Sensitivity of Venturia inaequalis to Trifloxystrobin and Difenoconazole in Uruguay

The sensitivity of Venturia inaequalis to trifloxystrobin and difenoconazole was studied in Uruguay. Populations of V. inaequalis were collected from apple orchards with different histories of trifloxystrobin use. Sensitivity of populations to trifloxystrobin was analysed using a method for testing spore germination published by FRAC, using a discriminatory concentration of 2.0 mg a.i./l. Resistance to trifloxystrobin was widespread in the region of commercial apple production with resistance detected in all orchards examined, the incidences ranging from 3% to 95%. The highest frequencies were found in orchards with the most intensive use of Quinone outside inhibitors (Q_oI) fungicides. Sensitivities of isolates of V. inaequalis to difenoconazole were assessed at five concentrations using a mycelial growth assay on isolates (33 isolates per orchard) from one non‐commercial (baseline orchard) and two commercial orchards having differing histories of difenoconazole use. Populations in both commercial orchards exhibited reduced sensitivities to difenoconazole compared to the baseline orchard. Resistance factor (RF) values of 6.6 and 11.74 were measured in the orchards with moderate (up to 4 sprays per season) and intensive use (more than 5 sprays per season) of difenoconazole, respectively. A single‐assessment concentration (SAC) was identified for assessing difenoconazole sensitivity of V. inaequalis isolates by fitting linear regressions between log₁₀ EC₅₀ and relative growth (RG) of the isolates at each fungicide concentration testing, and comparing the goodness‐of‐fit of the regression lines. Comparable results were obtained based on EC₅₀ values and RG values at a SAC of 0.05 mg of active ingredient per litre (a.i. per l). Populations from both commercial orchards differed from the baseline population, in that isolates with RG ≥70 were present at substantial levels in the former but absent from the latter.