Identification of Isolates from Soybean with Resistance to Quinone Outside Inhibitor Fungicides.

Brown spot, caused by , is a common foliar disease of soybean (). Applications of fungicide products that contain quinone outside inhibitor (QoI) active ingredients to soybean fields have contributed to the selection and development of QoI-resistant populations of . We investigated the molecular mechanisms of QoI-resistance in these populations through targeted analysis of the cytochrome gene. Isolates of collected from several soybean fields over different seasons varied in sensitivity to QoI fungicides. Characterization of the cytochrome gene revealed a mutation that changed an amino acid from glycine to alanine at codon 143 - one that is generally associated with QoI fungicide resistances. A PCR assay was developed that allowed successful discrimination of QoI-sensitive and -resistant isolates based on the G143A mutation. Results of this study demonstrated that 47.5% of isolates tested were resistant to QoI fungicides. Accurate monitoring of this mutation will help slow the spread of QoI resistance and will be important for fungicide resistant management in this pathosystem.