Will Triazoles Still Be of Importance in Disease Control of <i>Zymoseptoria tritici</i> in the Future?

Septoria tritici blotch (STB), caused by <i>Zymoseptoria tritici</i>, is one of the most important foliar wheat diseases worldwide. Current control strategies of STB rely mainly on fungicides, whereby triazoles (demethylation inhibitors; DMIs) have been the backbone in the control of <i>Z. tritici</i> in the last decades. However, in recent years a gradual loss of sensitivity of <i>Z. tritici</i> to several active ingredients of the triazole group has been reported in several European wheat-growing areas. Nevertheless, a new triazole fungicide, namely, mefentrifluconazole, has recently become available in disease management of STB, which belongs to a completely new triazole subclass, the so-called isopropanol triazoles. In this study, the trend in sensitivity development of <i>Z. tritici</i> towards older triazoles (tebuconazole, prothioconazole, and propiconazole) and the new isopropanol triazole mefentrifluconazole was determined in microtiter assays using <i>Z. tritici</i> field populations isolated in 1999, 2009, 2014, and 2020 in a high-disease-pressure and high-fungicide-input area in northern Germany in order to investigate whether the loss of sensitivity of <i>Z. tritici</i> to older triazoles also applies to mefentrifluconazole. For the three triazole fungicides tebuconazole, prothioconazole and propiconazole, a significant shift towards decreasing sensitivity of <i>Z. tritici</i> field populations was observed from 1999 to 2020, whereas the efficacy of mefentrifluconazole in reducing the in vitro fungal growth by 50% (EC₅₀) remained unchanged over the investigated period, demonstrating a stable sensitivity of <i>Z. tritici</i> towards mefentrifluconazole. Although older triazoles are suffering from a loss of sensitivity of <i>Z. tritici</i> field populations due to the selection and spread of less triazole sensitive strains within the <i>Z. tritici</i> population, the efficacy of the new triazole mefentrifluconazole with its unique isopropanol unit was not affected by these changes within the <i>Z. tritici</i> population. Thus, the introduction of such new molecular units could also represent an important contribution for older groups of active ingredients, which previously suffered from a loss of sensitivity.