No abstract

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.

No abstract

The effect of the quinone outside inhibitors (QoI) azoxystrobin and pyraclostrobin on yields of winter wheat where QoI resistant Mycosphaerella graminicola isolates were dominant was investigated in field trials in 2006 and 2007. Pyraclostrobin significantly increased yields by 1AE57 t ha )1 in 2006 and 0AE89 t ha )1 in 2007 when compared to the untreated controls, while azoxystrobin only provided a significant increase of 1AE28 t ha )1 in 2006. These yield increases were associated with reduction in septoria tritici blotch (STB) development as determined by weekly disease assessments over a 7 week interval. The effect of pyraclostrobin on STB was studied in controlled environment experiments using wheat seedlings inoculated with individual M. graminicola isolates. Pyraclostrobin significantly reduced STB symptoms by up to 62%, whether applied 48 h pre-or post-inoculation with resistant M. graminicola isolates containing the cytochrome b mutation G143A. Extremely limited disease (<1%) was observed on similarly treated seedlings inoculated with an intermediately resistant isolate containing the cytochrome b mutation F129L, while no disease was observed on seedlings inoculated with a wild-type isolate. Germination studies of pycnidiospores of M. graminicola on water agar amended with azoxystrobin or pyraclostrobin showed that neither fungicide inhibited germination of spores of resistant isolates containing the mutation G143A. However, pyraclostrobin significantly reduced germ tube length by up to 46% when compared with the untreated controls. Although the QoIs can no longer be relied upon to provide effective M. graminicola control, this study provides an insight into why QoIs still provide limited STB disease control and yield increases even in situations of high QoI resistance.

No abstract