Baseline sensitivity and biochemical responses of Valsa mali to propamidine

In the current study, baseline sensitivity of Valsa mali to propamidine was determined using 80 strains collected from apple orchards in Shaanxi Province, China. The median effective concentration (EC₅₀) values for propamidine inhibiting mycelial growth ranged from 0.086 to 0.852 μg/mL, with a mean of 0.405 ± 0.137 μg/mL. After treated with propamidine, mycelia were contorted with an increased number of branches, loss of fruiting body production, and decreased cell membrane permeability. Moreover, the enzyme activities of the complexes I, II, IV and ATPase in the mitochondrial respiratory chain were increased significantly, while the enzyme activities of complexes III decreased. Importantly, both on detached leaves and branches of apple trees, propamidine applied at 100 μg/mL exhibited over 75% protective and curative efficacies, which were even better than the efficacies obtained by carbendazim at the same concentration. These results indicated that propamidine could be used as an alternative compound in controlling Valsa canker and mitochondrial respiratory chains might be correlated with the action mode of propamidine. This study encourages further investigation for the action mechanism of propamidine against plant pathogens and the information could be valuable for synthesis of new antifungal drugs with novel modes of action.