Pythiaceous root pathogens associated with Eucalyptus gomphocephala decline in Western Australia

Eucalyptus gomphocephala is a keystone canopy species endemic to a narrow (5-10 km wide) coastal strip approximately 300 km in length in south-west Western Australia. E. 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% mortality. Multiple factors, including soilborne pathogens, have been identified as possibly contributing to the decline. Less fine roots are associated with trees on declining sites compared to those on healthy sites. Foliar analysis indicates that declining trees have lower concentrations of some micronutrients, including zinc, which uptake is typically impaired by fine feeder root loss. A range of Pythiaceous microorganisms have been isolated from declining roots, including a new isolation of a yet described Phytophthora species. The Phytophthora isolates appear morphologically similar to the Phytophthora citricola holotype although are distinct using molecular analysis of the internal transcribe region. The exact phylogeny of the new Phytophthora isolates is being determined using sequence analysis of other gene regions. These isolates may be contributing to the loss of fine roots. Glasshouse trials are currently underway to determine whether these isolates are indeed pathogenic