Research Highlights

Exploring Soybean Soil-Borne Diseases and Soil Health in Kansas

By Carol Brown

In Dr. Seuss’s children’s story “Horton Hears a Who,” there is an entire world residing on a speck of dust. Suess may have studied soil biology to find inspiration for that story because soil is indeed its own world. Researchers continue to discover new things about soil composition and the life within its micro-communities, and when agricultural practices enter the picture, it can change the entire system. Agronomists, including Gretchen Sassenrath, have been studying soil and its relationship with cropping systems for decades.

A Kansas State University cropping systems agronomist, Sassenrath’s recent research showed that mustard plants can help to reduce Macrophomina phaseolina, the organism that causes charcoal rot in soybeans. In a new project funded by the Kansas Soybean Commission, she is looking at soil-borne diseases affecting soybeans. 

“We’re looking at improving the good bacteria and fungi — the ‘good guys’ in the soil — so we can better manage the bad guys,” Sassenrath says. “Diseases including charcoal rot, Fusarium, sudden death syndrome, and Phytophthora are all endemic to our area. There aren’t fungicides to control them, and management options are limited as well.”

Sassenrath and co-investigator Chris Little, a plant pathologist at Kansas State, are exploring several conditions that affect soil health and soybean disease: crops that can host diseases including soybean residue and corn stubble, the application of cow manure, solarization, and planting mustard as a cover crop. Sassenrath oversees the field research plots, and Little is conducting controlled tests in the greenhouse.

“What happens underground is really fascinating. Plant roots secrete different sugars to encourage specific organisms to deliver nutrients to the plant,” explains Sassenrath. “It’s a very symbiotic relationship, and we’re learning about how the plants manage that biology to encourage the good guys and control the bad guys.”

Building on Sassenrath’s previous charcoal rot research results, this project includes plots with a mustard cover crop to explore the interaction between soil health and disease pressure. M. phaseolina populations will be determined by counting the number of colony-forming units. The researchers will then measure charcoal rot severity by selecting plants at the R7-R8 growth stage and rating the root and stem severity. 

“We don’t have data yet on microbial community structures or colony-forming units of charcoal rot,” she says. “Soil samples were taken in the plots before treatments were applied, at mid-season and again after harvest. The samples are being examined now for microorganism populations.”

Soil Temperature Could be a Key

Sassenrath and Little are exploring how solarization promotes or reduces organisms in the soil. Solarization involves increasing the soil temperature — basically baking the soil — to try to control microorganism populations. They covered a plot with clear plastic and compared soil temperatures across the different research plots. In Little’s greenhouse study, he added moisture levels to measure the combination of varying moisture and temperature, which could be critical for microbial survival. 

“In agriculture, methyl bromide was used to sterilize the soil, but it is extremely toxic and has been phased out,” says Sassenrath. “Scientists are looking at other ways to manage diseases in the soil, and solarization could be another way to do this safely.”

The soil temperatures were measured continuously during the growing season on the plots with corn stubble, manure, plastic cover, and the control plot that was left fallow. 

“The temperatures are quite variable. The plastic sheets raised the soil temperature more than 10 degrees above the fallow plot, and temperatures remained elevated at night,” explains Sassenrath. “The corn stubble greatly reduced soil temperatures as the stubble acted like a solid canopy cover.”

The manure plot fluctuated the most, with daytime temperatures as high as the plot with plastic cover and dropping as low as the fallow plot in the night. Over the course of the experiment, the manure was composting, or breaking down, Sassenrath says, noting high levels of microbial activity and therefore high temperatures with this process. 

“There are a lot of microbes in manure and basically, it feeds the bacteria and fungi and introduces new species to the mix,” she says. “Carbon from organic matter is another food source for microorganisms, and both corn stubble and the manure contain organic matter. We’re expecting those plots to have higher amounts of microbes overall because there are more food sources. We’re finding that the more food provided to the soil microorganisms, the more there are, and the better they grow.”

Management for Beneficial Results 

As Sassenrath and Little explore the communities of soil microorganisms under these treatments, she reminds farmers there are some things they can do to keep their soil world thriving. 

“I’m not a fan of tillage and we’re learning more about negative effects of this practice all the time,” comments Sassenrath. “Tillage breaks all the microbial and root interconnections in the soil and breaks the extensive networks of the fungal hyphae. These networks are critical for establishing the soil structure and are important for plants to extract nutrients and water. I have measured no-till soils that have double the amounts of microbes compared to tilled soil.”

She has found that tillage also increases the incidence of charcoal rot in soybeans. No-tilling with the addition of a mustard cover crop could reduce charcoal rot incidence even more. 

The previously described research plots are being harvested, and yield data is being collected now for this project. The team hopes to find results that are in line with their hypothesis that management practices such as manure application, cover crops, or increased soil temperatures could support beneficial microorganisms and reduce disease-causing organisms to help farmers fight soil-borne diseases better.

Researcher Profile — Sassenrath

Published: Dec 5, 2022