First detection of a SDH variant with reduced SDHI sensitivity in Phakopsora pachyrhizi

Abstract

Strains of Phakopsora pachyrhizi with a lower SDHI sensitivity have been found for the first time in monitoring studies in the season 2015–2016. Such strains needed to be characterized with respect to the underlying mechanism and for the development of a molecular genetic monitoring method. A mutation in the SdhC gene, which causes the amino acid substitution C-I86F, was identified in strains with a lower SDHI sensitivity. A species-specific real-time PCR for the detection and quantification of alleles which cause C-I86F exchange in diseased plant tissue was developed. The frequency of C-I86F was highest when samples were first selected on SDHI-treated detached leaves, and there was ~50% mutated allele. The SDHIs included in this study were affected to a greater or lesser extent by C-I86F in detached leaf assays. This is the first report of a causal resistance mechanism toward SDHIs in P. pachyrhizi. The practical relevance of the C-I86F exchange on the field efficacy of SDHI containing fungicides needs further investigation.