Fungicide spraying intensity in the field drives the selection of amino acid alteration conferring resistance in Zymoseptoria tritici

Zymoseptoria tritici, the causal agent of septoria tritici blotch (STB), is one of the most destructive fungal pathogens on wheat. To control the disease and limit yield losses, farmers rely mainly on uni-site fungicides belonging to two modes of action: demethylase inhibitors (DMIs) and succinate dehydrogenase inhibitors (SDHIs). DMIs have been used extensively for STB control for the last 40 years, resulting in a gradual sensitivity shift, which has been observed worldwide, due to the stepwise accumulation of resistance mutations in the CYP51 gene. SDHI fungicides have been used for just over ten years to control STB. Several mutations conferring resistance to this group have been detected in several countries to varying degrees. Fungicide resistance in Sweden and Denmark has generally been less prominent because of the lower fungicide input and restricted fungicide availability. Microtiter plate assays and allele-specific qPCRs (targeting C-T79N and C-N86S in SdhC and S524T in CYP51 ) confirmed that the Danish and Swedish Z. tritici populations still remain sensitive to SDHI and DMI fungicides, but that the frequency of resistant isolates is increasing. The target site alterations were further used as markers to evaluate the selection potential imposed by different control strategies in the field. Results indicated that efficacious SDHIs, including fluxapyroxad, imposed a higher selection of resistance mutations compared to less effective SDHIs. Treatments including prothioconazole increased the frequency of S524T more than the ones containing mefentrifluconazole. Splitting the dose into two treatments also selected for more resistant isolates compared to single-treatment strategies. This investigation also tackles the impacts of current anti-resistance strategies, including the use of lower doses, limiting the number of treatments, and mixing and alternating fungicides of a different mode of action under field conditions. These elements are essential to prolong the efficacy of current and future fungicides and delay resistance development in the Z. tritici population.