Cross-resistance between myclobutanil and tebuconazole and the genetic basis of tebuconazole resistance inVenturia inaequalis

BACKGROUND Myclobutanil is one of the most widely used demethylation inhibitor (DMI) fungicides for the management of apple scab, caused byVenturia inaequalis. Strains ofV. inaequalisresistant to myclobutanil have been reported across the world. Tebuconazole, another DMI fungicide, has been proposed as an alternative to myclobutanil, and the extent of cross-resistance with myclobutanil therefore needs to be evaluated. The sensitivity to tebuconazole and myclobutanil of a total of 40 isolates was determined. Half the isolates came from an isolated orchard which had never been sprayed with fungicides and half from orchards sprayed regularly with myclobutanil, but still with disease control problems. The progeny of a tebuconazole resistant (R) x sensitive (S)V. inaequaliscross were analyzed in order to improve understanding of the genetic control of tebuconazole sensitivity. RESULTS There is cross-resistance between myclobutanil and tebuconazole (r= 0.91;P< 0.001). Sensitivity to tebuconazole of the progeny of a R x S cross varied quantitatively in a pattern which implied at least two gene loci differing between the parental strains. In addition, the asymmetric distribution of the sensitivity in the progeny implied possible epistatic effects. CONCLUSION Resistance to myclobutanil and tebuconazole is strongly correlated. At least two genes are involved in the control of tebuconazole resistance inV. inaequalis.