Biological Methodologies on SDHI Fungicides to Assess Reductions of Sensitivity and Activity on Venturia inaequalis and Cross-Resistance Tests

Biological methodologies (in vitro and in vivo) were tested on Venturia inaequalis to assay sensitivity of populations treated with the SDHI penthiopyrad, fluxapyroxad and fluopyram that showed a field poor control in an experimental Centre. In comparison, sensible populations were tested. In vitro trials, the poorly controlled populations presented moderate increases of EC50 and RF values in all three years in which tests were carried out. The in vivo trials in glasshouse showed that the activity of SDHI fungicides on field poorly controlled populations was highly reduced. These populations can be defined as “resistant” as the high loss of sensitivity and activity were repeatedly assessed in field and glasshouse. Trials showed that these reductions were manifested by curative applications and not by preventive ones, as already ascertained for V. inaequalis populations resistant to AP and DMI fungicides. The cross-resistance of resistant populations to the three SDHIs was highly positive among them. In bibliography, there are only researches of cross-resistance among SDHIs starting from isolates resistant to boscalid, with results of different sensitivities and cross-resistance. These results with populations of V. inaequalis resistant to the three SDHI offer practical evaluations in cross-resistance.