Distribution of propiconazole-resistant Geotrichum candidum in peach orchards with or without cull fruit amendments

Resistance to propiconazole in G. candidum was reported previously in isolates collected from peaches after cold storage, but the origin of resistance was unclear. If resistance had been generated and selected in the packinghouse with postharvest propiconazole drenches, we would expect to find resistance in the sour rot pathogen only in orchards that had received cull fruit returned to the orchard floor from the packinghouse. In this study, 70 G. candidum isolates were collected from 7 orchards that had received cull fruit in the past from a packinghouse with documented resistance in G. candidum in stored fruit and six orchards that had never received cull fruit. Species identification was confirmed through species-specific PCR. Results showed a similar prevalence of sensitive, reduced-sensitive, and resistant isolates in cull and non-cull orchards. Sequencing of the GcCYP51B gene identified previously reported genotypes leading to E126K, Y143F, and G460S mutations and a newly described genotype consisting of a set of combined mutations, Y143F and L389V. Further investigation into alternate genetic determinants of resistance revealed that overexpression of the GcCYP51A or GcCYP51B genes was not involved in resistance to propiconazole. Our findings suggest that propiconazole resistance in G. candidum, first found in a peach packinghouse, originated in the field, likely from decades of demethylation inhibitor (DMI) fungicide applications to control preharvest diseases of peaches.