The mode of action of the antifungal agent phosphite¹

Abstract

Field studies have established that phosphorous acid (phosphite) is an effective control against a number of plant diseases. However, the mechanism of control is not known. We have used Phytophthora palmiuora to investigate the action of phosphorous acid as an antifungal agent. Phosphite is toxic to Phytophthoru in uitro , but concentrations which completely inhibit growth in vitro are rarely ever achieved in phosphite‐treated plants. Phosphite concentrations (1 m mol 1 ‐1 ) which do not inhibit fungal growth in vitro cause changes in the metabolism and cell‐wall components. We suggest that phosphite, when present in plants in concentrations too low to control fungi in vitro , may cause modification of the fungal cell surface in such a way that the plant is able to recognize it as foreign and respond by normal defence mechanisms.