Effects of phosphite on the in vitro growth of Verticillium nonalfalfae and Verticillium dahliae and on their in vivo ability to infect potato plants

Verticillium spp. are soil-borne fungi containing many pathogenic species that affect a wide range of cultivated plants. In potatoes, Verticillium dahliae , V. nonalfalfae and V. albo-atrum cause Verticillium wilt or, together with other pathogens, are responsible for the occurrence of the potato early dying disease. Verticillium pathogens are notoriously difficult to control, and techniques proved to be efficient, such as chemical soil fumigation, are causing environmental concerns worldwide. Phosphites (Phi) represent a class of chemicals with a good environmental footprint, that are currently used to control oomycete pathogens, such as the potato late blight agent, Phytophtora infestans . To determine the potential inhibitory effects of Phi on Verticillium spp., isolates of V. dahliae and V. nonalfalfae were tested in vitro and the IC 50 of Phi was found to vary between 60.9 and 481.9 μg/mL. The effects of Phi were further tested in field trials, carried for two years, using Phi-treated potato plants infected with a V. nonalfalfae isolate. Infection progression was assessed in both years by qPCR and was found to be significantly reduced in Phi-treated plants. The concentration of Phi in these plants, determined by high performance ion chromatography, was found to vary between 25.0 and 86.2 μg/mL in leaves and between 55.7 and 113.8 μg/mL in tubers. Even though the concentrations of Phi required to control Verticillium spp. were found to be relatively high compared to those needed to inhibit the development of oomycete pathogens, the results from this study indicate that Phi can limit the development of Verticillium spp .