Abstract/Summary

DMI (Demethylation Inhibitor) fungicides are the family of fungicides most commonly used for the post infection control of apple scab disease, which is produced by the fungus Venturia inaequalis. One of the most widely used DMI fungicides in the UK and many other countries is myclobutanil. Strains resistant to myclobutanil have been reported across the world, prompting a need to find new products effective against the disease and to improve the existing knowledge about the genetics behind the resistance and the evolution process. Attempts to design a new microplate-based assay to test for fungicide resistance were unsuccessful. A total of 40 isolates coming from two sources (a baseline orchard which had never been sprayed with fungicides and a collection of orchards with disease control problems thought to be a result from cases of fungicide resistance) were tested for resistance to fungicides myclobutanil and tebuconazole, a recently introduced DMI fungicide. Results confirm the trend of increased resistance to myclobutanil observed in other countries. Cross-resistance was discovered between the two chemicals, which lowers the prospects of tebuconazole to be used as a substitute to myclobutanil. Sequencing of the target CYP51A1 gene did not find any mutations linked to resistance in the gene sequence. An analysis of the progeny of a RxS cross for sensitivity to tebuconazole revealed quantitative control of the sensitivity, involving at least two genes, in the studied cross and possibly epistatic effects, reflected by the asymmetric distribution of the sensitivity in the progeny. Computer modelling of the evolution of the resistance helped identify some factors that can influence the rate of evolution. Most of the factors tested had a small effect on the evolution (epistasis and inclusion of overwintered conidia in the primary inoculum). The biggest effects were for fungicide coverage and fungicides with a strong post-symptomatic antisporulant activity.