Identification of Quinone Outside Inhibitor Fungicide-Resistant Isolates of Zymoseptoria tritici from Kentucky Wheat Fields

Soft red winter wheat (Triticum aestivum) production in Kentucky often utilizes no-till farming, a practice that can be favorable for fungal plant pathogens that overwinter in crop residue. One such pathogen, Zymoseptoria tritici, is the causal agent of Septoria tritici blotch and a prominent pathogen of wheat in Kentucky. The limitations of inoculum management and lack of fully resistant wheat cultivars increase the importance of foliar fungicide applications as a tool for management of Septoria tritici blotch. Quinone outside inhibitor (QoI) fungicides (Fungicide Resistance Action Committee, FRAC, Group 11) are one of the main classes of foliar fungicides used in wheat. A single mutation in the fungal cytochrome b gene can confer resistance to this class of fungicides. In this study, 79 Z. tritici isolates collected from 11 Kentucky counties during 2020 were screened for QoI resistance. Amplifying and sequencing a section of the cytochrome b gene spanning a section where mutations can occur that confer QoI resistance, revealed that 19 out of the 79 isolates tested (24.1%) had the G143A mutation. In vitro assays conducted on a small selection of isolates confirmed that isolates with the G143A mutation were resistant to QoI-fungicides. This study is the first to identify isolates of Z. tritici with the G143A mutation in Kentucky. Wheat producers in Kentucky and the surrounding areas should implement management practices that include rotating to nonhost crops, planting wheat cultivars with moderate resistance to Septoria tritici blotch and applying fungicides that contain active ingredients from other FRAC groups.