Red crown rot is a growing threat in Missouri
by Scott Wilburn, Sr. Staff Agronomist
Visual symptoms of red crown rot include small, red fruiting structures called perithecia on the base of an infected soybean plant.
Red crown rot of soybeans was confirmed in Missouri last summer. This disease was first discovered in the 1970s in the southeastern United States. It spread to the Midwest in 2017 beginning with Illinois and then to Kentucky and Indiana a few years later. Red crown rot can cause yield losses as high as 70-80%, so there is potential for this to become an economically significant disease.
Red crown rot (RCR) is caused by a fungus that overwinters in the soil and in diseased soybean roots. The disease favors wet, warm conditions at planting. Identification can be challenging because it shares characteristics with other common soybean diseases, namely sudden death syndrome (SDS) and brown stem rot (BSR). Foliar symptoms of those diseases—yellow spots that develop into yellowing tissue between the veins—are similar to the foliar symptoms of a red crown rot infection.
To help differentiate between SDS and BSR, standard protocol is to split the soybean plant’s stem when symptoms appear. With the discovery of red crown rot in our territory, going forward it will be crucial to also carefully inspect the roots and lower stem. Red discoloration and raised fungal fruiting bodies (perithecia) are distinguishing characteristics of RCR, but be aware that perithecia are not always present. Also, there are other diseases such as rhizoctonia that can cause red discoloration on the stem. If perithecia are not found, a lab confirmation may be needed to conclusively determine which disease is causing the symptoms. Fortunately, the diagnostic clinic at the University of Missouri is available to do this testing.
The first confirmation of RCR in Illinois was just across the Mississippi River in Pike County, so it is not surprising that Missouri’s first confirmed case occurred nearby in Marion County, northwest of Hannibal. Another case was confirmed later in the summer in Audrain County. The two Missouri sites are about an hour’s drive apart, so it’s likely other undetected cases exist.
Furthermore, in 2023 MFA participated in a study in which the DNA of plant pathogens were identified in soil samples. Several samples on the extreme western side of the state tested positive for the fungus that causes red crown rot, Calonectria iliciola. It’s important to note that the pathogen’s presence does not necessarily mean RCR is imminent. The disease triangle requires host (soybean), pathogen (fungus) plus favorable environment, which essentially covers everything else.
I share this information not to raise alarm but awareness so that we can identify and manage red crown rot effectively. Similar to SDS, there is no treatment for the disease once it appears. However, because it is believed to spread via equipment, knowledge of where it exists could help slow its escalation. It could possibly spread by wind, but even if this is the case, minimizing direct transfer of infected soil from one field to another would appear to be prudent with what is known now.
I visited the infected field in Marion County last July with some members of the Crop-Trak team. We were able to find examples of leaves with SDS-like symptoms around the edge of the affected area, but these were not prominent. We were also able to find the red fruiting bodies. What was most striking to me, though, was the speed of the disease’s progression in affected areas. Soybeans that are infected with SDS or BSR often stand late into the season despite the infection. The soybeans with red crown rot, however, were already extremely wilted with the most advanced nearing death. Because of this rapid progression, extreme wilting or death in soybeans during reproductive stages should prompt inspection for RCR. Simply relying on SDS-like symptoms may not suffice because the plants may have already moved past that stage.
Seed treatments have been shown to slow the disease, but they do not provide complete control. No varieties at this time are marketed as resistant. Identification of the disease, followed by steps to avoid spreading it to other fields, crop rotation away from host plants and seed treatments are the management tools we have at this time. If you suspect RCR this season, contact your local MFA and the Crop-Trak team for assistance in assessing plants and submitting samples for lab analysis if needed.
***
CLICK TO READ more articles from this issue.
SUBSCRIBE to Today's Farmer Magazine today.