Multiple mechanisms account for resistance to sterol 14α‐demethylation inhibitors in field isolates of Mycosphaerella graminicola

Abstract

Sterol 14α-demethylation inhibitors (DMIs) have been widely used in many European countries to control septoria leaf blotch, which is caused by Mycosphaerella graminicola (Fückel) J Schrot (anamorph Septoria tritici Berk & MA Curtis). However, treatment efficacy has declined, and significant shifts in population susceptibility have occurred in recent years, with the isolation of particularly highly resistant strains from French, English and Irish populations. The present aim was to determine the phenotypic characteristics of these field isolates and to identify the possible resistance mechanisms. Target alteration, linked to 11 possible changes in the gene encoding 14α-demethylase (Cyp51), was the basic resistance mechanism in weakly, moderately and highly resistant strains. Changes in Cyp51 combined with the overexpression of drug efflux transporters probably result in multidrug resistance in some of the most resistant phenotypes. Finally, some moderately or highly resistant isolates were found to harbour an insertion in the Cyp51 promoter and/or new combinations of known mutations in the target gene. An updated overview of M. graminicola field strains displaying low to high resistance to DMIs is provided here. The management of field resistance and efficacy should be adapted to take these findings into account.