The present study provides insights into evolving dynamics of SDHI fungicide resistance in Zymoseptoria tritici population from Estonia. While sensitivity to bixafen and fluxapyroxad remained stable from 2020 to 2023, a clear shift towards decreased sensitivity was observed in 2024, as evidenced by increased median EC50 values and the rapid increase in the prevalence of Z. tritici isolates with effective alterations in SDH-C and SDH-B subunits. The most widespread alteration in Estonian Z. tritici isolates was C-N86S, with the highest frequency of 24.4% in 2023. The three other alterations in SDH-C first appeared in 2022 and their frequency increased up to 17.1% for C-T79N, 9.9% for C-H152R, and 3.9% for C-W80S in 2024. The alteration with the greatest impact on the sensitivity to fluxapyroxad and bixafen was C-H152R. In SDH-B, alterations B-T268I, B-I269V and B-N225T were detected at low frequencies. Also, five novel double mutants combining SDH-B and SDH-C alterations were isolated from Estonian field population. Comprehensive longitudinal surveillance of this nature is critical, as resistant strains, once established, may gain a selective advantage and rapidly dominate local populations, thereby undermining effective disease control strategies. The findings corroborate existing evidence from Europe that, despite the essential role of SDHI fungicides in managing STB, increasing adaptation by pathogen populations is reducing their effectiveness. The increasing frequencies of key SDH gene alterations associated with SDHI resistance highlight a growing threat to the sustained efficacy of these compounds.
Andresen et al. (Tue,) studied this question.