Dynamics of the response of barley mildew to the use of sterol synthesis inhibitors1

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Abstract
Since 1980, the increase in the use of triazole fungicides to control Erysiphe graminis on barley in the UK has been accompanied by an increase in the frequency of pathogen isolates with different levels of reduced sensitivity to the fungicides. In each year, the rate of change varied, probably in relation to the degree of selection at different times in the host life cycle. In the first years, isolates with reduced sensitivity were less pathogenic than sensitive strains on a range of host cultivars. These differences were less obvious on cultivars with the Mla6 and Mlv resistance genes. More recently, reduced sensitivity has become widespread in the pathogen population and the general pathogenicity of this part of the population has increased. As a result, the yield benefit previously associated with mildew control by the triazoles is becoming severely diminished. This effect is not due primarily to the increase of highly insensitive isolates, but to the high frequency of isolates with intermediate insensitivity that are able to cause epidemic increase of the pathogen soon after fungicide application.

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