The Mode of Action of Phosphite: Evidence for Both Direct and Indirect Modes of Action on ThreePhytophthoraspp. in Plants

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Abstract
Phosphite, applied as a root drench, provided protection against invasion by Phytophthora cinnamomi, P. nicotianae, and P. palmivora in lupin, tobacco, and paw-paw, respectively. Protection was expressed as a reduction in the rate of lesion extension after wound inoculation. Phosphite concentrations at the site of inoculation were sufficient to reduce mycelial growth in vitro. There was a close relationship between the concentration of phosphite present at the invasion site and the extent to which protection was expressed, although phosphite concentrations were never fugnitoxic. Once accumulated, phosphite remained in the plant for extensive periods. Results suggest that the concentration of phosphite present at the infection site influenced the degree to which phosphite protection was observed in treated plants. In the three fungal-plant combinations examined, phosphite concentrations were sufficient to reduce fungal growth by direct action. However, plant defenses would be important in completely arresting pathogen invasion.