Effects of a strobilurin-based spray programme on disease control, green leaf area, yield and development of fungicide-resistance in Mycosphaerella graminicola in Northern Ireland

The effects of a range of strobilurin fungicides in programmes with the demethylation-inhibiting (DMI), epoxiconazole, or with chlorothalonil, were assessed in field experiments in Northern Ireland in 2004 and 2005 using the winter wheat cultivars Robigus and Savannah, partially resistant and moderately susceptible, respectively, to septoria tritici blotch. The development of strobilurin resistance was quantified using substitution of glycine by alanine in the target cytochrome b as a marker (G143A). Addition of strobilurins to epoxiconazole did not significantly reduce disease, increase green leaf area (GLA) or increase yield compared with epoxiconazole alone. However, azoxystrobin alone, despite failing to control disease or increase GLA, did increase yield compared with the untreated control. A mixture of azoxystrobin and chlorothalonil gave a significantly higher yield than either azoxystrobin or chlorothalonil alone. Higher frequencies of the strobilurin resistant allele (A143) were always selected for by treatments that included a strobilurin compared with those without. Frequencies of resistance were higher at the end of 2004 than at the start and there was a further increase in 2005. Field surveys of commercial winter wheat crops in 2004 and 2005 confirmed this trend of increasing strobilurin resistance in Mycosphaerella graminicola.