Articles
SUSCEPTIBILITY TO PHYTOPHTHORA AND NUTRITION OF BANKSIA SPECIES
Only one of the 73 species of Banksia occurs naturally outside Australia. 58 species are endemic to south-western Australia (George, 1981).
The vigorous root-rot and stem collar pathogen Phytophthora cinnamomi causes extensive death in some Australian Eucalyptus forests.
It is particularly pathogenic to the proteaceous understorey, which includes Banksia species as a major component.
Considerable controversy and discussion surround the question of the geographic origin of Phytophthora cinnamomi. While there are some indications of resistance in a range of species having developed in NE Australia (Pratt & Heather, 1973), the large number of susceptible hosts in Western Australia (WA) indicates that the fungus was introduced, probably within the past 60 – 70 years.
Proteaceous hosts include the genera Banksia, Conospermum, Dryandra, Grevillea, Hakea, Isopogon, Lambertia, Persoonia and Petrophile.
Two major areas in WA are affected by P. cinnamomi: the jarrah (Eucalyptus marginata) – banksia forest, and the heath-land of the south coast. (Figure 1)
E. marginata is highly susceptible to P. cinnamoni, yet is the most valuable timber component of the forest. Banksia grandis, a major understorey tree, is even more susceptible than jarrah, and is used by foresters as an indicator species to map P. cinnamomi spread.
Death is characterised by ‘dieback’ from the crowns, often suddenly in Banksia. Large tracts of the forest are quarantined to halt the spread of P. cinnamomi.
While proteaceous species are one major component of forest understorey, in the heath-lands of the south-coast they are dominant and account for up to 60% of total species.
Although P. cinnamomi is an important pathogen in Australia and elsewhere (Zentmyer, 1980) the mechanisms by which damage is caused to host plants are not well understood.
Little is known of the interactions between P. cinnamomi and inorganic mineral nutrition.
Soils for avocado cultivation (P. cinnamomi-suppresive) are characterized by high ammonium-N (Broadbent & Baker, 1974), while Marks et al (1972) found that fertilization increased the