The status of fungicide resistance inSouth American Zymoseptoria triticipopulations

Abstract

Septoria tritici blotch, causal agent Zymoseptoria tritici, is one of the most important wheat diseases and has a worldwide distribution. Fungicides are essential for the maintenance of reliable wheat yields in areas under high incidence of Z. tritici. Most modern fungicides have single-site modes of action (MoA). Prolonged use of the same MoA to control the pathogen population leads to the selection of isolates with reduced sensitivity. The emergence of resistance to these fungicides can, therefore, compromise disease control and reduce yields. This research aimed to determine whether South American Z. tritici field populations are evolving reduced fungicide sensitivity and to characterize underlying resistance mechanisms in comparison to worldwide field populations of Z. tritici. The results from in vitro sensitivity assays indicate that there is reduced sensitivity to methyl benzimidazole carbamate (MBC), azoles, quinone outside inhibitor (QoI) and succinate dehydrogenase inhibitor (SDHI) fungicides in South American isolates of Z. tritici. Isolates with highly resistant MBC and QoI phenotypes carried the target site amino acid substitutions E198A and G143A, respectively. Target site mutations conferring SDHI resistance were not identified, but some isolates had fluopyram resistance phenotypes possible associated with the presence of a previously-reported additional paralogue of succinate dehydrogenase subunit C in worldwide populations of Z. tritici. Reduced azole sensitivity was conferred by different combinations of amino acid substitutions in the target site sterol 14α-demethylase (CYP51). Population genetic analyses with microsatellite markers and sequence data of the azole target site identified that the European CYP51 encoding gene has introgressed into South American populations of Z. tritici. The results of this research indicate that South American populations of Z. tritici have the potential to evolve levels of fungicide resistance similar to those observed in Europe, which could compromise the future management of Septoria tritici blotch in South America.