Control of Phytophthora cinnamomi by the fungicide phosphite in relation to in planta phosphite concentrations and phytotoxicity in native plant species in Western Australia

Low volume aerial phosphite applications has been used in recent years at rates of up to 24 kg ha" to protect native plant species and communities threatened by Phytophthora cinnamomi while the recommended rate for spray to run-off phosphite application is 5 g L·'. Phosphite uptake and in planta phosphite concentrations in native plant species may vary considerably between species and with application rate as may the effectiveness of disease conlrol and the duration of this control. Phytotoxicity symptoms post-spray include foliar necrosis, defoliation, growth abnormalities and chlorosis, reduced root growth and reproductive effects and these may also vary considerably between species. Phytotoxicity symptoms increase with increasing application rate but are generally mild at recommended rates. However, a percentage of species in the plant communities assessed show greater sensitivity to phosphite. Aerial phosphite application rates for native plant communities aims to maximise in planta phosphite concentrations in Phytophthora-susceptible speeles for disease control while minimising phytotoxicity symptoms in the species present.