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Stem Canker In Soybeans

As you’re starting to consider soybean seed selections for the upcoming season, monitor your crop for stem canker. By properly identifying fully expressed signs of stem canker now, you can get ahead of the disease by making a plan for varietal selection and other cultural management practices in the year ahead. But as you’re scouting for stem rot, keep in mind symptoms of charcoal rot and other diseases can be mistaken for Diaporthe fungi, which causes stem canker along with pod and stem blight and Phomopsis seed decay. 

The fungal pathogens that cause northern and southern stem canker, Diaporthe caulvora and aspalathi, can overwinter in soil or residue for many years. It can also be seedborne, and weeds such as black nightshade, curly dock and morningglory can serve as hosts.  

Stem Canker Symptoms
Infection usually occurs in patches, and stem lesions start as small, reddish-brown spots at the base of a branch or leaf petiole. After infection, the fungus releases a toxin that causes leaf interveinal chlorosis, which can also be confused with either brown stem rot or sudden death syndrome. Then when lesions start expanding, they form slightly sunken reddish-brown cankers with reddish margins. As you inspect the stems, you’ll notice zone lines within the lower part of stems. These are associated with Diaporthe fungi, but not charcoal rot as some believe. To learn more about zone lines in soybeans, the Crop Protection Network has compiled a resourceful fact versus fiction reference guide. 

As pictured right, “zone lines” are shown near a canker on a soybean stem. During the late growing season, lesions are found on the lower stem.
When you see these aboveground symptoms, evaluate the lower third of the soybean stem and check for stem canker, as well. Cankers, or lesions, can be found at the base of the branch or leaf petiole and are more easily spotted when stems are green. Cankers can extend over several nodes on one side of the stem, girdling the stem so that the flow of nutrients and water to the plant are disrupted. 

Stem canker can easily be identified by the lower green stem in the photo at left. On the right, Phomopsis is beginning to infect the soybean seeds.

As the growing season continues, patches of dead plants with leaves attached may result, but take care to not confuse these symptoms with Phytophthora root rot or white mold. Lastly, plants with stem canker don’t have interior taproot discoloration. If you aren’t sure after scouting, you may need a laboratory diagnosis.

Conditions Favoring Infection
Rainy weather that splashes spores onto susceptible soybean plants during the early vegetative stages promotes infection. Northern stem canker is likely to occur when there are cool, extended periods of rain early in the growing season. In comparison, southern stem canker can start developing after only 24 to 96 hours of leaf wetness and warm temperatures of 70-85° F. Soybean fields that have been hit by hail damage also tend to show greater disease pressure. 

Cankers start showing during reproductive stages and become visible from mid-July to harvest, particularly in susceptible varieties. Along with plant susceptibility, early infection timing and how cankering affects reproduction will determine yield loss. There have only been a few cases where high yield loss is caused by this disease, but if environmental conditions are favorable, a susceptible variety can experience up to 50% yield loss when this disease is present.  

Management Tips
Although stem canker is an agronomic challenge in many areas where soybeans are grown, there are several steps that can be taken to help reduce infection levels.
  • Variety selection: One of the best steps you can take to prevent stem canker in 2020 begins now and the weeks ahead. If stem canker has been an issue in your soybean fields this year, check genetic ratings and choose resistant varieties to be planted on acres with high pressure.
  • Cultural practices: Fields with minimum or no-till as well as high fertility soil or organic matter are at higher risk for stem canker. Tillage and crop rotation to a non-host such as corn, wheat and sorghum can help to reduce inoculum for the next soybean crop. If stem canker infection is severe, a 2-year rotation may be needed. Another helpful management practice is to mix infected crop residue into the soil, which reduces available inoculum for the next soybean crop.
  • Fungicide applications: Since fungicide effectiveness in the early vegetative stages has received mixed reviews on susceptible stem canker varieties, they generally are not recommended. If applied at R5, a fungicide application could result in higher seed quality but not impact yield.
For more information, download the Crop Protection Network’s stem canker guide online under the “Additional Information” section, or contact your Golden Harvest Seed Advisor.

References:Photos are either the property of Syngenta or used under agreement.

Syngenta hereby disclaims liability for third-party websites.

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