Phosphite and nutrient applications as explorative tools to identify possible factors associated with Eucalyptus gomphocephala decline in South-Western Australia

Tree declines are increasingly being reported around the world. Since the 1990’s Eucalyptus gomphocephala (tuart) has suffered a significant decline in the Yalgorup region, approximately 100 km south of Perth Western Australia. The complexity of many tree declines makes diagnosis difficult. Robust tools are needed to help identify factors contributing to tree declines. Two experiments tested the effect of trunk applied phosphite, nutrients and combined phosphite and nutrient treatments on wild declining E. gomphocephala. Treatment efficacy was tested as a management option to mitigate crown decline and as an explorative tool to help determine disease causality. Experiment 1 assessed the efficacy of combined treatments of trunk injections of different phosphite concentrations, trunk nutrient implants of different compositions and combined phosphite and nutrient treatments. Experiment 2 assessed the efficacy of different phosphite concentrations. In Experiment 1, phosphite, together with nutrient treatments, increased the average crown condition by 21 %, as measured using a crown health score (CHS) averaged over 4 years, with the greatest improvements evident 6 months after application. Injection of 25 g phosphite/L combined with 0.3 g zinc sulphide gave the greatest increase. In Experiment 2, application of 75 to 375 g phosphite/L increased the CHS compared to the control treatment, with the greatest improvements in trees injected with 150 g phosphite/L. Foliar analysis for Experiment 2 confirmed a significant uptake of phosphite for all phosphite treatments. The increase in the CHS and significant flushes in new growth resulting from phosphite and nutrient treatments highlight the possible involvement of Phytophthora species in the decline, as Phytophthora species are known to be controlled by phosphite application. Further work on combined phosphite and nutrient applications, with a particular emphasis on zinc, is required to help understand and potentially mitigate the E. gomphocephala decline