Research Highlights

Mustard as a Cover Crop Shows Potential in Reducing Charcoal Rot

By Carol Brown

Charcoal rot is a large problem for soybean growers everywhere, but particularly in Kansas where the disease has been labeled the third greatest cause of yield loss in the state. Annual losses have been recorded in the millions of bushels (Allen et al, 2017). Southeast Kansas has been found to have higher concentrations than other parts of the state.

Researchers at Kansas State University are working to find ways to lessen the impact of the disease for soybean farmers in the state and possibly nationwide. Gretchen Sassenrath, crop production agronomist, and plant pathologist Christopher Little are focusing on using the mustard plant as a cover crop for charcoal rot control.

“Mustard is a brassica, like turnips or rapeseed, but mustard is high in glucosinolate,” said Sassenrath. “This is the same plant used to make the condiment — the glucosinolates are what provides mustard’s tang. We have found these high levels of glucosinolates can reduce charcoal rot.”

Sassenrath and Little have shown that growing mustard prior to planting the soybean crop can help reduce the presence of the colony-forming units of Macrophomina phaseolina, the fungus that causes charcoal rot. The researchers are determining the impact of charcoal rot in the soybean plants by measuring the number of colony-forming units of Macrophomina.

Their results have been promising. They saw a 50 percent reduction of the colony-forming units in the soybeans that were in the plots with a prior mustard cover crop. These units were reduced by 8 percent in the soil as well.

In each of the research plots, they planted the mustard the same way and terminated it with herbicide but varied the handling of the cover crop after termination, which made the most difference in the number of charcoal rot colony-forming units.

“We found that the more complete we left the mustard after termination, the better it performed,” Sassenrath said. “Rolling the mustard to lay it down and leaving the residue on the surface had the best results in reducing charcoal rot. Incorporating the mustard residue into the soil through tillage seemed to increase charcoal rot infection.”

They did not find issues planting soybeans into the mustard residue, as it is a fairly small plant and only a light layer of residue remains. Nor did the mustard cover crop reduce yield in any of the research plots.  

To determine the amounts of colony-forming units, they collected soil samples in the spring after cover crop termination and prior to soybean planting. Soil and soybean plant samples were collected again at growth stages R7-R8 to measure the amount of charcoal rot pressure. Yield was also measured at harvest. The research included a control plot without a mustard cover crop.

Sassenrath cautions farmers that other brassicas such as turnips or canola may not be as effective in reducing charcoal rot. The canola plant has been bred to reduce the amount of glucosinolate to keep the flavor of canola oil mild and not have the tang like mustard, she said.

Adding mustard into the crop rotation to increase diversity in the field can help control charcoal rot. Mustard and other brassicas support a different microbiome than traditional crops, so it’s good to get them into the rotation somewhere, Sassenrath said. This can help break the disease cycle for charcoal rot and there is evidence it may work for sudden death syndrome as well.

Published: Nov 23, 2020