Research Highlights

Research Highlights
Getting to the 'Root' of Charcoal Rot in Soybeans

Charcoal rot in soybean. Photo: Adam Sisson, Crop Protection Network

By Barb Baylor Anderson 

Charcoal rot is increasingly an economic soybean production concern in the United States. Its mounting prominence parallels a rise in average summer temperatures and a growing number of extended summer dry periods when soybeans are in critical reproductive stages.

The bad news is that no fungicide seed treatments offer consistent charcoal rot control and foliar fungicides are ineffective. However, researchers are getting to the root of the problem. 

“We are making progress understanding how to prevent charcoal rot and why disproportionately it occurs in Mississippi and neighboring states,” says Maria Tomaso-Peterson, Mississippi State University research professor and principal investigator of the study funded by the Mississippi Soybean Promotion Board. “A better understanding of the types of mycotoxins used to facilitate soybean root infection may allow development of improved methods to prevent root infection from the fungus Macrophomina phaseolina and other fungi that use this same mechanism.” 

Tomaso-Peterson leads a team that includes graduate student Vivek Khambhati, Hamed Abbas from the USDA Laboratory in Stoneville and Tom Shier from the University of Minnesota.  

M. phaseolina causes charcoal rot disease by infecting soybeans through their roots, spreading from other infected plants or decaying material in the soil. M. phaseolina appears to locate meristematic, or young, actively growing tissue near soybean root tips. During the root infection process, M. phaseolina releases a mycotoxin that kills the dividing cells of the meristematic tissue near the root tip, which exposes the soybean’s vascular system to infection. This creates a place where the fungus can enter the root and infect the rest of the soybean plant.

Effects of various toxins produced by M. phaseolina on soybean seedling foliage show no phytotoxic effect in the healthy plant control (A), and the following phytotoxic responses in others: chlorosis (B), photobleaching (C), necrosis (D), vascular discoloration (E), intervascular discoloration (F) and death (G).

The root rot that develops in soybeans appears as small, black, fungal structures, known as microsclerotia, hence the name charcoal rot. These structures form in and on the lower stem and roots of the infected soybean plants. Yield loss from charcoal rot can be highly variable, but the earlier hot and dry conditions appear, the greater the yield loss will likely be.

Better understanding of how these fungi infect soybeans can help scientists develop resistant cultivars and improve agronomic practices to minimize infection, says Tomaso-Peterson. Their studies initially identified the mycotoxin, botryodiplodin, which the majority of M. phaseolina isolates in Mississippi produce during infection. However, some isolates of M. phaseolina appear to use other toxins. Studies are ongoing to determine what other toxins are used in root infection.  

“Research is underway to learn more about how M. phaseolina finds soybean root tips in the soil, what signals it identifies to induce toxin release, and the process used to find and enter vascular systems of damaged root tips,” she says. “Each step is a potential target for developing a method to block fungal entry into the root. Studies are also being carried out to identify potential remediation strategies using biochar binding of mycotoxins in soil around root tips.”  

Tomaso-Peterson notes that studies were also planned to determine if any known mycotoxins contaminate seeds in soybean plants symptomatic of charcoal rot disease.   

Published: Apr 26, 2021

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.