but relatively poor protective control. Curative control was not affected following the residue stripping effect of post-dip brushing. Imazalil application in wax coatings demonstrated very good protective control, but poor curative control. This supported the recommendation of a double application of imazalil in dip and subsequently in wax coating application. More recently, we investigated imazalil application using a flooder applicator, which demonstrated excellent curative as well as protective control and sporulation inhibition. In South Africa, the imazalil sulfate formulation is applied most commonly, and the pronounced pH and temperature effects of application solutions were modeled to allow prediction of residue loading and green mold control. Fungicide resistance led to control failure and integrated programmes for fungicide application were recommended to South African packhouses.