By JORDAN STRICKLER
KNOXVILLE, Tenn. — An emerging soybean disease has caused significant losses in the southern United States over the past 10-15 years and could take up to 30 percent of a crop in the most severe situations.
The fungus-based disease, taproot decline (TRD), shows symptoms similar to other soybean diseases, most notably sudden death syndrome (SDS), but is a year-round threat and has no known treatment. The disease is caused by a previously undescribed fungal species, and foliar symptoms may be easily confused with other soilborne diseases.
Sightings first started to appear in 2007 in the Delta regions and Rachel Guyer, research specialist in the Plant Pathology Program at the University of Tennessee Institute of Agriculture, said the disease is starting to make northbound progress.
“TRD has an increasing northern distribution, evidenced by our first sighting of it in Tennessee in 2017 and again this past summer,” she said. “It has so far been confirmed in Alabama, Mississippi, Louisiana, Missouri, and Arkansas.”
While TRD mimics SDS, in that the leaves will yellow while the veins stay green, SDS is usually not found on heavier soils. TRD, however, can be found on every soil class from heavy clay to sandy soils.
While SDS infects plants early in its life, with visual symptoms generally not appearing until later in the season, TRD can be spotted when soybeans are first emerging and developing leaves and is generally found at the mid R5 to R6 growth stages. By May or early June, some fields are already showing symptoms.
“The biggest difference (between TRD and SDS) would be the presence of fungal signs on the main root system,” said Tom Allen, Mississippi State University Extension Service’s plant pathologist and Mississippi Agricultural and Forestry Experiment Station researcher. “In addition, plants removed from the soil profile exhibiting the symptoms associated with taproot decline generally have a degraded taproot and the presence of stroma, which is a black fungal reproductive structure.”
However, Allen doesn’t believe it is necessarily spreading, as much as education is improving.
“I’m not sure that what we are seeing now has to do with spread. I think we have more people familiar with the disease and therefore the map, or areas where the disease has been observed appear to be ‘spreading,’ but that’s not normally the case. Clearly the area where the disease is observed will increase in size as more people that work with soybeans in other states become aware of the symptoms and are able to culture the fungus involved in producing symptoms.”
Guyer said while no management strategies have been released on preventive measures, she recommends three experiments to see if the plant might be infected with TRD.
•Check the ground surrounding the symptomatic plant(s) for reproductive structures called stromata that typically grow on last season’s debris; these are branching structures covered in white powdery spores. These could be present or absent on the ground at the base of diseased plants growing out of previous crop debris.
•Check around symptomatic plant(s) for plants that died prematurely which can be caused by TRD.
•Pull up the entire plant to view the root mass. Diseased plants will exhibit rotten roots, with the taproot usually breaking off below the soil line, root necrosis throughout the xylem, and the pith may be filled with white mycelium.
Trey Price, associate professor of agronomic crop pathology at Louisiana State University, said one of the best defenses is good management practices.
“The pathogen causes seedling disease and death in addition to during vegetative stages, which often goes unnoticed as surrounding rapidly-growing soybeans cover diseased and dead plants. Fields in soybean monoculture with reduced tillage operations will have higher incidence of taproot decline. Rotation, tillage, and resistant varieties are current management options,” he explained.