Phosphite is an essential tool for managing Phytophthora diseases around the world and is often applied by trunk injection or foliar spray. Kauri (Agathis australis) can be infected by Phytophthora agathidicida causing kauri dieback, which is usually fatal. Kauri can be treated by phosphite trunk injection to effectively manage the symptoms of kauri dieback. However, this involves someone physically visiting and wounding the tree to administer the treatment. Foliar application of phosphite may provide a useful alternative for treating infected kauri without wounding or the need for ground-based disturbance, particularly in sites that are difficult to access. Results from a previous experiment showed that foliar application of 7.5 g/L Foschek® + 0.2% v/v Du-Wett® significantly reduced water use and stomatal conductance of kauri saplings, but it was unclear whether this was caused by the Du-Wett® or phosphite. The current experiment aimed to investigate the leading cause and mechanism of reduced stomatal conductance following foliar application of Foschek® and Du-Wett® on kauri seedlings in the glasshouse. Kauri seedlings were treated with foliar phosphite (Foschek®), Du-Wett®, phosphite + Du-Wett®, or distilled water. Stomatal conductance and water use were measured, as well as stomatal aperture. Results of this study will be discussed.