Maintaining the effectiveness of DMI fungicides in cereal disease control strategies

Abstract

A series of field trials carried out over three years 1997 to 2000 in different parts of the UK, formed the basis of an evaluation of  fungicide mixtures as anti-resistance strategies to maintain effectiveness of DMI (triazole) fungicides against Septoria disease, caused by Mycosphaerella graminicola (=Septoria tritici). Trials focussed on mixtures of either epoxiconazole (Opus) or tebuconazole (Folicur) with azoxystrobin (Amistar), at doses ranging from recommended  rate to ¼ field rate. Despite the wide range in sensitivity to DMIs in M. graminicola, monitoring sensitivity of isolates from these trial sites to several DMIs showed no changes in sensitivity during the three years, regardless of the rate of fungicide applied.  Epoxiconazole was the best fungicide when used alone, but adding a strobilurin improved disease control, and yields, especially when mixed with less effective DMIs than epoxiconazole. Reduced rates of both mixture partners were sufficient to achieve these benefits.

In a separate study, contour maps linking samples with similar sensitivities to cyproconazole, flusilazole and flutriafol highlighted the lack of uniformity of DMI sensitivities within a M. graminicola population. Despite an underlying similarity between these three DMIs, there were significant differences in sensitivity levels between them, and this probably reflected severaldifferent mechanisms contributing to sensitivity levels. Analysing these data using the geo-statistical technique, 'kriging', indicated that samples should be collected at least 20 metres apart to ensure they were independent of each other. This provided a guide to sampling protocols  needed to define DMI sensitivity distributions, but after selection with epoxiconazole too few lesions were available to construct maps with the same precision as generated prior to spraying. Although there was no evidence of any change in the contour maps following selection, this illustrates a feasible approach to monitoring the effects of anti-resistance strategies on a field scale, without replicating the treatments.

Applying PCR diagnostic techniques to follow the development of M. graminicola revealed that azoxystrobin had significant curative as well as preventative effects, and these lasted for up to one month after treatment. By slowing down development of the pathogen,  azoxystrobin enhanced the curative activity of the DMI partner. This not only ensures that DMI/strobilurin mixtures are a good anti-resistance strategy, but the prolonged action of the mixture means that spray intervals can be increased, such that a two-spray programme should be sufficient to adequately control Septoria.