Over the 2016-17 season there were reports of difficulty controlling Ramularia leaf spot in barley crops. Recent laboratory tests carried out by Plant and Food Research have identified Ramularia collo-cygni (Rcc) isolates from Canterbury and South Otago with low sensitivity to the succinate dehydrogenase inhibitor (SDHI) fungicides in microplate assays. Three mutations which may impact the performance of SDHI fungicides have been confirmed. What impact these mutations will have on the field performance of this group of chemicals is yet to be determined, with further testing ongoing.
Resistance to strobilurins has also been confirmed in New Zealand Rcc field isolates. Strobilurins have not been effective against Ramularia in field trials for several seasons. So far no shifts in sensitivity to the triazole group of fungicides has been detected. Although, now that the efficacy of SDHI fungicides against Ramularia is questionable, other diseases, including scald, net blotch and leaf rust, remain key threats and also need to be controlled. The only guidance that can be given at this time is around the importance of spray timings, fungicide mixing partners, and a maximum of two SDHI applications to the crop in a growing season (see Cereal disease management 2016, FAR Cropping Strategies).
If Ramularia mutants are confirmed to be completely resistant to SDHI fungicides, then a mix of triazole and strobilurin fungicides would become another option but we are not at that point at present.