Phosphites attenuate Sclerotinia sclerotiorum-induced physiological impairments in common bean

Abstract

Phosphites, marketed as foliar fertilizers and resistance activators, have been shown to be useful for the control of diseases in many profitable crops. Despite the importance of white mold, caused by Sclerotinia sclerotiorum, to reduce common bean yield, knowledge of the phosphites´ effect on disease control at the physiological level is still missing. In this study, the leaf gas exchange and chlorophyll a fluorescence parameters variable-to-maximum chlorophyll a fluorescence ratio (Fv/Fm), photochemical yield [Y(II)], yield for dissipation by down-regulation [Y(NPQ)], yield for non-regulated dissipation [Y(NO)], and electron transport rate (ETR) as well as the concentrations of photosynthetic pigments in common bean plants that were sprayed with zinc (Zn) or copper (Cu) phosphites and challenged or not with S. sclerotiorum were determined. Based on the in vitro assays, Zn and Cu phosphites inhibited fungal mycelial growth in a dose-dependent manner, but the Cu phosphite showed to be more fungitoxic. Lesion area and white mold severity were reduced by Zn and Cu phosphites, but the Zn phosphite was more effective. Fungal infection dramatically decreased the values of net carbon assimilation rate, stomatal conductance to water vapor and transpiration rate on non-sprayed plants. Increases in internal CO2 concentration indicated that fungal-induced photosynthetic impairments were chiefly governed by biochemical limitations, but these impairments were greatly abrogated in the Zn and Cu phosphite-sprayed plants. Similarly, the photochemical dysfunctions stemmed from S. sclerotiorum infection were limited in the Zn and Cu phosphite-sprayed plants. Concentrations of chlorophyll a + b and carotenoids decreased on inoculated plants, but lower reductions were recorded on Zn and Cu phosphites-sprayed plants. In conclusion, the potential of Zn and Cu phosphites in attenuate the S. sclerotiorum-induced physiological impairments in common bean leaflets was demonstrated and may be an effective mean for managing this disease under field conditions.