Mutations in target genes of succinate dehydrogenase inhibitors and demethylation inhibitors in Ramularia collo-cygni in Europe

Ramularia collo-cygni is a devastating plant pathogen of barley with an increasing importance in Europe and several other barley growing regions worldwide. R. collo-cygni is a pathogen with potential to adapt rapidly to environmental changes and is classified as a high-risk phytopathogen with respect to the evolution of fungicide resistance. Quinone-outside inhibitors (QoIs), demethylation inhibitors (DMIs) and succinate dehydrogenase inhibitors (SDHIs) represent three major fungicide classes that are frequently used to control a broad range of relevant cereal pathogens. However, in recent years R. collo-cygni has acquired resistance to QoIs; in addition reduced efficacy of SDHI- and DMI-containing products has been observed. In this study, we have investigated the frequency of SDHI- and DMI-adapted isolates in a European population of R. collo-cygni and evaluated the underlying resistance mechanisms towards both fungicide classes. Several mutations in the target genes of SDHIs were detected in the population of R. collo-cygni under investigation (B-H266Y/R, B-T267I, B-I268V, C-N87S, C-H146R, C-H153R and some others) with increasing frequencies since 2014. SDHI resistance in R. collo-cygni is mainly driven by both the presence of mutations and the high frequency of such mutations in the population. Additionally, DMI-adapted isolates of R. collo-cygni were found at a high frequency in the countries under investigation. Fifteen different Cyp51 haplotypes were detected in the set of isolates from 2009 to 2017. The most frequent haplotype in 2017 was C1 haplotype, which comprises three mutations in Cyp51 and shows a substantial increase in EC50 values to DMIs compared with other isolates. As R. collo-cygni has adapted to several groups of fungicides in many regions in Western Europe, future control of this highly diverse and adaptable pathogen must focus on new molecules, choosing resistant varieties and the improvement in seed hygiene standards.