The dilution of phosphite in rapidly growing plants and how soil and plant phosphate levels interact with phosphite and its ability to control Phytophthora cinnamomi

The soil borne plant pathogen Phytophthora cinnamomi has irreversibly altered the make-up and diversity of the plant communities found in Australia. Recently, the fungicide phosphite has been used to effectively reduce the impact of this pathogen in natural plant communities. However, little is known (a) about how rapidly phosphite is diluted in the tissues of rapidly growing plants and (b) how soil and plant phosphate levels interact with phosphite and its ability to induce host-resistant responses when challenged by P. cinnamomi. This study examined the effects of different phosphite rates (0, 24 and 48 kg/ha phosphite) applied as a mist application on three size classes of Banksia grand is, as well as the interaction of phosphate status on two Eucalyptus marginata forest vegetation types differing in soil phosphate status with phosphite. It also examined, under controlled glasshouse conditions, the effects different soil phosphate levels had on in planta phosphite and phosphate levels in B. hookeriana, and the subsequent control of P. cinnamomi. This study was the first to look at the role of phosphate in the soil and the plant, and its interaction with phosphite and the subsequent control of P. cinnamomi in planta. Results from the field trial indicated that phosphate in the soil did not play a role in the reducing the uptake of phosphite by the plant. It did suggest that stem and root colonisation was increased when phosphate in the soil was more plentiful. Further research is needed into this area. This study was also the first to look at the distribution of phosphite in planta. The highest concentration of phosphite was in the leaves, followed by the stem and then roots. Ph9sphite in the plant tissue was found to increase as the phosphite applied to 5 the plants increased. Plants classed as seedlings showed more phytotoxic symptoms than the intermediate and semi-mature plants. The concentration of phosphite in the roots of the intermediate sized plants was more than double the amount found in the seedling and semi-mature plants. The concentration of phosphite in the whole plant, as well as in the leaves and stems per plant, increased as the plant size increased. This was supported by results that showed that as the dry weight of the leaves increased so did the amount of phosphite in the leaves. The same was seen with the dry weights of the stems and roots that correlated with phosphite in the stem and the roots, respectively. Lesions and P. cinnamomi colonisation in the stems of non-phosphite treated plants were more than double those in stems of plants treated with 24 and 48 kg/ha phosphite. There was very little difference in the visible lesion lengths and P. cinnamomi colonisation between plants treated with 24 and 48 kglha of phosphite even though plants sprayed with 48 kg/ha phosphite had significantly more phosphite in their tissues than plants sprayed with 24kg/ha phosphite. This suggests that the phosphite in the plant may have been metabolised into another substance and that this substance was acting on the pathogen and/or the plant to reduce colonisation. This was further supported by no observed correlation between phosphite in the plant tissue and the extent of colonisation or visible stem lesion caused by P. cinnamomi. This was contradictory to other results in this study (Chapter 2) that clearly showed that phosphite did restrict the colonisation of the pathogen. Further research is needed into the mode of action of phosphite. In the glasshouse trial, a non-invasive inoculation technique failed to infect B. hookeriana plants with the pathogen. However, this is likely due to very high ambient temperatures experienced during the trial, since a preliminary trial 18 days earlier resulted in extensive colonisation of all plants inoculated. As phosphate levels increased, stem colonisation by the pathogen increased in the presence of phosphite. There was no difference in the concentration of phosphite in the leaves. As phosphite applied increased, so did the concentration of phosphite in the root tissue. This study shows that phosphate does interact with phosphite and the subsequent expression of P. cinnamomi, and as phosphate levels increased in planta so did the extent of colonisation by the pathogen. The exact nature of this interaction is still unknown and further research is required to better understand the nature of this relationship