Ability of phosphite applied in a glasshouse trial to control Phytophthora cinnamomi in five plant species native to Western Australia

The ability of phosphite to control Phytophthora cinnamomi Rands in five Western Australian native plant species was examined. Foliar application of phosphite slowed, but did not completely inhibit, colonisation of stems by P. cinnamomi. For example, in Banksia hookeriana Meisn. inoculated 2 weeks after phosphite application, 5 g phosphite/L inhibited the growth rate of P. cinnamomi by 57% compared with the non-phosphite-treated plants. The longevity of phosphite efficacy varied with plant species. Foliar application of 5 and 10 g phosphite/L decreased the growth rate of P. cinnamomi in Dryandra sessilis (Knight) Domin. for at least 12 months after it was applied. Application rates of 5 and 10 g phosphite/L for Banksia grandis Willd. and 10 g/L for B. hookeriana were effective for at least 18 months after application. In Hibbertia commutata Steud. and Dampiera linearis R.Br., phosphite was effective for less than 6 and 12 months, respectively. In a second trial, plants were inoculated with P. cinnamomi at different time periods after phosphite was applied and time to death was recorded. There was a range of responses depending on the plant species and time of year they were inoculated. The initial levels of phosphite in roots and stems of B. grandis, B. hookeriana and D. sessilis and the rate of decrease of phosphite in these tissues differed between plant species. In general, concentrations of phosphite in stems were higher or equivalent to those in roots. This study indicates that the long-term efficacy of phosphite depends on both the plant species treated and the time of year the plants are infected with P. cinnamomi.