White mold, caused by Sclerotinia sclerotiorum, is one of the most important diseases affecting soybean and its control has been difficult to achieve. This study aimed to investigate the potential of manganese (Mn) phosphite and fluazinam in protecting soybean plants against S. sclerotiorum infection by examining the photosynthetic performance (leaf gas exchange and chlorophyll (Chl) a fluorescence parameters), activities of defense enzymes [chitinase (CHI), β-1,3-glucanase (GLU), phenylalanine ammonia-lyase (PAL), and polyphenol oxidase (PPO)] as well as those related to the antioxidant metabolism (superoxide dismutase (SOD), catalase (CAT), peroxidase (POX), and ascorbate peroxidase (APX)) and the concentrations of hydrogen peroxide (H₂O₂), superoxide (O₂⁻), and malondialdehyde (MDA). White mold development was completely inhibited by fluazinam. Soybean metabolism was not changed by fluazinam. White mold severity was significantly reduced on plants sprayed with Mn phosphite, which showed a better photosynthetic performance than the non-sprayed plants. Mycelial growth of S. sclerotiorum was inhibited by Mn phosphite. Activities of CAT, POX, and SOD decreased while CHI, GLU, and PAL activities increased at 96 hai for Mn phosphite-sprayed plants compared to non-sprayed plants. In conclusion, Mn phosphite affected white mold development and pathogen-induced physiological impairments in soybean leaflets due to its dual mode of action.
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