Soil drenches and foliar sprays of potassium phosphonate protected glasshouse-grown Xanthorrhoea minor and X australis against root rot caused by Phytophthora cinnamomi. Three concentrations of potassium phosphonate (0.1, 1.0 and 5.0 mg/mL), applied by both methods, reduced the severity of root rot symptoms and increased plant survival rates. Soil drenches were more effective than foliar sprays. The highest rate of foliar spray (5.0 mg/mL) was phytotoxic to both species, causing burning of leaf tips.