We determined the effect of phosphite application as a high-volume spray to the understorey, and injection of overstorey species, on disease front extension and rate of extension of Phytophthora cinnamomi in Banksia woodland. Burning of the trial site by the owner gave an unplanned effect of fire on phosphite treatment. The five phosphite treatments were no phosphite (treatment 1), all trees injected with 50 g/L phosphite and the understorey sprayed with 2 g/L phosphite once or twice (treatments 2 and 3, respectively); or all trees injected with 50 g/L phosphite and the understorey sprayed with 5 g/L phosphite once or twice (treatments 4 and 5, respectively). The first phosphite spray occurred in summer (mid-February) 1993, the second spray was 6 weeks later, in April 1993. Overstorey trees of Banksia attenuata, B. ilicifolia and B. menziesii were injected with 50 g/L phosphite the next day after the first spray treatments. The first assessment of the disease front position occurred 0.5 year after the first spray, assessment 2 occurred 4.3 years after the first spray and 3.2 years after the fire and assessment 3, 5.3 years after the first spray and 4.1 years after the fire. Phosphite treatment significantly reduced disease front extension by an average of 0.9 ± 0.1 m, 4.0 ± 0.2 m and 4.1 ± 0.2 m, for assessments 1, 2 and 3, respectively. There were no consistent significant differences in disease front extension between phosphite treatments 2–5. Rates of extension 1 and 2 (calculated between assessments 2 and 1, and 3 and 1, respectively) were reduced by a half to a third by phosphite treatment. There were no consistent significant differences in rate of disease front extension between phosphite treatments 2–5. Residual action of phosphite would not be expected to last in understorey vegetation destroyed by fire, but probably persisted after the fire in the woody roots of injected overstorey trees. This study indicates that injection of overstorey should accompany spray of foliage to ensure long lasting protection by phosphite.