The long-term ability of phosphite to control Phytophthora cinnamomi in two native plant communities of Western Australia

This study examined the ability of foliar applications of the fungicide pho sphite to contain colonisation of Phytophthora cinnamomi in a range of plan t species growing in natural plant communities in the northern sandplain an d jarrah (Eucalyptus marginata) forest of south-western Australia. Wound in oculation of plant stems with P. cinnamomi was used to determine the effica cy of phosphite over time after application. Colonisation by P. cinnamomi w as reduced for 5-24 months after phosphite was applied, depending on the co ncentration of phosphite used, plant species treated and the time of phosph ite application. Plant species within and between plant communities varied considerably in their ability to take up and retain phosphite in inoculated stems and in the in planta concentrations of phosphite required to contain P. cinnamomi. As spray application rates of phosphite increased from 5 to 20 g L-1, stem tissue concentrations increased, as did the ability of a pla nt species to contain P. cinnamomi. However, at application rates of phosph ite above 5 g L-1 phytotoxicity symptoms were obvious in most species, with some plants being killed. So, despite 10 and 20 g L-1 of phosphite being m ore effective and persistent in controlling P. cinnamomi, these rates are n ot recommended for application to the plant species studied. The results of this study indicate that foliar application of phosphite has considerable potential in reducing the impact of P. cinnamomi in native plant communitie s in the short-term. However, in order to maintain adequate control, phosph ite should be sprayed every 6-12 months, depending on the species and/or pl ant community.