Oomicide treated soybean seeds reduce early season stand loss to Phytophthora sojae

The causal agent of Phytophthora stem and root rot (PRR), Phytophthora sojae Gerdemann and Kaufman, is a major pathogen of soybean (Glycine max (L.) Merr.). Integrated pest management strategies for PRR consists of planting varieties with effective single-dominant resistance genes (Rps genes), partial resistance (field tolerance) and seed treatments containing oomicides such as ethaboxam and mefenoxam. To investigate the interaction of PRR partial resistance and seed treatment on soybean stand, vigor and yield, soybean plots were inoculated infurrow with P. sojae isolates able to overcome the Rps genes present in the planted soybean varieties in Michigan in 2018 as well as Michigan, Indiana, and Minnesota in 2019. Nine commercial soybean varieties with contrasting Phytophthora resistance profiles received three different seed treatments: 1) non-treated, 2) base (ipconazole and clothianidin), and 3) Intego Suite (base + ethaboxam and mefenoxam) in Michigan, with four of the same varieties and seed treatments being planted in Indiana and Minnesota. Soybean stand was significantly protected with Intego Suite in the nine varieties tested in Michigan 2018, eight of nine in Michigan 2019, and two of four in Indiana 2019. Significant differences in yield between seed treatments were observed in three varieties of nine in Michigan 2018. The effect of partial resistance on plant health and final yields was not apparent given the supplied company partial resistance scores across locations and years. In conclusion, this study finds that oomicide seed treatments are useful in protecting soybean stand under PRR disease pressure in multiple locations in the Midwest United States.