ANKENY, Iowa (DTN) -- An emerging soybean disease is threatening severe localized yield losses as it appears in more Midwestern states. Researchers say early detection and sound management practices are the best defenses against the relatively new foe.

Red crown rot (RCR), caused by the soilborne fungus Calonectria ilicicola, has steadily expanded beyond its traditional range in the South and along the East Coast. First detected in North Carolina in 1972, the disease reached Illinois in 2018 and is now widespread there. This year, Iowa joined the growing list of Midwestern states reporting cases of RCR. Find the latest CPN red crown rot map at https://cropprotectionnetwork.org/….

The fungus infects soybean roots and can lead to poor plant growth, premature death, and yield loss.

"There's horror stories out there from people losing 50% to 70% of yields in patches where it's bad," said Daren Mueller, an Iowa State University Extension and Outreach plant pathologist and professor. He was part of a team that scouted for and confirmed Iowa's first two cases of RCR in mid-August and mid-September.

"The field with red crown rot in Des Moines County (located in southeast Iowa) had a lot of plants with no pods. If there were pods, there were no beans in the pods," Mueller continued. "The level of yield loss potential should get people a little bit nervous."

However, he added farmers shouldn't panic. They can take steps to manage and slow the spread of the disease.

SCOUT AND CONFIRM

Foliar symptoms of RCR -- premature yellowing, interveinal chlorosis and necrosis, wilting and plant death -- are most visible during the soybean's reproductive stages. Stem symptoms include a reddish discoloration at the base or crown of the roots, hence the disease's name. In humid conditions, this area may become covered with white fungal growth and reddish-brown spore-producing structures.

Red crown rot can resemble sudden death syndrome (SDS) and other diseases. This makes RCR tough to spot during the growing season and even harder at harvest when leaves have dropped.

Roots of soybeans infected with RCR are often severely rotted, making plants easy to pull out of the ground, which is another indicator of the disease. "It takes effort to pull a mature soybean plant out that doesn't have red crown rot. The ones that do, it's like pulling a pencil out of the soil," Mueller said.

Horacio Lopez-Nicora, an associate professor of soybean pathology and nematology at The Ohio State University, urged farmers to scout fields and watch the combine yield monitor for significant yield drops. Dig up suspected plants and have them tested. In Ohio, free diagnostic testing is available through the university's Soybean Pathology and Nematology program. Learn more at https://soypath.osu.edu/….

"Don't assume soybeans with SDS-like symptoms have SDS. It could be red crown rot," Lopez-Nicora said. "That's the mindset I encourage farmers to have now. Identify the pathogen so you can work with your agronomist, consultant and/or plant pathologist to come up with a management plan."

RCR was first detected in central Ohio in 2025, and one case was reported this year.

REDUCE THE SPREAD

Once RCR is established in a field or region, researchers say eradication is unlikely because the pathogen survives in the soil. Taking steps to limit soil movement is the key to slowing its spread.

"Conservation practices can help," Mueller said.

Planting cover crops and adopting no-till and conservation tillage can help control wind and water erosion, keeping topsoil in place.

"There's some old school thought that if we bury the pathogens with deep tillage, that will get rid of them. Actually, you're moving and spreading them," Lopez-Nicora said.

In known fields with RCR, researchers recommend harvesting those fields last if possible. Also, cleaning equipment before moving between fields can help prevent its spread. This is especially important this year given the wet field conditions across much of the Midwest.

"One culprit for sure is equipment," Mueller said.

ROTATE CROPS

If RCR is confirmed, rotate to a non-host crop the following year or two to help reduce pathogen levels in the soil. These crops include corn, sorghum and small grains, such as wheat and oats.

Avoid alfalfa as a rotation crop as it is a host.

CONSIDER SEED TREATMENTS

Damon Smith, a University of Wisconsin-Madison Extension field crops pathologist, said some fungicide seed treatments, such as Saltro and Victrato from Syngenta, have shown promising results curbing RCR severity and yield losses.

Wisconsin has two reported cases of RCR in the past two years, according to the CPN.

"At this point, we're going to rely heavily on seed treatments to combat the disease," Smith said.

MOVING FORWARD

Research is ongoing to improve RCR management, seed treatment options and find less susceptible soybean seed varieties. Learn more about this work at https://soybeanresearchinfo.com/….

"Pay attention to research, and more varieties are being bred with RCR resistance," Smith said. "Right now, it's important to scout and diagnose the disease to have serious conversations about your management plans in the years to come."

Read more about the emerging pathogen at https://www.dtnpf.com/….

Matthew Wilde can be reached at matt.wilde@dtn.com.

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