Reduced field efficacy and sensitivity of demethylation inhibitors in the Danish and Swedish Zymoseptoria tritici populations

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Zymoseptoria tritici causes septoria tritici blotch (STB), the predominant fungal disease in wheat in Denmark and Sweden. Disease control is highly reliant on fungicides in the group of demethylation inhibitors (DMI). The use of DMIs has increased steadily since their introduction in the 1970s. Epoxiconazole and prothioconazole were the most widely used active ingredients in the last ten years. The goal of this investigation was to survey the resistance development of Z. tritici towards these two compounds. In total, EC50 values were determined for 3472 Z. tritici isolates from 2012 to 2019. Also, the field performance of the most used DMI compounds was tested in field trials. EC50 values of epoxiconazole and prothioconazole increased in the testing period. A significant shift was observed for epoxiconazole in 2016 and again 2018 with average EC50 values >1 ppm in Denmark. In Sweden, average EC50 values for epoxiconazole reached 1 ppm in 2017. The sensitivity towards prothioconazole remained stable at a high level. Following the decline in sensitivity in vitro, field efficacies of epoxiconazole and prothioconazole decreased from 80 - 90% to 30 - 40% in Denmark and Sweden. Currently, the Danish and Swedish Z. tritici populations are highly adapted to epoxiconazole and prothioconazole. At the same time, a recovery in the sensitivity of tebuconazole and metconazole is observed. Our studies reinforce the view that the principles of anti-resistance management should be applied upon the launch of a new active ingredient to prolong the effective life of both the old and new products.