Phytophthora agathidicida is a water mold that causes a disease known as kauri dieback in Agathis australis, commonly known as the kauri tree. This pathogen infects the roots and lower trunk of the tree, leading to decline and eventual death. It thrives in moist environments and can spread through soil, water, and human activity. The disease poses a significant threat to New Zealand's native forests, particularly in the North Island, where kauri trees are culturally and ecologically important. Efforts to manage and control the spread of Phytophthora agathidicida include hygiene practices, monitoring, and research into resistant tree varieties.