As soybean moves through vegetative growth into the reproductive phases, it is very common to note that a field appears "uneven," with variable stands. When you see a field like this, you probably ask whether this is due to something biotic, abiotic, or another issue? This is an excellent time to look closely and determine what may be causing the issue, especially if it is due to a soybean stem disease. We also recommend that as the season continues that you pay attention to pockets or areas of the field where the stand does not appear to be improving. We know that different stem diseases can negatively impact soybean. Still, proper management begins with proper identification since, without that information, it is difficult to quantify the potential impact at harvest and make long-term decisions related to variety selection and crop rotation.
Figure 1. Soybean field showing variable growth and development at the R1-R2 (flowering) growth stage. Photo Credit: P. Esker.
Scouting for soybean stem diseases can be tricky. Proper identification begins with knowledge about your soybean variety and any resistance to specific soybean stem diseases, such as Rps genes for resistance to Phytophthora root and stem rot. Nonetheless, we can also use a symptom key to help with the proper identification of the different stem diseases. We have created a symptom key guide adapted from the Crop Protection Network, "Scouting for Soybean Stem Diseases" (Fact sheet, CPN 1002).
Diagnostic key to help with the identification of important soybean stem diseases
Step 1. Determine if there are external symptoms on the stems.
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If "No", the focus will shift to looking at the pith tissue (see Step 2).
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If "Yes", attention will focus on identifying the external symptoms on the stem (Step 3).
Step 2. Is the pith tissue white or brown?
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If "White", we will focus on foliar disease symptoms (see Step 2B).
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If "Brown", we can identify the disease as brown stem rot:
Brown stem rot. Credit: M. Draper, Bugwood.org
Step 2B. On the leaf, is there yellowing (early symptom) and necrosis (advanced symptom) between leaf veins?
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If "Yes", we can identify the disease as sudden death syndrome:
Sudden death syndrome. Credit: A. Collins
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If "No", and foliar symptoms include chlorotic middle and lower leaves and wilting and scorching of the upper leaves, we can identify the stem disease as Fusarium wilt:
Fusarium wilt. Credit: D. Mueller, Bugwood.org
Step 3. Do you see specks/dots or blotches on the stem?
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If "Specks/dots", go to Step 3B.
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If "Blotches", go to Step 3C.
Step 3B. Are the specks/dots in rows, or are they random and on lower stem or upper taproot?
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If "Rows", we can identify the disease as pod and stem blight:
Pod and stem blight. Credit: A. Collins
Pod and stem blight signs. Credit: A. Murillo-Williams.
If "Random and localized to lower stem or upper taproot", we can identify the disease as charcoal rot:
Charcoal rot. Credit: A. Collins
Step 3C. We now need to identify the type and location of the symptom to provide the best identification:
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Dark brown to purple blotches that originate from the soil line = Phytophthora root and stem rot:
Phytophthora stem rot. Credit: D. Mueller, Bugwood.org
Phytophthora stem rot. Credit: A. Collins
Water-soaking and discoloration of internal stem tissues caused by Phytophthora stem rot. Credit: A. Collins
Dark red-brown/sunken blotches, not at soil line but around a node and without girding the stem = stem canker:
Stem canker. Credit: D. Mueller, Bugwood.org
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White, fluffy growth or bleached white stem = white mold:
Stem discoloration caused by soybean white mold. Credit: A. Collins
Soybean white mold signs. Credit: A. Murillo-Williams.
Dark blotches that extend to petioles, leaves can have a Shepherd's crook appearance = anthracnose:
Anthracnose. Credit: A. Collins
