3.7.9
PHOSPHITE: ITS PHYTOTOXICITY AND EFFECTIVENESS IN THE PROTECTION OF EUCALYPTUS MARGINATA FOREST FROM PHYTOPHTHORA CINNAMOMI

RA BENNALLICK1, IJ COLQUHOUN2, BL SHEARER3 and GE ST J HARDY1

1Murdoch University, Murdoch 6150, Western Australia; 2Alcoa of Australia Limited, Booragoon 6154, Western Australia; 3Department of Conservation and Land Management, Como 6152, Western Australia

Background and objectives
Recent trials conducted by the Western Australian Department of Conservation and Land Management have shown that phosphite protects trees in E. marginata forest from P. cinnamomi. However, phytotoxicity has been observed in some plant species, and phytotoxic concentrations of phosphite have increased the susceptibility of Banksia coccinea to P. cinnamomi.

The aim of this project was to examine the effect of phosphite concentration on phytotoxicity and on colonisation by P. cinnamomi in three understorey species of the E. marginata forest.

Materials and methods
Adenanthos barbiger, Daviesia decurrens and Xanthorrhoea preissii plants at Alcoa of Australia Limited's Jarrahdale mine were sprayed to run-off with 0, 0.2, 0.5 and 2% phosphite. Plants were monitored regularly for phytotoxicity symptoms. One week after phosphite treatment, stems of A. barbiger and D. decurrens were underbark inoculated with P. cinnamomi. In X. preissii, roots were underbark inoculated with P. cinnamomi 30 days after treatment. The stems and roots were harvested between 23-31 days after inoculation, then plated sequentially on Phytophthora-selective agar. The leaves of A. barbiger and D. decurrens and roots of X. preissii were analysed for phosphite content using gas chromatography and a flame photometric detector.

Results and conclusions
The foliar application of 0.2, 0.5 and 2% phosphite was effective in restricting colonisation by P. cinnamomi in stems of A. barbiger and D. decurrens, but not in the roots of X. preissii. However, treatment with 2% phosphite resulted in severe phytotoxicity symptoms. Plants with severe phytotoxicity symptoms recovered by producing new growth. Leaf necrosis developed in all three species at a phosphite concentration as low as 0.2%, which is in contrast with the reported low phytotoxicity of phosphite [1]. However, the observed phytotoxicity symptoms were not severe in the 0.2 or 0.5% phosphite treatments. Phytotoxic concentrations of phosphite did not predispose A. barbiger or D. decurrens to more colonisation by P. cinnamomi.

Although phosphite is phloem mobile and has been detected in the roots of treated plants [2], very little is known about the distribution of phosphite after foliar application. Very low concentrations of phosphite were detected in the roots of X. preissii in comparison with the phosphite concentration measured in the foliage of A. barbiger and D. decurrens plants treated with phosphite. This suggests that phosphite was not translocated from the leaves to the roots in X. preissii.

The results indicate that phosphite has the potential to contain P. cinnamomi in native plants. It is generally accepted that phosphite does not eradicate the pathogen, but it may slow the destruction of native plant communities long enough for a more permanent solution to be found.

References
1. Guest DI and Grant BR, 1991. Biological Review 66, 159-87.
2. Ouimette DG and Coffey MD, 1989. Plant Disease 73, 212-15.