Soybean (Glycine max L.) and corn (Zea mays L.) are the two main field crops grown in the Midwest, and they are usually used in a rotation system (FAOSTAT, 2018). This cropping system contributes to the elevated nitrate levels found in the surface waters in the Midwest (David et al., 1997; Dinnes et al., 2002), along with higher risk of soil erosion (Pimentel et al., 1995) and loss of nutrients and organic matter (Burkart and James, 1999; David and Gentry, 2000). Cover crops, grown between soybean or corn harvest in the fall and planting of the subsequent crop in the spring, may reduce erosion and nutrient loss, thus improving or maintaining soil health as well as protecting water quality (Lu et al., 2000; Sarrantonio and Gallandt, 2003; Kaspar et al., 2007; Qi and Helmers, 2010; Kaspar and Singer, 2011). However, little is known about the impact of cover crops on soybean seedling and root rot diseases, as well as on their causal agents, including Fusarium graminearum and Pythium sylvaticum. This study was conducted to: (i) determine the susceptibility of different cover crop species to infection by F. graminearum and P. sylvaticum; (ii) test the effect of long-term winter rye (Secale cereale) cover crop, established for at least 10 years, on soybean seedling and root rot diseases, growth parameters, and grain yield; and (iii) evaluate the effect of different cover crops species on soybean grown in soil artificially infested with F. graminearum and P. sylvaticum.

In vitro and greenhouse experiments were conducted to evaluate the susceptibility of cover crop species to infection by F. graminearum, F. oxysporum, and P. sylvaticum. Seventeen cover crop species belonging to the Brassicaceae, Poaceae, and Leguminosae families were tested in addition to a resistant and susceptible soybean cultivar and a corn hybrid. For the in vitro study, eight seeds of each plant species were placed in petri dishes colonized with F. graminearum or P. sylvaticum, and non-colonized petri dishes were used as controls. After 7 days, crops were assessed for germination rate and disease severity. For the greenhouse experiment, seeds of each plant species were sown to pots containing sand:soil mixture (2:1 [v/v]) infested with F. graminearum, F. oxysporum, or P. sylvaticum. Non-infested sand:soil mixture was used as control. Pots were incubated for 4 to 5 weeks, then all plant species were assessed for percent of plant emergence, dry biomass weight, and root rot severity (% root length showing discoloration). Our findings from an in vitro experiment indicated that most of the species were susceptible to F. graminearumand P. sylvaticum,except pea, which was the only species that was not affected by P. sylvaticum. Under greenhouse conditions, we observed that soybean, alfalfa, pea, red clover, false flax, radish, rapeseed, corn oat, rye, ryegrass, and wheat are susceptible to infection by F. graminearum. False flax, radish, rapeseed, corn, millet, oat, rye, ryegrass, sorghum, and wheat are susceptible to infection by F. oxysporum. Soybean, alfalfa, hairy vetch, red clover, false flax, turnip, oat, rye, ryegrass, sorghum, and triticale are susceptible to infection by P. sylvaticum. It is important to know the cover crops that are susceptible to infection by these soilborne pathogens, because susceptible cover crops can increase pathogen populations in the soil, leading to a higher risk of soybean diseases and yield loss; therefore, soybean farmers can make a better decision on what cover crops to plant.

The effects of winter rye cover crops (Secale cereale L.) on soybean population, biomass, root morphology, seedling and root diseases, pathogen incidence, canopy reflectance, and yield were assessed over two years at four field locations (eight site-years). Plots without a rye cover crop were compared to plots with early-kill rye and late-kill rye cover crops, which were terminated 34 to 49 days or 5 to 17 days before soybean planting, respectively. Soybean shoot dry weight, root rot severity, and incidence of Fusarium spp. and Pythium spp. on roots were not influenced by the treatments. Soybean grain yield and plant population were reduced in the presence of rye at two site-years, increased in one site-year, and not changed in the remaining five site-years. Soybean canopy reflectance, measured at 810 nm at approximately 120 days after planting (DAP), differed by treatments but was not consistently associated with the presence or absence of a rye cover crop. Our field studies suggest that soybean farmers can use winter rye as a cover crop in soybean fields with low disease pressure without increasing the risk of soybean seedling and root diseases or suppressing soybean yield and plant populations.

From our field microplot and in vitro experiments testing cover crop species, we observed that cover crops rarely affect soybean diseases caused by F. graminearum and P. sylvaticum, as well as soybean growth and productivity. In the field microplot study, cover crops of rye, oat, triticale, hairy vetch, red clover, brown mustard, and rapeseed were tested in 2016 and 2017, and the site was infested or not with F. graminearum or P. sylvaticum. Soybean yield, plant population, plant height, and shoot weight were sometimes influenced by the cover crops; but the results were inconsistent between years. Filtrates of whole plants of cover crops of rye, oat, red clover, and alfalfa were tested in vitro. Filtrates of alfalfa, rye, and red clover stimulated the mycelial growth of F. graminearum,and the oat filtrate treatment was not different from the non-amended control. The mycelial growth of P. sylvaticum was inhibited by rye filtrates; however, no disease suppression was observed in field conditions.

The results from this study have improved our understanding of the impact of cover crops on soybean diseases caused F. graminearum and P. sylvaticum, and on these soilborne pathogens. Although we observed that most of the cover crops tested in this study may be susceptible to infection by F. graminearum, F. oxysporum and P. sylvaticum, they rarely have detrimental effects on soybean health, measured as plant weight, shoot height, root length, and canopy reflectance as well as and production in field conditions.