Abstract
Since the early 1990's there has been a significant decline of Eucalyptus gomphocephala and more recently E. marginata, in the tuart forest in Yalgorup National Park SW Western Australia, although no satisfactory aetiology has been established to explain the decline. Characteristics of the canopy dieback and decline distribution are reminiscent of other forest declines known to involve Phytophthora soil pathogens and indicate that a Phytophthora species may be involved in the decline. In 2007 isolates of a Phytophthora species were recovered from rhizosphere soil of declining or dead trees of E. gomphocephala and E. marginata, and were described as P. multivora. For E. gomphocephala and E. marginata, the pathogenicity of P. multivora was tested: ex situ on seedlings using a soil infestation method; and in situ on stems using an under bark inoculation method. Trials suggest that P. multivora can be significantly aggressive to both E. gomphocephala and E. marginata, although further research is needed. The soil infestation trial indicates that P. multivora is a pathogen of E. gomphocephala and may be a significant soil pathogen in the field, although there is variation in the pathogenicity between P. multivora isolates. The under bark inoculation trial confirms that P. multivora is a pathogen to E. gomphocephala, and especially pathogenic to E. marginata, where it is able to colonize the vascular tissue in the field.