Propiconazole sensitivity of

Green mold and sour rot, caused by Penicillium digitatum and Geotrichum citri-aurantii, 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 P. digitatum 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 G. citri-aurantii and P. digitatum from South African packhouses, a juice factory and orchards from Eastern Cape and Western Cape citrus production regions. Isolates of G. citri-aurantii and P. digitatum from three unsprayed orchards (biological production) were used to establish the baseline sensitivity in 56 isolates of each pathogen. The sensitivity of 182 G. citri-aurantii and 157 P. digitatum 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 G. citri-aurantii the mean baseline EC50 was determined at 0.53 mg/l (0.12-1.39 mg/l) and for P. digitatum 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 P. digitatum isolates were recovered (22.2%), while only one packhouse was identified to harbour propiconazole resistant G. citri-aurantii isolates. The resistant G. citri-aurantii 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 P. digitatum isolate (EC50 = 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.