A combination of target-site alterations, overexpression and enhanced efflux activity contribute to reduced azole sensitivity present in the Irish Zymoseptoria tritici population

Abstract

Control of septoria tritici blotch in Irish winter wheat crops has been reliant on the application of azoles fungicides for over a decade. The resulting intensive applications of azole fungicides have placed the Irish Zymoseptoria tritici populations under immense pressures to adapt. Evidence of this adaptation is observed in the decreasing sensitivity of the Irish population to the azole fungicide epoxiconazole, where the mean sensitivity has decreased almost 16x over the period 2006–2015. This decrease in sensitivity has occurred in a stepwise manner, with the sensitive proportion of the population being gradually replaced by those more resistant. The decrease in sensitivity was also accompanied by an increase in frequency of Z. tritici strains with an insert (120, 862 or 866 bp) in their CYP51 promoter region. Neither the 862 bp nor 866 bp inserts impacted the expression of CYP51 in either absence or presence of epoxiconcazole. Sequencing of the CYP51 gene in a sub-sample of the 2015 collection identified 25 different CYP51 haplotypes, the majority of which combined previously identified mutations, including V136A, I381V and S524 T. The presence of the 862 or 866 bp insert in the different strains, whilst not exclusive was strongly associated with specific CYP51 haplotypes. This was also reflected in their sensitivity to a range of azoles, most notably metconazole and tebuconazole. Strains with inserts in their MFS1 gene promoter region and identical in size to those known confer multiple drug resistance (MDR) phenotypes were also identified in the 2015 sub-collection. Although the frequencies of strains with those inserts known to confer moderate-high levels of MDR were extremely low, their presence further highlights the adaptability and continued erosion of azole sensitivity in the Irish Z. tritici population.