Baseline Sensitivity of Leptosphaeria maculans to Succinate Dehydrogenase Inhibitor (SDHI) Fungicides and Development of Molecular Markers for Future Monitoring

Succinate dehydrogenase inhibitor (SDHI) fungicides are widely used in Australia for the control of blackleg disease (caused by Leptosphaeria maculans, also called Plenodomus lingam). Populations of L. maculans are highly variable and therefore at risk of evolving fungicide resistance. The baseline sensitivities of L. maculans isolates towards the SDHI fungicides pydiflumetofen and bixafen were determined through in vitro mycelial growth assays, and the mean EC50s were found to be 4.89 and 2.71 ng mL−1, respectively. L. maculans populations were also screened against three commercial SDHI fungicides, Saltro®, ILeVO®, and Aviator®, using an in planta assay to reveal very low levels of resistance. Nineteen of these ascospore populations from 2022 were analysed in a deep amplicon sequencing (DAS) assay and showed no mutations in the genes likely to be associated with resistance to SDHI chemistries. This study establishes baseline sensitivities of L. maculans isolates towards commonly used SDHI fungicides, importantly before and during the introduction of these new chemistries for blackleg control, and outlines monitoring techniques to allow timely identification of resistance if it evolves.