LSU AgCenter plant pathologist Trey Price said soybean growers in Louisiana have been finding a fungal disease called black root rot in their crops.
"This disease started showing up here about five years ago, and we called it a mystery disease," Price said.
The disease makes leaves turn yellow, orange-brown or mottled even though their veins remain green, said Tom Allen, an extension plant pathologist at Mississippi State University. If a plant is pulled out, much of the root will be broken off below the surface, and the main root turns black.
Cutting a stem to the pith will cause a dark brown stain on the pith, suggesting a major problem with the vascular system that carries water and nutrients throughout the plant, he wrote.
Price said while black root rot is a minor problem statewide in Louisiana, some soybean fields have lost 5 to 10 percent of their plants.
It's still a bit of a mystery: scientists don't know just what fungus causes the disease. They suspect it's one that has been known for years in cotton. But many people around the country have tried unsuccessfully to grow the fungus from infected roots, Allen wrote on the Mississippi Crop Situation blog Aug. 1.
University of Arkansas pathologists are trying to recover the fungus from numerous samples from a field in Leflore County, Mississippi, he wrote.
Price said the fungus can survive on plant residue and build up in the soil, and fields continuously planted in soybeans appear to be the most susceptible.
With corn prices low and soybean prices a bit more steady right now, he said, growers are less likely to rotate their fields.
"At this time we don't have the data to prove that rotation with corn helps," Price said. "But I have seen a decrease in the presence of the disease where soybeans have not been continuously planted."