Large-scale study reveals regional fungicide applications as a major determinant of resistance evolution in the wheat pathogen Zymoseptoria tritici in France.

Research rationale: In modern cropping systems, the quasi-systematic use of plant protection products selects for resistance in pest populations. The emergence and evolution of this adaptive trait threaten treatment efficacy. We identified determinants of fungicide resistance evolution and quantified their effects at a large spatiotemporal scale. Methods: We focused on Zymoseptoria tritici, which causes leaf blotch in wheat. Phenotypes of qualitative or quantitative resistance to various fungicides were monitored annually, from 2004 to 2017, at about 70 sites throughout France. We modelled changes in resistance frequency with regional anti-Septoria fungicide use, yield losses due to the disease and the regional area under organic wheat. Key results: We found that the major driver of resistance dynamics was fungicide use at the regional scale. We estimated its effect on the increase in resistance and apparent relative fitness of each resistance phenotype. The predictions of the model replicated the spatiotemporal patterns of resistance observed in field populations fairly accurately. Main conclusion: The evolution of fungicide resistance is determined at very large scales. There is therefore a need for the collective management of resistance, which could be guided by the results of studies like this.