Sensitivity of the soil-borne pathogen Phytophthora agathidicida, the causal agent of kauri dieback, to the anti-oomycete fungicides ethaboxam, fluopicolide, mandipropamid, and oxathiapiprolin

The oomycete Phytophthora agathidicida is the causal agent of kauri dieback, which threatens the survival of endemic kauri (Agathis australis) forests in Aotearoa|New Zealand. Current chemical control of P. agathidicida involves the application of either a mixture of halogenated tertiary amines or phosphite salts with some success, but neither treatment cures the disease. Recently, four anti-oomycete fungicides, all with different modes of action, have become commercially available. Here, we determined the inhibition potential of these fungicides on three P. agathidicida isolates, using agar dilution assays. The average concentration required to inhibit mycelial growth by 50% (EC50) for ethaboxam, fluopicolide, and mandipropamid was 0.0916, 0.372, and 0.0196 µg/mL, respectively. Inhibition of P. agathidicida mycelia by oxathiapiprolin and its commercial formulation, Zorvec® Enicade®, was 0.000152 and 0.000309 µg/mL, respectively. Based on the EC50 values reported in this study, these fungicides are the most effective inhibitors of P. agathidicida mycelia when compared to previously screened fungicides, natural products, and plant extracts. Thus, their performance in this initial screening supports further research into their potential use as a kauri dieback management tool.