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Septoria tritici blotch (STB), caused by the pathogen Zymoseptoria tritici, is one of the most devastating diseases of wheat worldwide. Foliar disease management relies mainly on the application of fungicides. The demethylation inhibitors (DMIs) have been and continue to be one of the core components in STB control. A continuous shift of the Z. tritici population toward reduced sensitivity to DMIs is known for several years. This shift is mainly associated with the accumulation of target site mutations in the CYP51 gene, resulting in distinct CYP51 haplotypes, but also with CYP51 overexpression and increased efflux activity. The occurrence and distribution of the different DMI adaptation mechanisms were monitored in 2021 and compared in relation to previously published results from a survey in 2016 and 2017. In 2021, 54 different CYP51 haplotypes were identified across Europe; however, 15 CYP51 haplotypes represented 90% of the population. The frequency of different CYP51 haplotypes is dependent and variable between countries, with more complex CYP51 haplotypes found in Ireland and the United Kingdom carrying mostly the alteration S524T. In Germany, France, and Poland, a diverse pattern of moderately adapted CYP51 haplotypes together with complex haplotypes carrying S524T was identified, whereas the least complex haplotypes were found in the Eastern- and Southern-European countries. A comparison in the frequency of the different haplotypes for Germany, Poland, France, Ireland and the United Kingdom from 2016 to 2021 revealed a decrease in the frequency of less and moderately adapted haplotypes and an increase in the frequency of complex CYP51 haplotypes carrying the S524T alteration indicating an ongoing shift in DMI sensitivity. An increase in the occurrence and further distribution of CYP51 overexpression based on the ~ 120 bp promotor insert was found from 2017 to 2021. The frequency was shown to vary between countries, with higher frequencies found for example in Poland, Germany and France compared to the United Kingdom and Ireland. An increase in the frequency of increased efflux activity based on the 519 bp insert in the MFS1 promotor was mainly found in Ireland in 2021. In other countries, a stable situation or slight increase in the frequency of this insert was detected. In those countries, the frequency of increased efflux was found at lower levels comparable to findings from 2017. Altogether, the results imply that underlying mechanisms for DMI adaptation are getting more complex and diverse. Regular monitoring to understand the DMI adaptation mechanisms is crucial to implement appropriate anti-resistance strategies in order to slow down further shifts in sensitivity. [ABSTRACT FROM AUTHOR]