Baseline and temporal changes in sensitivity of Zymoseptoria tritici isolates to benzovindiflupyr in Oregon, USA, and cross-sensitivity to other SDHI fungicides

Authors: HAGERTY, CHRISTINA - Oregon State University; KLEIN, ANN - Former ARS Employee; Reardon, Catherine - Kate; KROESE, DUNCAN - Oregon State University; GRABER, KACI - Oregon State University; MELLE, CAROLINE - Former ARS Employee; MUNDT, CHRISTOPHER - Oregon State University

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/24/2020
Publication Date: 6/30/2020
Citation: Hagerty, C.H., Klein, A.M., Reardon, C.L., Kroese, D.R., Graber, K.R., Melle, C.J., Mundt, C.C. 2020. Baseline and temporal changes in sensitivity of Zymoseptoria tritici isolates to benzovindiflupyr in Oregon, USA, and cross-sensitivity to other SDHI fungicides. Plant Disease. Available: https://doi.org/10.1094/PDIS-10-19-2125-RE.
DOI: https://doi.org/10.1094/PDIS-10-19-2125-RE

Interpretive Summary: Zymoseptoria tritici is the causal agent of septoria tritici blotch (STB), a disease of wheat (Triticum aestivum) that results in significant yield loss worldwide. In the Willamette Valley of Oregon, fungicide use has increased over the last decade due to relatively high grain prices and reduced fungicide cost. Succinate dehydrogenase inhibitor (SDHI) fungicides (Group 7) were not widely used to control STB in the Willamette Valley until 2016. Field isolates of Z. tritici collected in the Willamette Valley at dates spanning the introduction of SDHI (2015-2017) were screened for resistance to four SDHI active ingredients: benzovindiflupyr, pethiopyrad, fluxapyroxad, and fluindapyr. Fungicide resistance was determined based on the fungicide concentration which fungal growth is decreased by 50%. Resistance to benzovidiflupyr increased significantly following the adoption of SDHI fungicides in Oregon. Additionally, moderate cross-resistance was observed between pethiopyrad and benzovindiflupyr, and weak cross-resistance between pethiopyrad and fluxapyroxad. No cross-resistance was observed with Fluindapyr, which has not yet been labeled in the region. The results document an increase in SDHI resistance in Z. tritici isolates following the introduction of the active ingredients to the Willamette Valley. The cross-resistance observed between SDHI active ingredients highlights the notion that careful consideration is required to manage fungicide resistance and suggests that within-group rotation is insufficient for resistance management.

Technical Abstract: Zymoseptoria tritici is the causal agent of septoria tritici blotch (STB), a disease of wheat (Triticum aestivum) that results in significant yield loss worldwide. Z. tritici’s life cycle, reproductive system, effective population size, and gene flow put it at high likelihood of developing fungicide resistance. In the Willamette Valley of Oregon, fungicide use has increased over the last decade due to relatively high grain prices and reduced fungicide cost. Succinate dehydrogenase inhibitor (SDHI) fungicides (Group 7) were not widely used to control STB in the Willamette Valley until 2016. Field isolates of Z. tritici collected in the Willamette Valley at dates spanning the introduction of SDHI (2015-2017) were screened for resistance to four SDHI active ingredients: benzovindiflupyr, pethiopyrad, fluxapyroxad, and fluindapyr. Fungicide resistance was determined by an increase in the EC50 values, or the fungicide concentration at which fungal growth is decreased by 50%.The benzovidiflupyr EC50 values increased significantly following the adoption of SDHI fungicides in Oregon (p < 0.0001). Additionally, significant cross- resistance among SDHI active ingredients was also observed with a moderate and significant relationship between pethiopyrad and benzovindiflupyr (p = 0.0002), and a weak relationship between pethiopyrad and fluxapyroxad (p = 0.0482). No cross-resistance was observed with Fluindapyr, which has not yet been labeled in the region. The results document an increase in SDHI resistance in Z. tritici isolates following the introduction of the active ingredients to the Willamette Valley. The cross-resistance observed between SDHI active ingredients highlights the notion that careful consideration is required to manage fungicide resistance and suggests that within-group rotation is insufficient for resistance management.