Calcium sulphate soil treatments augment the survival of phosphite-sprayed Banksia leptophylla infected with Phytophthora cinnamomi

The application of either phosphite or calcium salts to plants that are susceptible to Phytophthora spp. protects them from infection and the development of disease symptoms. This suggests that there may be an additive protective-effect when they both are applied together. The combined effect of foliar phosphite and soil calcium levels on the health and survival of Banksia leptophylla seedlings infected with Phytophthora cinnamomi was investigated. Six month-old Banksia leptophylla plants grown in sand supplemented with 0, 3, 10 or 30 mM calcium sulphate were sprayed with 0, 0.1 or 0.3 % phosphite and inoculated with P. cinnamomi. Plant survival and health were recorded for 12 months after inoculation. The combination of foliar-phosphite spraying with the supplementation of sand with calcium sulphate significantly increased the survival and health of plants infected with P. cinnamomi. There was 2.7 % survival of plants with no phosphite or additional calcium, 8.3 % survival with 30 mM calcium alone, 53 % survival with 0.3 % phosphite alone and 100 % survival of plants given 0.3 % phosphite and 30 mM calcium. The pathogen survived in the sand of all treatments for the 12-month period of the trial. Combining foliar-application of phosphite with addition of calcium sulphate to soil is a cheap and practical way of significantly increasing the efficacy of phosphite in controlling the development and spread of Phytophthora dieback disease. A mechanism involving inhibition of calcium-dependent ATPases by phosphite and pyrophosphate, and the subsequent disruption of calcium ion signaling, is discussed.