Sequence conservation in the mitochondrial cytochrome b gene and lack of G143A QoI resistance allele in a global sample of Rhynchosporium secalis

Barley scald caused by Rhynchosporium secalis is often controlled by fungicides, most recently by those in the strobilurin-based (QoI) class. Since the launch of QoIs in 1996 a range of important plant pathogens, including Blumeria graminis, Mycosphaerella fijiensis and Plasmopara viticola, developed resistance. Present monitoring data indicate that R. secalis populations remain sensitive. The primary molecular mechanism of QoI resistance in several fungi is a point mutation at codon 143 in the mitochondrial-encoded cytochrome b gene (cytb), which causes the substitution of glycine by alanine (G143A). We characterised the cytb gene of R. secalis, assessed the intraspecific and interspecific sequence diversity, developed a PCR-RFLP diagnostic tool to detect the most common allele associated with QoI resistance, screened a global collection of 841 R. secalis isolates for this allele and tested a representative sample of isolates for QoI resistance in vitro. The results indicated a high degree of conservation for the cytb gene at both intra- and interspecific levels, and complete QoI sensitivity in all R. secalis populations tested.