Preconditioning Hormesis of the Fungicide Dimethachlone on Mycelial Growth and Aggressiveness of <i>Sclerotinia sclerotiorum</i>

ABSTRACT Preconditioning hormesis refers to the phenomenon that early exposure (preconditioning) to low doses of a toxicant may reduce the potency of the subsequent high doses of the same toxicant. This paper investigated the preconditioning hormetic effects of the dicarboximide fungicide dimethachlone on mycelial growth and aggressiveness of the necrotrophic phytopathogen Sclerotinia sclerotiorum . S. sclerotiorum was cultured on potato dextrose agar (PDA) amended with different doses of dimethachlone. After the preconditioned mycelia were transferred onto PDA amended with a highly challenging concentration of dimethachlone, significant stimulation of mycelial growth was detected. The aggressiveness of the preconditioned mycelia to rapeseed leaves sprayed with a high concentration of dimethachlone was also enhanced, and the maximum stimulation magnitude for lesion size averaged 19.1% for the six isolates tested. Mycelia with the maximum stimulation in the preconditioning treatment were transferred onto PDA amended with challenging concentrations of dimethachlone, and mycelial growth of the second generation displayed a similar stimulation magnitude of around 10% for different challenging concentrations of dimethachlone. After dimethachlone‐preconditioned mycelia were subjected to similar in vitro and in vivo tests with high concentrations of challenging iprodione, carbendazim, and boscalid, only iprodione induced significant growth and aggressiveness stimulations. These results indicate that preconditioning hormesis could occur for fungicides with the same mode of action. For S. sclerotiorum mycelia preconditioned with dimethachlone, control efficacies of dimethachlone and iprodione were reduced slightly, whereas no decreases in control efficacies were found for carbendazim and boscalid.