Soilborne oomycete pathogens of soybean are a constant concern, especially at the seedling growth stage. In a survey for Phytophthora sojae, 126 isolates of Phytophthora sansomeana were recovered through soil bating between 2016 and 2018, of which 42 were from Indiana, 9 from Kentucky, and 75 from Ohio. A subset of P. sansomeana isolates along with check isolates of Phytophthora sojae, Pythium irregulare, and Pythium ultimum var ultimum were selected for pathogenicity testing using Petri plate, tray test, and greenhouse cup assays. The average seed rot score for P. sansomeana infected seeds in the Petri plate assay was 2.3 on a scale from zero to three with three indicating complete colonization with little to no germination of seeds. In the tray test assay, the mean lesion development on the seedling tap root for P. sansomeana was 4.9 mm, compared to 46.0 mm for P. sojae. In the greenhouse cup assay the average root rot score of P. sansomeana was 2.1 on a scale from one to five where a score of five indicates no germination. Based on these assays, P. sansomeana isolates were more aggressive seed rotters than P. sojae and Pythium isolates. Conversely, they developed smaller tap root lesions on soybean seedlings than P. sojae, and less severe root rot compared with isolates of Pythium irregulare and Py. ultimum. This research reinforces the importance of selecting a diverse panel of isolates to screen for resistance in host plants, as well as selecting effective management strategies against multiple seedling pathogens.