Propiconazole sensitivity of and isolates in the Western and Eastern Cape provinces of South Africa.

Green mold and sour rot, caused by and , respectively, are the two most important citrus postharvest diseases in South Africa (SA), which rely on postharvest fungicide application, although other practices such as minimizing injuries, using sanitizer washes, and managing temperatures of harvested fruit significantly contribute to minimizing losses. Propiconazole, a demethylation inhibitor (DMI) fungicide was registered in SA for sour rot control in 2015 and showed efficacy against green mold, but its use poses a risk of cross-resistance in to the widely used imazalil, since these two fungicides have the same mode of action. Thus, the aim of this study was to determine propiconazole baseline sensitivity from organic orchard populations, verify pathogen species identity and assess propiconazole sensitivity of and from South African packhouses, a juice factory and orchards from Eastern Cape and Western Cape citrus production regions. Isolates of and from three unsprayed orchards (biological production) were used to establish the baseline sensitivity in 56 isolates of each pathogen. The sensitivity of 182 and 157 isolates to propiconazole was assessed at discriminatory doses of 0.9 and/or 0.5 mg/l, respectively. Furthermore, inoculated citrus fruit was used to establish propiconazole efficacy in planta. For the mean baseline EC was determined at 0.53 mg/l (0.12-1.39 mg/l) and for at 0.10 mg/l (0.04-0.27), from untreated South African orchards. After three years of postharvest fungicide use, a high number of propiconazole-resistant isolates were recovered (22.2%), while only one packhouse was identified to harbour propiconazole resistant isolates. The resistant isolate showed a higher mean incidence causing sour rot at the registered propiconazole concentration (24.8 %) compared to the sensitive isolate (7.0 %); while a moderately sensitive isolate (EC = 0.147 mg/L) resulted in 17.0 % green mold incidence. It is important that the development of fungicide resistance be monitored, particularly following the intensive use of DMIs such as imazalil, so that fungicide resistance management strategies are put in place to prevent losing the use of the available fungicides.