Phosphite (Phi; H2PO3- or HPO3-2) or its conjugate phosphorous acid (H3PO3), a reduced form of inorganic phosphate (Pi; H2PO4- or HPO42-), has increasingly been used as a pesticide against various species of plant pathogens of importance in horticulture. Indeed, Phi may control and/or induce resistance against pathogenic bacteria such as Erwinia amylovora and E. caratovora, as well as the oomycete genera Peronospora, Plasmopara, Phytophthora and Pythium, the fungi genera Alternaria, Rhizoctonia and Macrophomina, and the nematode species Meloidogyne javanica, Pratylenchus brachyurus, Heterodera avenae and Meloidogyne marylandi, among others. In recent years, Phi has emerged as a potential biostimulator improving yield and quality of a number of crop species, and inducing better performance of plants exposed to abiotic stress factors. In conventional agricultural systems, Phi has not been proved to have a direct effect on plant nutrition, and should not be considered as a proper fertilizer. Nonetheless, novel genetic engineering approaches are currently allowing its use in alternative P fertilization and weed control, albeit its commercial application is still at issue. Though this innovative technology could address the imminent danger of phosphate reserve depletion and multiple herbicide tolerance in an increasing number of weeds, environmental and human health concerns need to be critically approached. Its role as inductor of beneficial metabolic responses in plants is more evident in conditions of Pi-sufficiency. Additionally, Phi applications are more efficient when its rate and utilization are properly timed to meet the requirements of crop plants in order to stimulate physiological processes, which in turn depend on plant genotypes, environmental conditions, agronomic management, source and dosage of Phi to be used. This paper outlines recent research advances on the impact of Phi as a pesticide, biostimulant, and a dual fertilizer and herbicide in horticulture, and discusses potentialities and challenges of its use, especially those related to its impact in the environment and human health.
Keywords: Phosphorus, phosphorous acid, pesticide, biostimulant, alternative fertilizer, herbicide