Identification and Characterization of Geotrichum candidum Isolates from Peach with Reduced Sensitivity to Propiconazole

Geotrichum candidum Link (1809) is a yeast-like fungus that causes sour rot of stone fruits, including peach (Prunus persica). The disease caused damage between 2021 and 2023 to the peach crop kept in cold storage at a commercial farm in South Carolina despite preharvest and postharvest applications of fungicides. Isolates from 41 symptomatic fruit, from a South Carolina (SC) commercial packing house, as well as other isolates obtained from South Carolina were identified to be G. candidum based on species-specific PCR amplifications of the β tubulin gene. Commonly used postharvest fungicide fludioxonil had no effect on G. candidum mycelial growth or germination and revealed minimum inhibitory concentration (MIC) values for mycelial growth and sporulation greater than 500 µg/ml. Isolates from a South Carolina commercial packing house (SC commercial packing house), from an orchard adjacent to the commercial packing house (SC commercial field), and from a 2004 collection (SC historic isolates) had EC 50 values for propiconazole ranging from 1.91 to 7.65 µg/ml, 5.14 to 7.9 µg/ml, and 4.34 to 5.54 µg/ml, respectively. In contrast, California G. candidum isolates had EC 50 values ranging from 0.05 to 0.22 µg/ml. MIC values for California isolates were \u3e5 andµg/ml, for SC commercial field isolates \u3e50 andµg/ml, and for SC commercial packing house isolates \u3e500 µg/ml. Cloning and analysis of CYP51 gene sequences revealed two homologous genes CYP51A and CYP51B. Only CYP51B from SC commercial packing house isolates revealed mutations leading to G460S and E126K amino acid changes. Detached fruit assays revealed that label rates of propiconazole did not control the mutant G. candidum isolates. Resistance to propiconazole was stable after 1 year of cold storage and 10 consecutive transfers on PDA medium and no fitness penalties were apparent for G. candidum mutants. While the CYP51B mutations were likely responsible for the control failure observed in the SC commercial packing house, additional mechanisms such as target gene over expression may exist that afford South Carolina isolates higher propiconazole-tolerance compared to California isolates