Anti-resistance strategies for fungicides against wheat pathogen Zymoseptoria tritici with focus on DMI fungicides

Septoria tritici blotch (STB) caused by the fungal pathogen Zymoseptoria tritici is a global threat to sustainable wheat production. Applications of fungicide against STB are regarded as an essential means to minimise yield losses, however, fungicide resistance is developing and affecting fungicide efficacy greatly. Only a few fungicide classes are available for STB control. DMI fungicides are seen as the main group, but increasing problems with resistance is challenging their use. In 2015 and 2016, field trials were conducted testing fungicide spray strategies using one, two or three applications, alternations, mixtures of different DMIs and DMI mixed with other modes of action including a SDHI and a multi-site inhibitor. The strategies were tested for disease control, their impact on yield and their effect on resistance build-up, measured as CYP51 alterations responsible for decreased sensitivity and efficacy of DMIs. Strategies consisting of three treatments provided adequate control of STB and significant yield increases. The best results with regard to yield and control were attained by a diversified DMI strategy also including the SDHI boscalid and the multi-site inhibitor folpet. Spraying once or twice lowered selection yet compromised STB control and yield. CYP51 alteration I381V was the most predominated in all samples. Frequencies of alterations D134G, V136A and S524T increased significantly following applications with DMI fungicides. The more diversified a strategy, the less it selected for CYP51 alterations. EC50 values for epoxiconazole showed a tendency to be higher post-treatment in 2015. The results presented in this study encourage the adoption of mixing and alternating fungicides into spray strategies to minimise the risk of resistance build-up and to prolong the effective life of fungicides.