Production Blog

Nematodes Linked to Sudden Death Syndrome in Soybeans

Pamela Smith
By  Pamela Smith , Crops Technology Editor
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If you want to see if soybean cyst nematode resistance is holding up in the varieties you planted, look for females feeding on roots. They look like small white pin dots and are much smaller than regular nodules. (DTN photo by Pamela Smith)

DECATUR, Ill. (DTN) -- The '80s rock band The Fixx likely did not have soybeans on their mind when they belted the hit song "One Thing Leads to Another." Still, the tune seems appropriate when considering news from The SCN Coalition that soybean fields infested with soybean cyst nematode (SCN) are at increased risk of sudden death syndrome (SDS).

This time of year, above-ground symptoms from these two perennial problems may not be evident. However, it is possible that both SCN and the fungal pathogen that causes SDS (Fusarium virguliforme) are already at work below ground on a one-two punch that doles out yield losses.

It is relatively easy to catch female nematodes in the act of gorging themselves on soybean roots in mid-to-late July. Read more about how to perform those digs here https://www.dtnpf.com/….

This midseason scouting for SCN females can offer valuable clues to whether the varieties you planted are working. Eventually, the adult SCN females fill with eggs and die, changing into hardened cysts that protect the eggs in the soil. Fall soil tests are used to determine SCN egg counts that help guide management strategies for the coming year. If you see lots of feeding in those fields now, it's a sign that fall testing is needed.

ONE THING LEADS TO ANOTHER

By the time SDS rears its head in the fall with those characteristic leaves with yellow blotches between the leaf veins that eventually turn brown at the center, soybean roots have been infected for months. SDS begins in wet springs when the pathogen, which is present in the soil, infects soybean roots. Rain during the reproductive growth stages allows the pathogen's toxins to move from the roots to the leaves.

In a 2019 Michigan State University study (https://apsjournals.apsnet.org/…), researchers drew the conclusion that SCN has significant association with SDS development.

SCN reproduction thrives in dry soils, while SDS prefers cooler and wetter conditions. However, Martin Chilvers, a Michigan State University plant pathologist, said soybean farmers need to understand that no matter the weather, these two pathogens appear to compete for nutrients within the root, either directly or indirectly.

"The data from the 2019 study reveals the added pressure of SCN can increase the risk of developing severe SDS in a density-dependent manner. In other words, SCN can exacerbate SDS," Chilvers said in The SCN Coalition release.

Severe SDS can also develop in the absence of SCN, but soybean producers who detect SDS during the growing season or have fields that have shown SDS in the past are encouraged to add SCN sampling to their list of fall management chores.

"I want to stress that SDS worsens as SCN population densities rise," he said. "We want soybean growers to understand this relationship, and I hope this gives them another reason to test soil for SCN."

Chilvers is hopeful a soil test assay will also be available to test for Fusarium virguliforme in the future. "The study that we conducted was a proof of principle which demonstrated that our assay could detect hotspots of SDS within a field. We are now collaborating with others to expand the application of our assay to many soil types and locations to see if the assay can be routinely used as an SDS risk prediction tool, which would aid as a decision tool for the selection of SDS-resistant varieties and SDS-specific seed treatments," Chilvers said.

WATCH THE WEATHER

There are many scenarios at play this year, but environment plays a role in SCN numbers, according to Greg Tylka, Iowa State University nematologist.

"We have data verifying that SCN is worse in hot, dry years," he said. "The nematode is reproducing much quicker, raising population densities.

"Many soybean growers have faced back-to-back years of hot and dry conditions," added Tylka. "Once again, I would expect SCN soil samples collected this fall to reveal higher SCN population densities. In a traditional corn-soybean rotation, accelerated nematode reproduction would be a consideration the next time you plant soybeans. I urge growers to work with their agronomist or crop consultant to fine-tune their SCN active-management strategy."

For more information on how weather influences SCN go to: https://www.youtube.com/…

For state-specific advice visit www.TheSCNCoalition.com

To learn more about why SCN varietal resistance isn't enough go to: https://www.dtnpf.com/…

Read more about how farmers are rotating varieties https://www.dtnpf.com/…

Pamela Smith can be reached at pamela.smith@dtn.com

Follow her on Twitter @PamSmithDTN

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