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
