Abstract
Foliar application of phosphite, a systemic fungicide, to Phytophthora cinnamomi infected plants results in the control of disease symptoms and a reduction in the spread and impact of the pathogenic in native plant communities. Calcium ions have also been shown to affect the interaction between Phytophthora species and their plant hosts and to reduce the impact and spread of disease caused by soil‐borne Phytophthora species. Calcium may enhance plant defence mechanisms or interfere with sporangial production, zoospore release and encystment on plant roots. Phosphite has been shown to have similar effects. The addition of calcium salts to soil inhibits the infection of plants by P. cinnamomi, and there is a correlation between the incidence in dieback disease caused by P. cinnamomi in natural ecosystems and the distribution of calcareous soil. This study used a susceptible Australian native plant species Banksia leptophilia, to investigate whether the disease control of P. cinnamomi by phosphite could be augmented by soil supplementation with calcium sulphate. The results showed that the effects of applying both calcium and phosphite were synergistic, and that the addition of calcium sulphate to the soil augmented and significantly prolonged the effect of foliar phosphite application. A mechanism involving the disruption of intracellular calcium signatures caused by phosphite induced accumulation of pyrophosphate in the cytosol of P. cinnamomi is discussed.