SENSITIVITY TO FENARIMOL OF VENTURIA-INAE QUALIS STRAINS FROM FRENCH ORCHARDS

The sensitivity to fenarimol of 150 Venturia inaequalis strains from 5 French orchards was tested by an agar plug assay with increasing rate s of fungicide. The 5 orchards had received 0 to 50 treatments with st erol demethylation inhibitor fungicides (DMI) during the 5 years prece ding the sampling. The results showed a great variability of the conce ntrations inhibiting 50% and 90% of the mycelial growth (ED 50 and ED 90) of the strains. The sensitivity of the strains from each orchard w as related to the number of treatments with DMI fungicides. The non-tr eated reference orchard had a sensitive population, significantly diff erent from the others; the mean ED 50 value of its strains was 0.14 mg /l of fenarimol. The treated orchards had mean ED 50 values ranging fr om 0.18 to 2.63 mg/l. Their populations varied from nearly sensitive ( for an orchard which had received 18 treatments) to entirely resistant to fenarimol (for an orchard which had received 50 treatments). The d ifferent sensitivities of the V inaequalis populations of these orchar ds are discussed in relation to the disease incidence on the year of s ampling and the fungicide schedule. The consequences of these results on the strategy for the use of DMI fungicides against apple scab are d iscussed.