Sensitivity, resistance stability, and cross-resistance of Plasmopara viticola to four different fungicides

Baseline sensitivity to fungicides was determined in 105 Plasmopara viticola isolates using the floating leaf disk test. The mean EC50 values were 0.372 ± 0.104, 0.604 ± 0.215, 0.306 ± 0.101, and 0.922 ± 0.209 μg mL−1 for fluxapyroxad, benthiavalicarb-isopropyl, ametoctradin, and famoxadone, respectively, which we regarded as the baseline sensitivity to the four fungicides. Resistant mutants can be generated using a median effective concentration or a minimum inhibitory concentration to fluxapyroxad, benthiavalicarb-isopropyl, ametoctradin, and famoxadone respectively, but the resistance stability of resistant mutants from the minimum inhibitory concentration (MIC) was significantly lower than that from the median effective concentration (EC50). In brief, high resistant mutants obtained was at the higher concentration, the resistance stability of the mutants recovered was more quickly. But lower resistant mutants obtained was at the lower concentration, the resistance stability of the mutants recovered was slower. Even if the resistance level of the mutants is low, it is more capable of stably heritable. These data indicate that the highest dosage is not used, resistance will still develop. In a correlation analysis, no cross-resistance to each other of these four fungicides was observed.