Emergence of resistance to succinate dehydrogenase inhibitor fungicides inPyrenophora teresf.teresandP. teresf.maculatain Australia

Abstract

AbstractThe net blotches are among the most economically significant diseases of barley worldwide. There are two forms of the disease: net-form net-blotch (NFNB, causal agentPyrenophora teresf. sp.teres[Ptt]) and spot-form net blotch (SFNB, causal agentPyrenophora teresf. sp.maculata[Ptm]). Alongside varietal choice and cultural practices, fungicides form an important part of the regime for net blotch control. The succinate dehydrogenase inhibitors (SDHIs) are a key class of fungicides used in net blotch management. However, resistance to this group of compounds has emerged in the net blotches in recent years. Here, we describe the first cases of resistance to SDHIs in Australian populations of net blotches. This study was prompted by reports of field failures of SDHI fungicides in controlling NFNB in South Australia and SFNB in Western Australia. Target site mutations in theSdhcomplex genes, previously associated with reduced sensitivity in European net blotch populations, were found in Australian isolates, and two mutations which have not been previously observed inP. teres, are also described. The mutations found inPttincluded H134R and S135R inSdhC; and H134Y and D145G inSdhD; theSdhC-H134R mutation was the most frequently observed. InPtm, the mutations found included H277L inSdhB; S73P, N75S, H134R and S135R inSdhC; and D145G inSdhD; theSdhC-N75S mutation was the most common. These mutations were correlated with reducedin vitroSDHI fungicide sensitivity by microtiter assay. The highest resistance factors to fluxapyroxad and bixafen, the most important SDHI fungicides for net blotch control in Australia, were associated with theSdhC-H134R andSdhC-S135R mutations inPtt, and with theSdhB-H277L,SdhC-H134R, andSdhC-S135R mutations inPtm. Modelling of theP. teresSdh complex showed that the two novel mutations, H277L inSdhBand H134Y inSdhD, result in a highly altered binding mode and lower binding affinity of the SDHI compound compared to the wild-type.