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

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 similar to 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.