This was suspected, as even though the yield response to the strobilurin mixtures was still good in a 2013 South Otago trial, the control of STB was relatively poor. Samples from other regions in New Zealand tested by Plant & Food Research and Massey University and verified at Rothamsted Research in the UK have found that this resistance is due to the mutation G143, which renders strobilurins completely ineffective against STB.

Unlike reduced sensitivity in triazoles, this is a single point mutation, which confers complete resistance. The same mutation and resultant resistance has been selected for in European STB populations through the use of strobilurin fungicides.

STB resistance to strobilurins means these fungicides will be ineffective against STB. It is suggested that when selecting fungicides for controlling STB in wheat, select one of the SDHIs e.g. Adexar, Aviator Xpro (co�formulated with triazole) or Seguris flexi plus triazole.