Four azoles’ profile in the control of Septoria, yellow rust and brown rust in wheat across Europe

Leaf diseases cause major yield losses in winter wheat every year across Europe. Septoria leaf blotch – STB (Zymoseptoria tritici) is the most serious leaf disease in Northern Europe, but also yellow rust (Puccinia striiformis) and brown rust (Puccinia triticina) are known to cause major problems in some regions and seasons. Problems with fungicide resistance in the populations of Z. tritici have caused concerns for future control options. With the aim of investigating the differences in azole performances against STB, yellow rust and brown rust, 40 field trials were carried out during two seasons (2015 and 2016) in 10 different countries across Europe covering a diversity of climatic zones and agricultural practices. Four single triazoles (epoxiconazole, prothioconazole, tebuconazole and metconazole) and two mixtures of azoles (epoxiconazole + metconazole; prothioconazole + tebuconazole) were tested at full and half rates. Regarding control of yellow rust and brown rust similar control patterns were seen across Europe and treatments with epoxiconazole and tebuconazole provided between 80 and 100% control. In contrast lower levels of control and major variations in azole performances against STB were seen across Europe, with ranking of the azoles tested varying significantly across the continent. Similarly, the CYP51 mutation frequencies varied greatly across Europe with a clear pattern of decreasing frequencies from west to east of all investigated mutations except I381V and A379G. Azoles were most effective against STB when used as mixtures, either as epoxiconazole + metconazole or prothioconazole + tebuconazole. This was especially clear in the western parts of Europe with high frequencies of CYP51 mutations D134G, V136C and S524T in local Z. tritici populations. Effectiveness of all single azoles decreased from 2015 to 2016 except for tebuconazole and azole mixtures, the mixtures providing more robust control across all sites and sensitivities. The average EC50 values for Z. tritici from the trial sites measured for the four azoles showed different levels of cross-resistance and similarly did the efficacy ranking from the azoles. Across all trials full rates of azole mixtures were best at increasing yields, by up to 20%. Single azoles increased yields between 14 and 18%. The greatest yield responses were measured at the sites where yellow rust was the primary disease controlled.