Sensitivity variation and cross-resistance of Zymoseptoria tritici to azole fungicides in North America

Though delayed relative to Europe, fungicide resistance in Zymoseptoria tritici is now present in North America. We assessed azole fungicide sensitivity and cross-resistance in Zymoseptoria tritici isolates from the Willamette Valley of western Oregon, USA. Isolates were grown in the presence of 12 serial dilutions of four different azole fungicides (cyproconazole, propiconazole, tebuconazole, and prothioconazole) and their EC50 values calculated. Mean sensitivity of Z. tritici to the four fungicides differed by three orders of magnitude. Significant correlation coefficients ranging from to 0.42 to 0.75 were found among the three triazole fungicides (cyproconazole, propiconazole, and tebuconazole). Prothioconazole, a triazolinthione and more recently introduced fungicide in Oregon, showed evidence of weaker cross-resistance with the three triazoles (r ranging from 0.26 to 0.42). Our results suggest that, in the early stages of fungicide evolution, growers may be able to conserve azole fungicides through conscientious and carefully selected fungicide applications and management practices.