Emergence of resistance to succinate dehydrogenase inhibitor fungicides in Pyrenophora teres f. teres and P. teres f. maculata in Australia

The 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 agent Pyrenophora teres f. sp. teres [Ptt]) and spot-form net blotch (SFNB, causal agent Pyrenophora teres f. 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 the Sdh complex 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 in P. teres, are also described. The mutations found in Ptt included H134R and S135R in SdhC; and H134Y and D145G in SdhD; the SdhC-H134R mutation was the most frequently observed. In Ptm, the mutations found included H277L in SdhB; S73P, N75S, H134R and S135R in SdhC; and D145G in SdhD; the SdhC-N75S mutation was the most common. These mutations were correlated with reduced in vitro SDHI 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 the SdhC-H134R and SdhC-S135R mutations in Ptt, and with the SdhB-H277L, SdhC-H134R, and SdhC-S135R mutations in Ptm. Modelling of the P. teres Sdh complex showed that the two novel mutations, H277L in SdhB and H134Y in SdhD, result in a highly altered binding mode and lower binding affinity of the SDHI compound compared to the wild-type.