Pyrrolnitrin in Strain AFS009 Metabolites Reduces Constitutive and Demethylation Inhibitor-Induced Gene Expression in .

Brown rot, caused by , is one of the most economically important diseases of peach. Demethylation inhibitor (DMI) fungicides play an important part in managing brown rot in the Southeastern United States, but over the last 20 years, reduced efficacy to DMIs has been reported in field isolates overexpressing the DMI target enzyme encoding the gene. Metabolites of the biocontrol agent (BCA) strain AFS009 suppressed the gene in sensitive and resistant isolates previously, but it is not known what molecule was responsible. The goals of this study were to determine the presence and role of pyrrolnitrin (PRN), a common metabolite of and chemical analog to fludioxonil with antifungal activity, in the suppression of the gene and to investigate whether expression can also be suppressed by (Theia). High-performance liquid chromatography detected PRN at 1.75 μg/mg in metabolites formulated as Howler EVO (Howler). PRN at 0.1 μg/ml, fludioxonil at 0.1 μg/ml, and Howler applied at a dose that contained 0.1 μg/ml PRN significantly reduced the gene expression at similar levels in DMI-resistant isolates. Furthermore, expression in DMI-sensitive and three DMI-resistant isolates treated with Howler (88.1 μg/ml), Theia (209.5 μg/ml), propiconazole (0.3 μg/ml), or the mixture of either Howler or Theia + propiconazole revealed that Howler significantly reduced the target gene expression in two of three sensitive and all three resistant isolates. On the other hand, Theia showed no suppressive effect and even increased the gene expression level in two of three resistant isolates. In detached fruit assays on apple with a DMI-resistant isolate, only the mixture of Howler + 50 μg/ml propiconazole resulted in synergism. The results indicate that suppression of target gene is BCA dependent and can be induced by PRN.