Can Venturia inaequalis populations show a reduced sensitivity to a multisite fungicide? The case study of Captan in french orchards

National audience ; Since 2000, the control of apple scab, which is mainly based on chemicals in French orchards, has faced several cases of control failure. One of the causes of this situation could be the emergence of a reduced sensitivity of V. inaequalis to multisite fungicides. As multisite fungicides are not known to induce resistance in fungal pathogens, such a possibility has not been investigated. Between 2002 and 2006, different experiments showed an in vivo reduced efficiency (27.6 to 48 % on incidence and severity of the disease) of Captan for the control of a V. inaequalis population from an orchard in which this fungicide failed to control scab. This efficiency was lower than that for Mancozeb (96.5 to 100 %) on the same population, and lower than the efficiency of Captan on other populations less exposed to the fungicide. The variability in sensitivity to Captan of V. inaequalis strains collected in 5 orchards differently exposed to Captan was assessed in vitro, and a significant difference of ED50 values, which ranged between 5.2 and 51.9 mg/l, was displayed. These results show consistent elements, but not clear evidence of a reduced sensitivity of V. inaequalis to multisite fungicides. They support the need for applied and basic research on this question.