The discovery of red crown rot in a southern Minnesota soybean field is sending shockwaves through the Midwest ag sector. The disease, caused by the soil-dwelling fungus Calonectria ilicicola, was found in Rock County and confirmed through lab analysis by University of Minnesota plant pathologist Dean Malvick. Prior to this case, the nearest confirmed infection was more than 400 miles away in northwest Illinois, raising serious questions about how far the pathogen has spread - and how long it may have gone undetected.

First identified in Midwest soybean fields in 2018, red crown rot has slowly expanded from Illinois into Missouri, Indiana, Ohio, Michigan, and recently south-central Wisconsin. Now, its arrival in Minnesota suggests it may be more widespread than currently recognized. The fungus is known to persist in soil for multiple years, making crop rotation largely ineffective as a mitigation strategy. In most trials so far, Midwestern soybean varieties have shown high susceptibility, and effective fungicide seed treatments remain limited and inconsistent.

Visual symptoms of RCR often resemble sudden death syndrome (SDS), stem canker, or brown stem rot, making early detection difficult. Common field signs include interveinal chlorosis, premature senescence, and a reddish discoloration at the stem base. The most characteristic feature - although not always present - is the appearance of bright red, spherical perithecia near the soil line. Severely infected plants often die early, with leaves remaining attached, contrasting SDS, where senescing leaves typically fall off.

As with SDS, symptoms usually appear after midseason rains, when toxins produced in infected roots move upward through the plant. According to Stephanie Porter, outreach agronomist with the Illinois Soybean Association, this visual similarity often leads to misdiagnosis, allowing RCR to remain hidden until significant damage has occurred. This diagnostic complexity makes laboratory testing critical, especially in new regions like Minnesota where growers may be unfamiliar with the disease.

Managing red crown rot is especially difficult due to the fungus's resilience in the soil. Once introduced, it can remain viable for years, making eradication virtually impossible under current practices. Some biological seed treatments have shown early promise, but widespread adoption is limited by cost, availability, and inconsistent efficacy data. Additionally, there are no commercial soybean varieties with strong genetic resistance currently available in the U.S.

Malvick warns that the disease's recent appearance in Minnesota may only be the beginning. Given the difficulty in visual diagnosis and the likelihood of asymptomatic infections, it is possible that the fungus is already present in other parts of the state. Extension researchers are now working to assess the potential geographic spread, while encouraging growers to intensify field scouting, especially in low-lying, poorly drained areas where the disease tends to establish.

For producers across the Upper Midwest, this confirmation of red crown rot in Minnesota represents more than just a new disease on the map - it's a warning. With input costs rising, yields under pressure, and disease management already complex, the arrival of RCR adds a critical challenge to 2026 soybean planning. The best line of defense, experts say, will be proactive field monitoring, early sample testing, and exploring integrated seed treatment options where feasible.

In the broader context of Midwestern soybean production, this development underscores the need for continued research, extension outreach, and adaptive management practices. As the USDA, land-grant universities, and agri-cooperatives focus on the evolving threat landscape, red crown rot will likely become a focal point in upcoming crop protection discussions, especially as more cases are identified across the region.