Foliar phosphite application to manage kauri (Agathis australis) dieback

Phosphite (HPO₃^2–) is an effective chemical treatment used to manage Phytophthora diseases in natural and horticultural environments. In New Zealand it is applied as a trunk injection to protect the threatened endemic foundation tree, Agathis australis (kauri), from the invasive pathogen P. agathidicida. It is applied as a foliar spray in natural ecosystems in Australia to protect against a range of Phytophthora-associated diseases and retain plant biodiversity and abundance. Foliar spray application could be a useful method for broad area treatment and to protect isolated and difficult-to-reach stands of kauri, or in plant production settings. In this study, we assessed the efficacy of phosphite when applied as a foliar spray to inhibit P. agathidicida colonisation, the phytotoxicity response and whether the phosphite was translocated to the roots using two glasshouse experiments with 2- and 6-year-old kauri plants. The foliar phosphite treatment effectively reduced colonisation by P. agathidicida in the 2-year-old plants and elevated concentrations of phosphite in the roots of the 6-year-old plants were detected. Phytotoxicity increased with increasing phosphite concentration in both trials. The addition of the surfactant Du-Wett® reduced phytotoxicity for older leaves in the 6-year-old plants. In both assays, phytotoxicity symptoms were most severe on young leaves but had no effect on the production of fresh growth following treatment, suggesting that any ill effects were short-lived. This study indicates that phosphite foliar sprays may be an effective tool to manage kauri dieback under the appropriate circumstances.