Silent Mutation: Characterization of its Potential as a Mechanism forSterol 14ñ-Demethylase Resistance in Cercospora beticola Field Isolatesfrom the United States

Sterol demethylation inhibitors (DMIs) are considered among the most effective fungicides used to control Cercospora leaf spot (CLS), caused by Cercospora beticola Sacc., in sugar beet. Resistance to DMI fungicides has been reported in the C. beticola population from the United States, but the molecular mechanism is not known. It is considered that genetic changes in the C. beticola 14α-demethylase (CbCyp51) gene may be contributing to DMI resistance. The study investigated a silent mutation (GAG to GAA) at codon 170 as a potential mechanism for C. beticola DMI resistance. The CbCyp51 gene was obtained from DMI-sensitive and -resistant isolates, cloned into a plasmid vector, transformed in an isogenic yeast R-1, and tested for DMI sensitivity. Transformed yeast showed low ED50 values (0.02 - 0.09 μg ml–1) as compared to high ED50 values from C. beticola DMI-resistant isolates (21 - 65 μg ml–1). The finding did not support our hypothesis that a silent mutation in the CbCyp51 gene may be associated with C. beticola DMI resistance. Furthermore, genetic analysis of the CbCyp51 gene found no mutation in 2 C. beticola DMI-resistant isolates from the Central High Plains. Further studies will be required to investigate additional mechanisms which have been associated with DMI resistance in fungi. Thus, we could not develop a molecularbased assay for the rapid detection of C. beticola DMI resistance, because no mutation was found in the CbCyp51 gene. Currently, fungicide sensitivity assay could be the best method screen for C. beticola DMI resistance.