Imazalil resistance management for sustainable citrus green mould control: limited options and alternatives

Imazalil (IMZ), a demethylation inhibitor (DMI) fungicide, with excellent curative, protective and anti-sporulant abilities is used by the postharvest citrus industry as the kingpin in green mould (caused by Penicillium digitatum) control strategies, Resistance development against IMZ is incremental and results in partially loss in ability to control green mould, as the major cause of decay losses. For sensitive isolates the effective residue level that will give 50% curative control (ER₅₀C) can be as low as 0.26 mg kg^‑1. In vitro sensitivity (EC₅₀ values) did not always correlate with practical resistance levels (ER₅₀ values): in some cases, resistant isolates could be partly controlled with higher residue levels, while control failure was observed for moderately resistant isolates in other cases. Improved IMZ residue loading is therefore not a sustainable resistance management option. The recent registration and implementation of another DMI, propiconazole (PPZ), for the control of sour rot further compromises the sustainable use of IMZ as cross resistance is possible and since PPZ is less effective against green mould. TBZ and FLU are co-application options, but field resistance of TBZ and relatively poor green mould control by FLU renders them less suitable partners. Pyrimethanil (PYR) stands out as a more suitable alternative to IMZ given very good curative control following dip treatments. When compared to other alternatives PYR was shown to be superior in controlling IMZ resistant isolates. PYR activity in drench and wax application was poor, but very good synergistic activity was seen when PYR was used with IMZ, TBZ and/or GZT. In cases where IMZ resistance is prevalent, inoculum levels should be managed through monitoring and effective sanitation, implementation of optimum fruit quality management, treating of fruit within 24 h after harvest and incorporation of PYR in treatment programs. Market demands limiting the number and level of actives may hamper the sustainable use of IMZ.