Decreasing azole sensitivity of Z. tritici in Europe contributes to reduced and varying field efficacy

Septoria tritici blotch (STB;Zymoseptoria tritici) is the most important leaf disease of wheat in Northern and Western Europe. The problem of fungicide resistance inZ. triticipopulations is challenging future control options. In order to investigate differences in azole performances against STB, 55 field trials were carried out during four seasons (2015-2018). These trials were undertaken in ten different countries across Europe covering a diversity of climatic zones and agricultural practices. During all four seasons, four single azoles (epoxiconazole, prothioconazole, tebuconazole and metconazole) were tested. Increasing variability in the performances of these azoles against STB was observed across Europe. The efficacy of the tested azoles varied considerably across the continent and between countries. The shifts in disease control from these commonly used azoles were confirmed by increasing EC(50)values for epoxiconazole, prothioconazole-desthio and metconazole. The sensitivity towards tebuconazole remained relatively low across the four years. The frequencies ofCYP51mutations varied substantially across Europe, with a clear pattern of significantly decreasing frequencies of D134G, V136A and S524T in the localZ. triticipopulations from west to east. In contrast, no major differences were seen forCYP51mutations V136C, A379 and I381V. The four azoles showed different levels of cross-resistance, which again depended on specificCYP51mutations. Across the four seasons, the single azoles increased the yields between 9 and 11% on average.

Automated summary

The study found significant differences in the efficacy of different fungicides against Septoria tritici blotch across Europe, indicating a challenge in controlling this disease. Variability in sensitivity of Z. tritici to different azoles further complicates disease control measures. The research also revealed varying levels of cross-resistance among different azoles, depending on specific CYP51 mutations.