Farmers seeing Sudden Death Syndrome (SDS) in soybeans this season are being urged to look closer for a dangerous mimic: red crown rot.
Alison Robertson, plant pathologist at Iowa State University, says the emerging disease can be easily mistaken for SDS if growers rely only on leaf symptoms.
“Red crown rot has similar leaf symptoms to SDS,” Robertson says. “The best way to check for red crown rot is to look at the base of the soybean stem, and if you see little bright red structures, like little balls, tiny little red balls — it almost looks like the soybean plant is bleeding — then you probably have red crown rot.”
Robertson says Iowa State’s plant disease clinic is actively seeking suspect samples this season, especially because a number of surrounding states have already confirmed the disease.
“We need that sample to come into the plant disease clinic at Iowa State because so far all the states surrounding us have confirmed red crown rot, but we still have not found it yet in Iowa,” she told AgriTalk Host Chip Flory on Tuesday morning. However, later that same day Iowa State did confirm its presence in the state.
“It was only a matter of time before this article needed to be written,” wrote Daren Mueller and team. “Yesterday, we scouted a field in Des Moines County in southeast Iowa. The short story: red crown rot (RCR) was identified in a swath stretching across part of the field. In talking with the farmer, he noted that the same area had shown up on the yield monitor map two years ago, but he assumed it was sudden death syndrome (SDS).”
Red crown rot confirmed in Iowa: https://t.co/VaVJUKrKYB.
— Iowa State Plant Protection (@ISU_IPM) August 25, 2026
For this season, there is nothing to be done from a management standpoint. However, confirming “SDS patches” are actually SDS will be important for making management decisions in future years.@ISUCropNews @IowaSoybeans pic.twitter.com/FBYAsnoORJ
Random Pockets of Disease, Not a Classic “March” North
Red crown rot has a long history in the South, but it was first detected in the Midwest, Illinois, less than a decade ago. It has since appeared in other Midwest states including Indiana, Michigan, Minnesota, Missouri, Ohio and Wisconsin, according to the Crop Protection Network (CPN).
“We’ve just had these random places where it’s occurred, like a county in southwest Minnesota, so it’s not spreading the way like we saw with tar spot… This is just random pockets that suddenly show up,” Robertson says.
That irregular pattern, combined with its visual similarity to SDS, raises the likelihood that red crown rot is already present but going unrecognized in some Midwest fields.
Soilborne Fungus With Staying Power
Robertson describes red crown rot as a soilborne fungal disease capable of surviving for years and infecting multiple hosts.
“It’s a fungus that lives in the soil. It survives as microsclerosia. So you’ll remember sclerotia from white mold; this is microsclerosia,” she says. “So it can survive in the soil for a pretty long time, and it also has a very wide host range… it likes warm, wet conditions for infection and for disease development.”
Those warm, wet conditions have been common across at least central and eastern Iowa this season, heightening the need for careful scouting in soybean fields showing SDS-like symptoms.
Infected fields often show a mix of healthy, yellowing and dead plants scattered together, rather than uniform patches. Another key difference is that the leaves in plants affected by red crown rot tend to stay attached to the plant even after it dies, unlike SDS, where leaves drop off.
The most definitive sign is often found at the base of the plant. To confirm red crown rot, farmers need to dig plants and look about an inch above and below the soil line for small, red, pinhead-sized structures, recommends the Iowa Soybean Association.
Fields affected with red crown rot typically show symptoms after the R3 reproductive stage, according to Mandy Bish, University of Missouri Extension state plant pathologist. She says patches of symptomatic plants often occur in low-lying or poorly drained areas of fields. RCR has caused yield losses of nearly 70% in affected areas. “In Missouri, we have estimates around 54% yield losses,” Bish reports.
More “typical” documented soybean losses to red crown rot are roughly 25% to 30% in fields with established disease, according to Pioneer, which has documented that in Louisiana and Mississippi fields where the disease has “been present for years.”
RCR became a much bigger issue in the U.S. in 2025: the Crop Protection Network estimated 7.7 million bushels of soybean production were lost nationally, versus only about 121,000 bushels in 2024.
Management Tools Are Limited, At Best
Robertson says farmers currently have few targeted tools to manage red crown rot.
“I don’t think that there are any resistant varieties available, so we can’t use that,” she says. “I believe that Carl Bradley at the University of Kentucky might be doing some seed treatment work underway, so I think there is some seed treatment that will help with it.”
With control options off the table and seed treatment data still emerging, early and accurate identification becomes even more critical for long-term management and rotation planning.
What Midwest Farmers Should Do Now
Robertson’s message to growers and agronomists this season is to move beyond windshield scouting when they see SDS-type symptoms in fields — whether based in Iowa or another state.
“If you’re driving by and you just look at the leaves, and you think it’s SDS. It could be, but get in there and look,” she says. “Take a look at the base of the plant. Look for those small red dots on there.”
In Iowa, samples from suspect plants can be submitted to the Iowa State University Plant Disease Clinic for diagnostic confirmation. Testing is currently offered at no cost to farmers.
“It’s available for free thanks to the Iowa [Soybean] Association,” Robertson says.
Free testing is also offered in some other states. Interested growers should check with their local Extension office for details.
Robertson emphasizes that confirming how widespread red crown rot is in Iowa now will depend directly on what farmers send in from their fields.
“Get out there and check your fields, and send those samples in,” she advises.
Hear the discussion between Robertson and Flory at the AgriTalk link below:
