Propiconazole Sensitivity of <i>Geotrichum citri-aurantii</i> and <i>Penicillium digitatum</i> Isolates in the Western and Eastern Cape Provinces of South Africa

Green mold and sour rot, caused by <i>Penicillium digitatum</i> and <i>Geotrichum citri-aurantii</i>, respectively, are the two most important citrus postharvest diseases in South Africa (SA). Management relies 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 <i>P</i>. <i>digitatum</i> to the widely used imazalil, since these two fungicides have the same mode of action (inhibition of the 14α demethylase during fungal sterol biosynthesis). 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 <i>G</i>. <i>citri-aurantii</i> and <i>P</i>. <i>digitatum</i> from South African packhouses, a juice factory, and orchards from Eastern Cape and Western Cape citrus production regions. Isolates of <i>G</i>. <i>citri-aurantii</i> and <i>P</i>. <i>digitatum</i> from three unsprayed orchards (biological production) were used to establish the baseline sensitivity in 56 isolates of each pathogen. Mycelial growth tests of 182 <i>G</i>. <i>citri-aurantii</i> and 157 <i>P</i>. <i>digitatum</i> isolates from commercial fruit were conducted at discriminatory doses of 0.9 mg/liter of propiconazole and/or 0.5 mg/liter of propiconazole. Furthermore, inoculated citrus fruit was used to establish propiconazole efficacy in planta. For <i>G</i>. <i>citri-aurantii</i>, the mean baseline EC₅₀ was determined at 0.53 mg/liter (0.12 to 1.39 mg/liter) and for <i>P</i>. <i>digitatum</i> at 0.10 mg/liter (0.04 to 0.27) from untreated South African orchards. After 3 years of postharvest fungicide use, a high number of propiconazole-resistant <i>P</i>. <i>digitatum</i> isolates were recovered (22.2%), while only one packhouse was identified to harbor propiconazole-resistant <i>G</i>. <i>citri-aurantii</i> isolates. The resistant <i>G</i>. <i>citri-aurantii</i> isolate showed a higher mean incidence of causing sour rot at the registered propiconazole concentration (24.8%) compared with the sensitive isolate (7.0%), while a moderately sensitive <i>P</i>. <i>digitatum</i> isolate (EC₅₀ = 0.147 mg/liter) 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.