The Mystery of SDS

As sudden death syndrome (SDS) took its toll in Midwest fields last summer, farmers first asked what they could do about it. Another logical question is, “What research is being done to solve the problem?” In fact, a lot is happening. Throughout the Soybean Belt, checkoff funds are being invested in research to understand and manage the disease.

SDS is caused by a soil fungus that infects the soybean root early in plant development; later, the pathogen rapidly kills the plant during pod set and seed fill. Infection of the soybean root is worsened by cool weather as well as soil compaction and poor drainage.

SDS first showed up in Arkansas in 1971. Its name developed naturally because the new disease was characterized by plants suddenly dying, without any previous symptoms. As SDS moved north and west, it arrived in Illinois in the early 1980s.

Since 1985, Southern Illinois University has been conducting SDS variety trials to evaluate both public and private soybean varieties, says Jason Bond, a plant pathologist.

Commercial variety trials have been funded annually by the Illinois Soybean Association. These trials evaluate more than 1,000 commercial varieties across central and southern Illinois. SDS trials that evaluate public germplasm from regional universities are funded by the North Central Soybean Research Program and involve sites across multiple states. The public efforts result in new lines with improved resistance to SDS that are released to the entire industry, which is the fastest way soybean checkoff dollars can be invested to make industry-wide improvements.

“Illinois didn’t have as much SDS in 2010 as Iowa,” Bond says. “Some SDS showed up early in the season, but the weather turned hot and dry so the disease didn’t progress like it has in past years. It takes all the pieces for SDS to develop – the pathogen, the weather and the host. When one factor changes, it prevents the problem from being catastrophic.”

Interestingly, Bond also says he has also seen the pathogen blow up in a certain area and bother for awhile, and then be not as prevalent, as resistant varieties are released. “Comparing the last five years to 2000, we see in southern Illinois that SDS is no longer the biggest worry,” he observes. “With current practices like early planting in cool weather, there may be big losses yet to come, but most years in Illinois the rain patterns determine how severe SDS will be. Hopefully, we’ll see more stability develop in the north, too.”

Bond recommends using seed treatments when planting early, even though they don’t specifically help with SDS.

As SDS moved north and west, Iowa Soybean Association farmer leaders identified the increasing threat in 2005 and committed additional checkoff funding to expand research at Iowa State University (ISU) on soil-borne pathogens. The result has been improved knowledge of the pathogen that causes SDS and newly gained hope for genetic resistance in maturity group II varieties.

ISU soybean breeder Silvia Cianzio leads a successful program to develop breeding lines with significantly improved field tolerance to SDS. “We’re making progress in moving sources of resistance from later maturity groups, where resistance to SDS is more common, to maturities suitable to Iowa, but it takes time,” Cianzio says. She recently released two additional breeding lines with high resistance to SDS to private companies.

One challenge in developing new lines is that they must be resistant not just to SDS but to other diseases as well. “If a plant is weakened by one disease, it is more susceptible to other stresses,” Cianzio says. “That is why we’re focused on developing lines with stacked traits like SDS and SCN. We hope to add brown stem rot and iron deficiency chlorosis resistance as well.”

In 2006, the Iowa Soybean Association worked with ISU to hire Leonor Leandro, a plant pathologist specializing in soil-borne fungal pathogens like SDS. Much of her work is focused on discovering mechanisms that protect the soybean plant against infection. Her work has shown that soybean seedlings are most susceptible to infection the first few days after planting.

“We have learned that the longer seedlings grow before they are infected, the fewer SDS symptoms we see in vegetative stages of plant growth,” Leandro says. “In seedlings, the fungus can penetrate and colonize the root, from where toxins are translocated up to the leaves. As the seedling gets older, a yet unknown protective mechanism slows down the pathogen in the vascular system. However, we do not know how these mechanisms affect disease development in mature field plants.”

Leandro discovered that the onset of SDS can be delayed by plucking flowers, a tactic used by researchers to delay the reproductive process. This suggests there is a link between the onset of reproduction and plant sensitivity to the pathogen toxins.

Leandro has teamed with ISU plant pathologist Gary Munkvold to study the infection of soybean roots by several Fusarium species. There are 10 species of Fusarium commonly found on soybean roots; Fusarium virguliforme, the pathogen that causes SDS, is one. This research team is determining which of the Fusarium species aggressively attack soybean roots and how they impact yield.

Leandro and Munkvold use a root scanner to measure how Fusarium species affect roots. “Analysis of a root scanner image tells us precisely how much root rot is present rather than relying on subjective observation,” Munkvold says. “We can measure the amount of root decay, as well as the root structure, so we know how many root tips are present, their size, and how much root surface is present. All of these may impact yield and these measures of root health may be improved by fungicides applied as a seed treatment.”

Based on what they’ve learned, Munkvold says, “We feel that we can contribute to SDS management with seed treatments, but we must find seed treatments that last long enough and are systemic enough to protect the plant when the fungus that causes SDS tries to enter the vascular system. In the growth chamber we are seeing some success in controlling it; we’re working to see improved yields in the fields, as a result.”

Will the problem of SDS be solved in our lifetimes? Munkvold says, “Just as there are other problems that we’ve been working on for decades, I expect that SDS may not be completely solved, but eventually it will be manageable without costing a lot of money. Soybeans and corn must continue to be highly productive crops for Iowa farmers, so we must continue the research to understand these pathogens and reduce the problem in the future.