Research Highlights

Seedling Diseases of Soybean: Characterization and Education

By Jason P. Bond, Southern Illinois University

Soybean crops are susceptible to soilborne seedling and root diseases, which reduce yields by more than 35 million bushels annually in the U.S. Seedling diseases rank among the top four pathogen threats to soybean; insidious nature makes them difficult to diagnose and control. It is difficult to predict when seedling and root diseases will take a heavy toll until it happens. This project received $268,113 in 2019 to address these critical challenges.

This project focuses on identifying the impact of environmental conditions, stress syndromes, cover crops and specific production practices on early season root pathogens. In addition, the interactions between root and seed pathogens and seed quality, emergence and seedling vigor are being evaluated.

Fusarium oxysporum and Fusarium graminearum are two of the most prevalent pathogens that infect soybean roots, but little is known about the conditions that favor infection and enhance disease development. This research is investigating how soil moisture, texture and pH affect the severity of seedling disease. Root rot caused by F. oxysporum was found to be enhanced in soybeans planted in warm conditions (25-27 degrees Fahrenheit) and in moderately acidic soils (pH 6). Root rot caused by F. graminearum was also favored by moderately acidic soil and was most severe in moisture-limiting soil conditions, particularly in sandy vs. loam soils.

Two additional Fusarium species, F. solani and F. tricinctum, are also common root rot pathogens. Both cause severe root rot and kill seedlings under high disease pressure. This research has shown that the amount of pathogen in the soil, the soil type, moisture and temperature are key factors in the development of root rot. In a sandy loam, disease is favored when the soil moisture content is greater than 40%. F. solani is the more aggressive pathogen and infects soybean roots over a broader temperature range and at cool soil temperatures where soybean germinates slowly. Lighter soils tend to favor infection and higher disease severity, while heavy clay soils are generally associated with lower disease severity. 

Soil type and contents can also favor the activity of one pathogen over the other. For example, the presence of soybean cyst nematode at high populations can increase seedling root rot caused by these pathogens. Salinity was thought to be a major factor predisposing seedling to root rot, but the research showed that the effect of salinity on root growth was more important than the effect of the pathogen. These new findings clarified will help soybean growers make more informed management decisions about how to reduce risk from these pathogens.

Another line of investigation looked at the role of Fusarium in the development of iron deficiency chlorosis (IDC). Three years of field studies comparing root colonization by Fusarium species in plants with and without IDC were conducted. There was no difference in the frequency of Fusarium species on roots of plants with and without IDC, suggesting that infection by Fusarium is not a key driver in the development of this syndrome.

The effects of seed treatment and soybean seed quality, as well as planting date, were determined in fields infested with soybean pathogens. Seed quality had the greatest impact on stands and yields at all locations and at both planting dates. Most seed treatments evaluated resulted in greater stands and yields than the untreated control with low-quality seed. There were no differences between treatments in stands or yields when high-quality seed was used. Seed treatments applied to seeds with low quality were equally effective May and June plantings. In some conditions, seed treatments actually lead to a reduction in stands and yields, especially when low-quality seed is used. 

The promotion of cover crops implements a different scenario for soybean growers. Cover crops can increase soil organic matter, water infiltration and soil structure; however, these conditions could be favorable for certain soilborne diseases. In cover crop trials, soybean seed treatments increased stands across different cereal rye termination treatments. Planting was more difficult into green cereal rye than into cereal rye that had been terminated by herbicide. 

Taproot decline is an emerging soybean disease caused by the Xylaria fungus. Members of this group are commonly observed in the forest producing a fruiting body known as “dead man fingers,” surviving on dead plant material and forest litter. In soybean fields, it has been associated with plant debris, rich in organic matter. Cover crops that increase organic matter could potentially increase the prevalence of pathogens like taproot decline. Also, cover crops can serve as alternative hosts for soilborne pathogens. Inoculation methods were adapted to study the potential of Xylaria to survive and colonize different cover crops. The cover crops studied included winter pea, radish, wheat, mustard and cereal rye. The responses varied significantly among the cover crops, either reducing, not affecting or increasing the cover crop biomass. The potential survival of Xylaria on cover crops is important in considering crop rotations for management of taproot decline.

The ultimate goals of this research project are to increase knowledge about seedling disease progression, to help farmers manage diseases more effectively and increase economic returns, and to improve yields and quality of the U.S. soybean. To this end, several webinars on seedling disease management were created, and research summaries are being developed in conjunction with the Crop Protection Network (https://cropprotectionnetwork.org/). In addition, interactive opportunities are in process, where soybean farmers can learn about soybean seedling diseases at trade shows and other events. By disseminating the results of this research widely, and in a variety of different formats, this project is helping growers manage these diseases more effectively and develop comprehensive management plans. More effective management will increase economic returns and improve yield and quality of the U.S. soybean crop. 

Published: May 14, 2020