VIEW ARTICLE

Disease Control and Pest Management

The Mode of Action of Phosphite: Evidence for Both Direct and Indirect Modes of Action on Three Phytophthora spp. in Plants. R. Smillie, Russell Grimwade School of Biochemistry, Melbourne University, Parkville, Victoria, 3052, Australia; B. R. Grant(2), and D. Guest(3). (2)Russell Grimwade School of Biochemistry, Melbourne University, Parkville, Victoria, 3052, Australia; (3)Department of Botany, Melbourne University, Parkville, Victoria, 3052, Australia. Phytopathology 79:921-926. Accepted for publication 5 April 1989. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/Phyto-79-921.

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 fungitoxic. Once accumulated, phosphite remained in the plant for extensive periods. Results suggest that the concentration of phosphate 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.