Fungicides should be used to the extent required to minimize economic costs of disease in a given field in a given season. The maximum number of treatments and maximum dose per treatment are set by fungicide manufacturers and regulators at a level that provides effective control under high disease pressure. Lower doses are economically optimal under low or moderate disease pressure, or where other control measures such as resistant cultivars constrain epidemics. Farmers in many countries often apply reduced doses, although they may still apply higher doses than the optimum to insure against losses in high disease seasons. Evidence supports reducing the number of treatments and reducing the applied dose to slow the evolution of fungicide resistance. The continuing research challenge is to improve prediction of future disease damage and account for the combined effect of integrated control measures to estimate the optimum number of treatments and the optimum dose needed to minimize economic costs. The theory for optimizing dose is well developed but requires translation into decision tools because the current basis for farmers’ dose decisions is unclear.

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 EC₅₀values 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.