Abstract
Eucalyptus gomphocephala is a Mediterranean forest canopy species endemic to a narrow (5- 10 km wide) coastal strip approximately 300 km in length in south-west Western Australia. Eucalyptus gomphocephala is undergoing a significant decline that was first identified as a spot decline in 1994 and now occurs throughout large sections of its remnant distribution within Yalgorup National Park, in some areas resulting in 100 percent mortality. The reduction of this keystone species represents a significant modification to the associated ecosystem. Modifications to hydrology, fire regimes, entomological pressures, and fungal and Pythiaceous soil pathogens have been identified as possibly contributing to the decline syndrome. The potential of phosphite (phosphonate), nutrient and insecticide treatments to reverse the decline in tree health was assessed as (a) a method for controlling the decline and (b) a method for diagnosing possible causal agents. Phosphite has been successfully used to control Phytophthora and Pythiaceous soil pathogens by inducing a host defense response within the plant. Stem injection of declining Eucalyptus gomphocephala in the present study has resulted in improved canopy health and vigor, indicating that Phytophthora and/or other Pythiaceous microorganisms may be playing a role in the decline. The impact of phosphate application on nutrient uptake and fine feeder root concentration was also assessed.