Propiconazole Resistance Phenotypes in <i>Geotrichum candidum</i> from South Carolina Peaches Are Linked to Point Mutations in the <i>GcCYP51B</i> Gene

<i>Geotrichum candidum</i> Link (1809) is a yeast-like fungus that causes sour rot of peach (<i>Prunus persica</i>). Outbreaks of the disease have occurred since 2021 in peach fruit kept in cold storage despite postharvest treatments with propiconazole at a commercial farm in South Carolina (SC). A total of 58 isolates, 40 from symptomatic fruit from cold storage in Saluda County (SC packing house isolates), 11 from three SC orchards in Saluda County, Spartanburg County, and Pickens County (SC non-packing house isolates), and 7 California (CA) isolates (at least 3 from packing houses) were evaluated for propiconazole sensitivity. Mycelial growth assays revealed that six of seven CA isolates had the lowest fungicide concentration causing inhibition of mycelial growth by 50% (EC₅₀) values and were considered sensitive (S) to propiconazole with an average EC₅₀ value of 0.02 μg/ml and minimum inhibitory concentration (MIC) values of >1 to <3 μg/ml. Isolate 02J018 from CA and all SC non-packing house isolates were considered reduced-sensitive (RS) to propiconazole with an average EC₅₀ value of 0.33 μg/ml and MIC values of >10 to <30 μg/ml. SC packing house isolates were considered resistant (R) to propiconazole and had an average EC₅₀ value of 3.55 μg/ml and MIC values of >300 μg/ml. Two <i>CYP51</i> genes, <i>GcCYP51A</i> and <i>GcCYP51B</i>, encoding two demethylase inhibitor (DMI) target enzyme 14α-demethylases were identified, sequenced, and characterized. Two <i>GcCYP51A</i> and three <i>GcCYP51B</i> variants were found. Although both <i>GcCYP51A</i> variants were linked to S isolates, the <i>GcCYP51B2</i> variant possessing the mutation Y143F was found in RS, and the <i>GcCYP51B3</i> variant possessing Y143F, E126K, and G460S mutations was identified in R isolates. The Y143F and G460S mutations had been associated with DMI fungicide resistance in other plant pathogens. No increased constitutive expression of <i>GcCYP51A</i> or <i>GcCYP51B</i> was observed in RS or R isolates. Detached fruit assays revealed that label rates of propiconazole controlled sour rot caused by S and RS but not R isolates. Our results suggest that sour rot outbreaks in an SC packing house were linked to target gene-induced propiconazole resistance in <i>G. candidum</i>.