Assessing the in Planta Efficacy of Oxathiapiprolin as a Potential Treatment for Kauri Dieback Disease

Phytophthora agathidicida, a plant pathogenic oomycete, causes fatal dieback disease in New Zealand kauri trees (Agathis australis). Currently, few treatments exist to prevent or cure this infection. Previous research has demonstrated the potent in vitro inhibition of multiple lifecycle stages of P. agathidicida by the oomycide oxathiapiprolin. In this study, we have evaluated the efficacy of oxathiapiprolin in planta as either a protective or curative treatment. Kauri seedlings (1-2 years old) were treated with 10 or 50 mg of oxathiapiprolin, in the form of Zorvec Enicade, per seedling as a soil drench either before (7 days) or after (15 days) inoculation with P. agathidicida NZFS 3770 to test for protective and curative activities, respectively. Results showed that oxathiapiprolin treatments successfully protected the kauri seedlings from disease, with the higher dose (50 mg) demonstrating greater efficacy. However, the treatments did not cure kauri seedlings already infected with P. agathidicida, likely because the infection was already well-established by the time of treatment. This study demonstrates that, while oxathiapiprolin shows protective effects against P. agathidicida infection in kauri seedlings, its lack of curative properties significantly limits its potential as a practical tool for managing kauri dieback disease.