Resistance of Zymoseptoria tritici to azoxystrobin and epoxiconazole in the lower North Island of New Zealand

In response to a suspected fungicide resistance problem in the spring of 2013, 23 isolates of Zymoseptoria tritici (also known as Mycosphaerella graminicola) were collected from wheat fields showing symptoms of speckled leaf blotch around the lower North Island. EC50 values for the collected isolates and three sensitive isolates against rates of azoxystrobin, epoxiconazole and isopyrazam were determined by growth in microplates as measured by light absorbance. Furthermore, greenhouse inoculations on fungicide-sprayed plants were conducted with selected isolates. Resistance to azoxystrobin appears widespread, as does reduced sensitivity to epoxiconazole. Most growers supplying the samples were using epoxiconazole as their standard cereal fungicide but at lower than label rates. A combination of conducive weather, low fungicide rates, resistance to azoxystrobin and reduced sensitivity to epoxiconazole is likely to have contributed to the disease epidemic.