The efficacy of phosphite applied after inoculation on the colonisation of Banksia brownii stems by Phytophthora cinnamomi

Low-volume phosphite application 2 days after stem inoculation significantly (P < 0.05) reduced colonisation of Banksia brownii by Phytophthora cinnamomi in a glasshouse trial at all phosphite application rates when compared with the control. There was a greater reduction in colonisation at rates of 24 and 96 kg/ha of phosphite, compared with the lowest rate of 12 kg/ha. The relationship between application rate and disease control was non-linear and suggested an optimum dose-response relationship. Foliar phytotoxicity at harvest, 9 days post-inoculation, was minimal at all application rates, although in planta phosphite concentrations were high. Growth of P. cinnamomi was not halted at any application rate at the time of harvest, but this may be due to the very high susceptibility of B. brownii to P. cinnamomi, the high virulence of the isolate used and the ideal temperature for growth of P. cinnamomi. The study suggests that low volume phosphite application to Phytophthora-infested plant communities may control the disease in individuals of plant species in the early stages of infection as well as protecting individuals that have avoided infection. Further studies on a range of native species are necessary to verify whether phosphite, applied post-infection, is effective in increasing plant survival.