Understanding evolution and selection of azole resistance mechanisms in UK populations of Mycosphaerella graminicola

Abstract

Septoria tritici blotch (STB), caused by the fungus Mycosphaerella graminicola, is the most important foliar disease of wheat in the UK. The pathogen has shown an ability to develop resistance to fungicides routinely used for its control. Survey and fungicide performance data over the past 15 years show a gradual erosion of azole efficacy against STB. A key question now is whether this erosion is likely to continue or even accelerate with the increased use of azole fungicides to control Septoria and how can the risk be reduced? The project addressed this question by seeking to understand the genetic basis and evolution of resistance to azoles in the UK M. graminicola population using a combination of molecular genetic and field-based approaches, with the aim of using this knowledge to develop robust strategies to maintain the effectiveness of this important group of chemicals in crop protection.

Detailed analysis of M. graminicola strains isolated from azole-treated and untreated plots during the course of the project revealed a large number of variants of the sterol 14?-demethylase (CYP51) target protein. M. graminicola isolates carrying these variants, with a range of amino acid substitutions and a deletion of two amino acids in at least 15 different positions in the protein, have reduced sensitivity levels to azoles in comparison with 'wild-type' isolates carrying an unchanged CYP51 protein. Pyrosequencing assays were developed to detect the underlying genetic changes in the CYP51 gene.

The majority of strains of most UK populations sampled since 2006 are represented by four different CYP51 variants. Strains carrying these variants are less sensitive in vitro and in planta to prochloraz and tebuconazole, depending on the presence of amino acid substitutions V136A and I381V respectively. There were no or only slight differences in sensitivities to epoxiconazole and prothioconazole for these variants. Continued monitoring will be needed as new CYP51 variants were detected in 2009 and 2010.

The chemical diversity of azoles was explored to improve STB control and to provide a wider spectrum of disease management. This is also reflected by the recent successful uptake of highly effective formulated azole mixtures on the UK market.