Moderate and High Propiconazole Resistance Characterized in the Citrus Postharvest Sour Rot Pathogen Geotrichum citri-aurantii Is Based on Point Mutations in CYP51A but Not CYP51B.

Moderate (MR) and high resistance (HR) in Geotrichum citri-aurantii to propiconazole (PPZ) have been detected in California citrus packinghouses. Fitness cost and resistance mechanisms were characterized in this study. No fitness cost was identified in resistant isolates, and they were highly virulent and competitive in the presence of sensitive (S) isolates. F1 ascospore progeny from crosses between MR or HR isolates and an S isolate mostly segregated in a 1:1 ratio of S to R isolates, indicating involvement of a single gene in resistance. Resistance was associated with point mutations in GcaCYP51A resulting in Y143F and L389V amino acid substitutions in HR isolates and an A125V substitution in MR isolates. There were no sequence differences in the GcaCYP51B paralog between S and MR isolates, but missense and silent mutations were identified in HR isolates. Segregation of GcaCYP51B alleles was observed in crosses between S and HR isolates: some PPZ-S progeny carried the GcaCYP51B-HR allele, whereas some PPZ-HR isolates carried the GcaCYP51B-S allele. Therefore, mutations in GcaCYP51B have no major role in PPZ resistance. The direct involvement of mutations in GcaCYP51A in resistance was demonstrated by transformation of a PPZ-S isolate with cloned alleles from MR or HR isolates, where transformants exhibited the expected resistance phenotype. Transformants carrying the GcaCYP51B allele of HR isolates, however, remained sensitive. No differences between S and R phenotypes were detected in the promoter sequences of GcaCYP51A or GcaCYP51B. Our data indicate that PPZ resistance in G. citri-aurantii is solely determined by point mutations in GcaCYP51A.