Phosphite concentration: its effect on phytotoxicity symptoms and colonisation by Phytophthora cinnamomi in three understorey species of Eucalyptus marginata forest

Pre-treatment of plants with foliar sprays of 0.2, 0.5 and 2% phosphite res tricted colonisation by Phytophthora cinnamomi in inoculated stems of Adena nthos barbiger and Daviesia decurrens, and led to a reduction in the isolat ion of P. cinnamomi from these stems in comparison with unsprayed plants. I n plants treated with 2% phosphite, P. cinnamomi was not isolated from D. d ecurrens but was isolated from 22% of the stems of A. barbiger. In Xanthorr hoea preissii, colonisation by, and isolation of, P cinnamomi from inoculat ed roots was not significantly affected by pre-treatment of the foliage wit h 0.2, 0.5 and 2% phosphite. Very low concentrations of phosphite were dete cted in the roots of X. preissii (maximum mean of 2.2 mu g/g dry weight), i n comparison with the phosphite concentrations measured in the foliage of A . barbiger and D. decurrens plants treated with phosphite (maximum means of 80 and 871 mu g/g dry weight, respectively). Treatment with 0.2% phosphite resulted in minimal phytotoxicity in each of the three species, whereas tr eatment with 2% phosphite led to the development of severe phytotoxicity sy mptoms. This study indicates that phosphite has potential for the managemen t of P. cinnamomi in native plant communities.