Restriction of potato and tomato late blight development by sub‐phytotoxic concentrations of boron

Boron is a microelement required for normal growth and development of plants but its positive effect is restricted to a narrow range of concentrations. The gradual increase in use of recycled water, which contains high concentrations of boron for irrigation, has already raised the level of boron in soils and plants in southern Israel. This research was conducted to examine the direct effects of sub‐phytotoxic boron concentrations on potato late blight epidemics and to explore the mode of action of boron against Phytophthora infestans. When boron was applied alone to field grown potato plants it did not affect the epidemic. However, together with a reduced rate of the fungicide Melody Duo (propineb + iprovalicarb), boron improved late blight suppression compared to plants treated with the fungicide alone. The ED₅₀ of boron against P. infestans (256·4 mg L^?1) was about 6400 times higher than the ED₅₀ value of the fungicide chlorothalonil (0·04 mg L^?1), indicating that boron does not have a direct fungicidal activity that would explain the level of protection seen in the field. In greenhouse experiments conducted with potted tomato plants, boron decreased late blight severity in both treated leaves and distant leaves not treated with boron. The results suggest that boron is active locally but also may induce systemic acquired resistance against P. infestans.