Discolored Soybean Seed


Environment, insects, diseases, and the interactions of these can cause soybean seed to be blemished or appear damaged. Economic injury depends on cause and timing of causal agent. Discounts may occur at elevators because of the discoloration.

Developing soybean seed can become discolored and damaged because of stressful environmental conditions, certain diseases, and from pod and seed feeding insects. Environmental conditions include rain, heavy dews, fog, and high relative humidity. Purple seed stain, Diaporthe species, frogeye leaf spot, anthracnose, and viral diseases can cause discolored seed. Pod-feeding insects, such as stinkbugs and bean leaf beetles, can expose the seeds to moisture and secondary infection.

  • Purple seed stain and leaf blight may be caused by Cercospora kikuchii, C. flagellaris, or C. sigesbeckiae. Infected seed is characterized by pink and varying depths of purple discoloration (Figure 1). The seed may have discolored specks to large blotches that cover the entire seed. Warm, humid weather favors development. Potential yield may not be reduced; however, under severe discoloration, dockage may occur. Cercospora can also infect leaves, stems, and pods. Leaf symptoms begin on the uppermost leaves as reddish purple to reddish brown angular to circular lesions. As the disease develops, the lesions may cover the entire uppermost trifoliolate leaf and petiole becoming blighted, yellow, and brown. Brown spots may be found within yellow tissue and pods on the uppermost nodes may develop circular reddish purple/brown lesions.

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    Figure 1. Purple seed stain.

  • Diaporthe longicolla and D. sojae (pod and stem blight) infected seeds are misshapen or oblong and may have a white mold growth (Figure 2). The seeds become infected when spores of Diaporthe longicolla or D. sojae penetrate the pod. Infected plants are generally identifiable by pycnidia that are in rows on the stems and scattered over the pods (pod and stem blight) or zone lines within cortical tissues (D. longicolla). Warm, wet weather over prolonged periods during flowering and pod fill favor the disease.

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    Figure 2. White coating on seed resulting from Diaporthe.

  • Frogeye leaf spot caused by Cercospora sojina is primarily a leaf disease. However, it can infect stems, pods, and seeds. Circular to elongate, sunken, and reddish brown fungal lesions on pods can penetrate into the seeds and cause light gray to dark gray or brown specks or large blotches to form on the seeds. Leaf symptoms initially appear as small, dark, water-soaked angular spots on upper leaf surfaces. Older leaf lesions have a dark reddish-brown border surrounding a light brown to light gray center. Leaves may drop prematurely. Lesions on stems appear deep red with a narrow, dark brown to black border initially and become brown to smoky gray later.

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    Figure 3. Varying degrees of stink bug damage to soybean pods and seeds. Soybean photo source: Ric Bessin, University of Kentucky.

  • Anthracnose caused by Colletortichum truncatum and other Colletortichum species, can infect seeds. Diseased seeds may be smaller, moldy, dark brown, and shriveled. Anthracnose is favored by warm, wet weather. Infected plants have irregularly shaped, light to dark brown spots, streaks or lesions on stems, pods, and petioles. Black fungal growth (setae) may be present within the lesions. A tip blight phase causes a yellowing or browning of the uppermost leaves and pods which may dry and die prematurely.
  • Bean pod mottle virus (BPMV) and soybean mosaic virus (SMV) can cause the hilum to “bleed” down the side of seed. The intensity of the black or dark discoloration can be a factor of environmental conditions. The discoloration can also be characteristic of some soybean products. The viruses infect plants through the feeding of insects such as the bean leaf beetle. Plant symptoms of BPMV include green to yellow mottling of young leaves and may cause the leaves to become distorted. Aphid species, including soybean aphids, are the usual vectors for SMV. Plant symptoms of SMV include stunting, reduced pod set, and mottled, distorted, or blistered leaves, and curled down leaf margins. The potential for infection is more severe during cool weather for both viruses. If both viruses are present, symptoms can be magnified.
  • Stink bugs pierce pods and suck fluids from developing seed causing the seeds to be shriveled, smaller, discolored, and low in oil content (Figure 3).

Sources: Sweets, L. 2011. Discolored soybean seed. Integrated Pest & Crop Management. University of Missouri. https://ipm.missouri.edu. Malvick, D. 2018. Bean pod mottle virus on soybean. University of Minnesota Extension. https://extension.umn.edu/. Giesler, L.J. Soybean mosaic virus. CROPWATCH. University of Nebraska-Lincoln. https://cropwatch.unl.edu/. Obermeyer, J. Green stink bug. Field Crops IPM. Purdue University. https://extension.entm.purdue.edu/. Web sources verified 10/18/18. 181008084607