Research HighlightsSoils May Hold Solutions for Taproot Decline
By Laura Temple
The recently identified pathogen Xylaria necrophora causes an emerging disease called soybean taproot decline, or TRD. As its name suggests, it attacks soybean plant roots. No commercial fungicides show activity against this disease. Plant pathologists are looking for other ways to manage TRD, found in the Delta region of the Mid-South.
Observations that individual soybean plants in infected fields had varying levels of susceptibility to the disease sparked the curiosity of Sorina Popescu, associate professor of plant pathology at Mississippi State University. Severely infected plants wilted right next to soybeans that appeared to be completely healthy. She wondered if the soil microbes in each plant root may help soybeans fight TRD.
With support from the Mississippi Soybean Promotion Board, she began investigating soil bacteria in soybean roots from fields known to be infected with TRD. She designed a new way to understand TRD progression by comparing the microbial communities of soybean plants at different stages of TRD disease and identified bacterial strains that may have an antifungal effect.
Popescu formed a team of Mississippi State University plant pathology researchers, including Tessie Wilkerson, assistant research professor of row crop pathogens; George Popescu, assistant research professor at the Institute for Genomics, Biocomputing and Biotechnology; and Maria Tomaso-Peterson, professor emeritus. The project team grew to include master’s graduate student Uyen Wesser, postdoctoral student Xin Ye, Ph.D. students Philip Berg and Josh Mitchell, and numerous undergraduate students. Mitchell has been working on this project since fall 2020.
“Many types of soil bacteria tend to form communities in and around plant roots,” Mitchell explains. “Those communities can be pathogenic, increasing the impact of fungi attacking the plant, while others can be symbiotic, supporting plant growth and health.”
Some of those microbes enter plant roots and form microbial communities in a region called the endosphere. Popescu’s lab spearheaded the in-depth characterization of soybean root microbial communities by creating a library of isolates. During the 2020 growing season, the research team collected both healthy and TRD-infected soybean roots and isolated their endophytic bacteria, creating a library of more than 230 samples of soil bacteria to study. They then screened many of those samples for activity on Xylaria and evaluated the biocontrol activity of selected isolates against TRD.
Identifying Anti-Fungal Soil Bacteria
Mitchell helped test each of these samples against Xylaria necrophora in petri dishes to see if any had anti-fungal activity against TRD.
“About 25% of the samples had significant activity against Xylaria and another 25% had moderate activity against it,” he says. “But other bacterial samples aided Xylaria growth or had no effect.”
The team selected and grew bacterial samples with the strongest anti-fungal properties to test with soybeans in growth chambers. They planted soybeans with each bacterial treatment in sterile soils with no other microbes and in natural soils containing typical levels of native microbial populations. Each treatment included trials with and without Xylaria necrophora inoculations.
“Some of the soil bacteria showed that they helped soybeans fight TRD, and other samples promoted soybean growth without any disease pressure,” Mitchell says. “With this data, we chose bacterial treatments for a field trial in 2022.”
In the field trial at Mississippi State’s Delta Research and Extension Center, soybean seeds were sprayed with bacterial treatments after planting. Due to dry conditions throughout the growing season, fungal pressure was low, and no TRD appeared.
While conducting the field trial, the team received gene sequencing data on the bacteria in some of the most promising sample treatments.
“We found that our original samples contained multiple strains of bacteria, so we separated and purified the bacteria, pulling about 116 individual bacterial isolates from the 48 library samples we wanted to sequence,” he explains. “Interestingly, gene sequencing revealed those samples consisted mostly of about 14 species of bacteria in varying ratios.”
Though Popescu succumbed to breast cancer in December 2022, this research continues under the leadership of other members of the research team, including her husband George and Wilkerson.
Combining all the data and observations gathered from the lab, growth chambers, gene sequencing and the first field trial, Mitchell says the focus for 2023 is looking at bacterial combinations likely to be most effective against TRD. They will test combinations that show potential in additional growth chamber studies in 2023 field trials.
“Our goal is to find helpful soil bacteria with anti-fungal activity that could eventually be commercialized as a treatment for TRD,” Mitchell adds. “And, we think the bacterial library created in 2020 may hold solutions for other soybean diseases. For example, we plan to leverage what we’ve learned here and test those samples on Southern blight, a disease that inflects many other crops in this region.”
Published: May 1, 2023
The materials on SRIN were funded with checkoff dollars from United Soybean Board and the North Central Soybean Research Program. To find checkoff funded research related to this research highlight or to see other checkoff research projects, please visit the National Soybean Checkoff Research Database.