First Report of Phytophthora sansomeana Causing Root Rot of Soybean in North Dakota

Across the United States, Phytophthora sansomeana has been reported as a soybean pathogen in 10 states, but until now, North Dakota has never had a confirmed documented case. During 2023, a statewide survey of oomycote pathogens associated with soybean (Glycine max) was conducted across North Dakota, United States. Soil samples were collected during the growing season (June - August) from soybean fields exhibiting symptoms consistent with Pythium damping-off and/or Phytophthora root and stem rot; these symptoms included poor emergence, root rot, stem lesions, and plant decline. In total, 133 soil samples were baited following the protocol of Dorrance et al. (2008). Isolates exhibiting oomycete-like growth were purified and subjected to DNA extraction using the DNeasy Plant Mini Kit (Qiagen). The internal transcribed spacer (ITS) region was amplified using primers ITS6/ITS4 and sequenced via Sanger sequencing. BLASTn analysis identified five isolates as Phytophthora sansomeana with ≥98% sequence identity. Pathogenicity of the five isolates was evaluated in a greenhouse assay using the susceptible soybean cultivar ‘Traill’ (Helms and Nelson, 1998). Seedlings at the VC growth stage were inoculated using the hypocotyl inoculation method described by Dorrance et al. (2008) and maintained under high humidity. Inoculated plants developed brown, necrotic lesions originating from the inoculation site within five days, whereas control plants remained symptomless. Three isolates of Phytophthora sansomeana were successfully reisolated from lesions and confirmed using PCR amplification and sequencing of the ITS region using primers ITS6/ITS4 and cytochrome c oxidase subunit I (COI) gene using primers OomCoxI-Levup/Fm85mod developed by Robideau et al., (2011). BLASTn analysis of ITS and COI sequences showed 100% identity and 100% query coverage with P. sansomeana ex-type sequences (GenBank accessions MG865585.1 and MH136977.1). Resulting sequences were deposited in GenBank under accession numbers PX387665, PX387666, PX387667, PX714926, PX714927, and PX714928. The detection and confirmation of P. sansomeana causing Phytophthora root rot highlights the presence of an additional Phytophthora species contributing to the root rot disease complex. Its presence may complicate disease diagnosis and management, emphasizing the importance of species-level identification and further research on pathogenicity, distribution, and management of soilborne pathogens causing root rot under North Dakota conditions.