Variation in soybean rhizosphere oomycete communities from Michigan fields with contrasting disease pressures

Although oomycete species can contribute to significant losses in soybean plant density, root mass and yield, they are often underrepresented in high-throughput sequencing studies. In this study, soybean oomycete rhizosphere communities were characterized over two years from locations with and without historical disease pressure. The goals of this research were to examine the effect of location, soybean genotype, and seed treatment on oomycete communities. Soybean oomycete rhizosphere communities were dominated by Pythium, but community composition differed depending on the location and year. Pythium ultimum var. ultimum was the most abundant oomycete OTU accounting on average for > 30% relative abundance in high disease pressure sites. However, sites without historical disease pressure were not devoid of oomycete plant pathogens indicating that historical disease pressure may be due to an imbalance of species, rather than simply the presence or absence of highly pathogenic species. High-disease pressure sites contained more oomycete taxa and were less even. There was no substantial evidence of seed treatment or soybean genotype impacting oomycete community composition or diversity, however, plant density and root biomass increased with the addition of neonicotinoid insecticides. Overall, this study represents an improvement of our understanding of oomycete communities in soybean rhizosphere and the impacts of agronomic factors on oomycete diversity.