Fungicide Sensitivity Shifting of Zymoseptoria tritici in the Finnish-Baltic Region and a Novel Insertion in the MFS1 Promoter

Septoria tritici blotch (STB) is caused by the ascomycete <i>Zymoseptoria tritici</i> and one of the predominating diseases in wheat (<i>Triticum aestivum</i>) in Europe. The control of STB is highly reliant on frequent fungicide applications. The primary objective of this study was to assess sensitivity levels of <i>Z. tritici</i> to different fungicide groups. The fungicides included in this study were epoxiconazole, prothioconazole-desthio, tebuconazole, and fluxapyroxad. A panel of 63 isolates from Estonia, Latvia, and Lithuania, and 10 isolates from Finland were tested. Fungicide sensitivity testing was carried out as a bioassay analyzing single pycnidium isolates on different fungicide concentrations. The average EC₅₀ value in Baltic countries and Finland to epoxiconazole was high ranging from 1.04 to 2.19 ppm. For prothioconazole-desthio and tebuconazole, EC₅₀ varied from 0.01 to 0.24 ppm, and 1.25 to 18.23 ppm, respectively. The average EC₅₀ value for fluxapyroxad varied from 0.07 to 0.33 ppm. To explain the range of sensitivity, the samples were analyzed for <i>CYP51</i> and <i>Sdh</i> mutations, as well as <i>cytb</i> G143A, <i>CYP51</i> overexpression, and multidrug resistance (MDR). Frequencies of <i>ZtCYP51</i> mutations D134G, V136A/C, A379G, I381V, and S524T in the Finnish-Baltic region were lower than in other European countries, but have increased compared to previous years. The frequency of <i>cytb</i> G143A conferring strobilurin resistance also augmented to 50-70% in the <i>Z. tritici</i> populations from Estonia, Finland, Latvia, and Lithuania. No <i>Sdh</i> mutations were found in this study, and neither strains of MDR phenotypes. However, we found a strain harboring a previously unknown transposon insertion in the promoter of the <i>MFS1</i> gene, involved in drug efflux and multi-drug resistance. This new insert, however, does not confer an MDR phenotype to the strain.