WEST LAFAYETTE, Ind. — With sudden death syndrome (SDS) just starting to show up in a few soybean fields, it’s time for growers to start scouting for the telltale first symptoms of yellowing and defoliation of upper leaves.
Ross Fear, a Fulton County, Ind., grower, noted a few areas are showing signs of SDS. "It’ll really show up in the next week," he said. "If it gets severe enough, it’s easy to see from the road. It’s best to monitor it with weekly scouting."
Considered one of the most important diseases of soybeans in the Midwest, SDS was first discovered in Arkansas in 1971.
It has since spread to Illinois, Indiana, Iowa, Kansas, Kentucky, Minnesota, Mississippi, Missouri, Nebraska, Ohio and Tennessee.
It is most severe when soybeans are planted early in cool, wet soils and when heavily saturated by heavy mid-summer rains. In other words, this year’s weather has been ideal for SDS.
Farmers have been planting soybeans earlier every spring for the past few years. If the early spring conditions are favorable for rapid soybean growth and saturating rains do not occur during early reproductive stage of growth, SDS risk is limited even though the fungus is in the soil. Purdue University advises growers to plant fields with no SDS history first, then move into fields where it has been a problem.
The fungus that causes SDS infects the roots of soybean seedlings early in the growing season, but foliar symptoms don’t typically appear until after the soybean plants reach reproductive growth stages. Angie Peltier of the University of Illinois said these symptoms begin with a yellowing of the tissue between leaf veins.
"But," she cautioned, "foliar symptoms of SDS can be confused with those of brown stem rot, which causes browning of the innermost stem tissue. Stems of plants with SDS remain healthy."
Purdue’s soybean experts note because the SDS fungus can overwinter in soil, areas of a field that show symptoms of the disease often grow larger with each growing season until most of the field is affected.
The obvious question is: "How can SDS be managed?" The answer, according to Purdue, is that such options are limited. Crop rotation does not appear to reduce SDS; severe outbreaks have occurred after several years of continuous corn.
Rotation does reduce other soybean pathogens, leading Purdue researchers to conclude that shifting to two-year rotations of corn and soybeans – compared with longer rotations involving small grains and forages – reduces other soybean pathogens.
While soybean breeders are developing SDS-resistant varieties, progress has been slow. Growers are advised to seek accurate information about how each variety reacts to SDS and to read variety descriptions carefully.
"When looking at a variety described as resistant, ask how extensively it was tested under conditions known to favor SDS," advised one researcher.
Purdue advises growers to keep detailed field records of when and where SDS and other soilborne diseases occur.
Growers should map problem fields or areas within fields. While handheld GPS receivers work well, even simple sketched maps will help record problem areas.
By documenting these areas, growers can determine the effectiveness of their management practices. Fields with severe SDS outbreaks should be earmarked for later planting, and every operation improving water permeability should be considered.
Last, but not least, varieties with some degree of resistance should be planted in those fields.
