Propiconazole resistance phenotypes in Geotrichum candidum from South Carolina peaches are linked to point mutations in GcCYP51B gene.

Geotrichum candidum Link (1809) is a yeast-like fungus that causes sour rot of peach (Prunus persica). Outbreaks of the disease have occurred since 2021 in peach fruit kept in cold storage despite post-harvest 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 6 of 7 CA isolates had the lowest 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 >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 >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 >300 µg/ml. Two CYP51 genes, GcCYP51A and GcCYP51B, encoding two demethylase inhibitor (DMI) target enzyme 14α-demethylases were identified, sequenced, and characterized. Two GcCYP51A and three GcCYP51B variants were found. While both GcCYP51A variants were linked to S isolates, the GcCYP51B2 variant possessing the mutation Y143F was found in RS, and the GcCYP51B3 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 GcCYP51A or GcCYP51B 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 a SC packing house were linked to target gene-induced propiconazole resistance in G. candidum.