Background: Oxathiapiprolin is a new isoxazoline fungicide developed by DuPont to control oomycete diseases. Although oxathiapiprolin has shown strong inhibitory activity against oomycete pathogens, little is known about its ability to control Phytophthora sojae.Results: Oxathiapiprolin showed high inhibitory activity against Phytophthora sojae, with 50% effective concentration (EC 50 ) values ranging from 1.15 × 10 −4 to 4.43 × 10 −3 ∼g mL −1 . Oxathiapiprolin inhibited various stages of Phytophthora sojae development, including mycelial growth, sporangium formation, oospore production, and zoospore release. Electron microscopy studies revealed that oxathiapiprolin caused severe morphological and ultrastructural damage to Phytophthora sojae. Oxathiapiprolin affected the cell membrane and wall of Phytophthora sojae, making it more sensitive to osmotic and cell wall stress. Oxathiapiprolin exhibited translocation activity; it was absorbed by soybean roots and then translocated to the leaves. It was effective at reducing soybean Phytophthora root rot under glasshouse and field conditions. Both fungicide seed treatment and foliar spray significantly reduced disease incidence and yield losses compared with untreated controls in the field. Conclusion:Oxathiapiprolin exhibits high inhibitory activity against Phytophthora sojae, and has multiple mechanisms of action including severe mycelial damage and modulation of osmotic and cell wall stress. These results indicate that oxathiapiprolin can be used at low concentrations for highly effective management of soybean Phytophthora root rot caused by Phytophthora sojae.

The oxysterol-binding protein inhibitor oxathiapiprolin is a new fungicide for controlling oomycetes diseases. Besides, laboratory mutagenesis oxathiapiprolin-resistance among phytopathogenic oomycetes in the field remains unknown. Here, the sensitivity of 97 P. colocasiae isolates to oxathiapiprolin was examined that were collected between 2011 and 2016. We obtained a baseline sensitivity with a mean EC50 value of 5.2639 × 10–4 μg mL–1. We showed that 6/32 isolates collected in Fujian Province from 2019 to 2020 were resistant to oxathiapiprolin without a significant fitness penalty on sporulation, vegetative growth, and virulence of the field isolates. The oxathiapiprolin resistance field isolates contained the point mutation glycine to valine at 699 in PcoORP1. The point mutation G699V was verified to confer resistance of P. colocasiae to oxathiapiprolin using the CRISPR/Cas9 system. The mutation G699V decreased the binding affinity between oxathiapiprolin and PcoORP1. These results will improve our understanding of the mechanism of P. colocasiae resistance to oxathiapiprolin under field conditions.