Research HighlightsIdentifying Symptoms and Solutions for Soybean Taproot Decline
By Laura Temple
Many soybean diseases have similar symptoms, though they are caused by very different pathogens. A problem easily can be misdiagnosed, especially if it’s something new.
Agronomists, farmers, consultants and researchers in southeast Arkansas, northeast Louisiana and western Mississippi have first-hand experience learning to recognize a new soybean disease: taproot decline.
“Several years ago, a soybean disease was brought to the attention of plant pathologists in this delta region where Arkansas, Louisiana and Mississippi come together,” says Terry Spurlock, Extension plant pathologist for the University of Arkansas System Division of Agriculture Cooperative Extension Service. “It appeared a bit like sudden death syndrome, but roots of affected soybeans looked more like black root rot, a cotton disease.”
A combination of scouting and research led Spurlock and his colleagues to the conclusion that they were seeing a new soybean disease. A collaborative multi-state working group went through the steps needed to prove that this was a previously unidentified disease, caused by a unique pathogen.
The group named the pathogen Xylaria necrophora, and the disease itself taproot decline, or TRD, because of the way it attacks the primary soybean root.
“In the world of plant pathology, it’s rare to identify and name a new disease, so it has been very interesting from a scientific perspective to be a part of the TRD working group,” Spurlock says. “We are now in the process of understanding where it is, why it becomes severe in some fields, and how farmers can manage it. We’ve been fortunate that our state soybean checkoffs have been very supportive in funding this work.”
To date, Spurlock says much of that work has focused on helping county Extension agents, consultants and farmers identify TRD and distinguish it from other problems.
“As I have visited fields, it’s become clear that this is an economically important disease in Arkansas and the other states where it has been found,” Spurlock says. “It can cause significant yield losses.”
While symptoms most commonly appear during pod fill, TRD can infect soybeans at any growth stage. Early signs of the disease can be seedling death and seed rot. At later growth stages, common foliar symptoms include chlorosis and dropping leaves. Root symptoms include black fungal growth, and the often the entire root system snaps off when an infected plant is pulled by hand. Humid conditions in fields with a history of TRD allow fungal growth called “dead man’s fingers” at the base of soybeans or on residue.
“In fields with TRD and other diseases side-by-side, it is easy to understand why it can be misdiagnosed,” Spurlock notes. “The foliar symptoms can sometimes appear with nutrient deficiencies, making them a bit atypical or ambiguous, but we are getting better at identifying it, and we have been training others to recognize it by root symptoms.”
While identifying the disease, Spurlock and the TRD working group have been documenting the distribution of the disease. They have been studying the soil system and microbial community in areas where the disease shows up, in an effort to learn why it appears in one spot and not in another spot in fields. A deeper understanding of Xylaria necrophora and how it interacts with its soil environment will provide direction for long-term control recommendations.
At the same time, Spurlock and others recognize that farmers need immediate options to manage TRD.
“Another key focus of our research is good old-fashioned product testing and variety screening,” he says. “We have been doing trials with seed treatment and in-furrow applications of fungicides to see if any existing products may be helpful. We’ve seen some seed treatment chemistries with some effectiveness. However, these have been inconsistent in field trials.”
Variety screening is identifying degrees of tolerance or susceptibility to TRD. Spurlock notes that while some varieties show a bit more tolerance, the patchy nature of the disease in fields can be very challenging for these trials. It’s their primary goal to identify very susceptible varieties and advise farmers to avoid planting them in fields with a history of heavy TRD pressure.
“We’ve seen severe cases of TRD in continuous soybean fields,” Spurlock says. “Right now, our best recommendation is to rotate crops, if possible, in those fields. While it hasn’t been completely successful, rotation does seem to make a big difference in reducing the presence of the pathogen.”
He adds that it takes time to learn how to design research to effectively get answers for this new disease, simply because the team still has much to learn about how it works. However, the working group freely shares what they are learning, so that they can move toward firm recommendations to protect soybean from this new disease.
This project was funded by the soybean checkoff. To find research related to this research highlight or to see other checkoff research projects, please visit the National Soybean Checkoff Research Database.