Are insensitivities of Venturia inaequalis to myclobutanil and fenbuconazole correlated?

In the UK, two demethylation inhibitor (DMI) fungicides, myclobutanil and fenbuconazole, are recommended for controlling apple scab. Experiments were conducted to determine whether the sensitivity of V. inaequalis to myclobutanil was correlated with that to fenbuconazole. There was a marked reduction in the sensitivity of V. inaequalis to myclobutanil; averaged ED50 values increased from 0.338 mg L−1 in the baseline population to 2.945 mg L−1 for isolates from a commercial orchard. Overall, the average ED50 value for fenbuconazole was only about 20% of that for myclobutanil for the baseline population. There was an overall significant positive correlation in the fungal sensitivities to myclobutanil and fenbuconazole, but such correlation only accounted for 40% of the total observed variability. In addition, the magnitude of this cross-resistance varied among orchards. The finding is discussed in the context of fungicide deposition in relation to disease control.