International Symposium on Cereal Leaf Blights 2019 | University College Dublin, Ireland | 22-24 May 2019
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Multidrug resistance, or MDR, is characterized in Zymoseptoria tritici by low levels of resistance to DMIs, QoIs and SDHIs. The associated mechanism is the overexpression of the membrane transporter MFS1, inducing a more effective efflux of these fungicides outside the fungal cell. Three insertions of nucleotide sequences (types I to III), varying according to their size and position in the promoter of mfs1, are responsible for the overexpression of MFS1 and therefore of the MDR phenotype. This resistance mechanism is independent from the more frequent fungicide target modification mechanism, and in Z. tritici, sexual reproduction allows the combination of these mechanisms. Crosses between isolates carrying either MDR or target site resistance revealed semi-isogenic progeny that we used to characterize resistance. We thus show a significant increase in the levels of resistance to DMIs and SDHIs, compatible with losses in efficacy in practice, when the two mechanisms are combined in the same isolates. Moreover, the in vitro and in planta growth of isolates bearing these resistance alleles, solo or combined, doesn’t seem to be affected, suggesting the maintenance of a fitness identical to that of sensitive individuals or presenting only one resistance mechanism. Field trials, carried out by the “Performance network” in France between 2004 and 2017, identified the programms most likely to be involved in MDR selection. Finally, we discuss management measures to limit the selection of different resistance mechanisms.