By Madalyn Shires and Connie Tande et.al
As soybean harvest began in South Dakota and the Midwest, there were multitudes of reports of soybean seeds that were abnormal. Some soybeans were stained a shade of pink or purple; others were shriveled or decayed in appearance (Figure 1). Many producers were left with questions about what was affecting their seed and how to manage seed from fields that were showing one or both of these symptoms.
Purple Seed Stain of Soybean
Purple Seed Stain (PSS) of Soybean is a fungal disease caused by several Cercospora species, including Cercospora flagellaris, the primary pathogen associated with Cercospora leaf blight (Figure 2). Other members of the Cercospora genus can also cause Frog-eye Leaf Spot (FLS); however, FLS and PSS are caused by different pathogens and are not directly linked despite belonging to the same fungal group. 2026 weather conditions were ideal for the establishment of this fungal disease. To cause infection, the fungus needed warm, humid conditions for several days around the time of bloom. With very warm and humid conditions prevailing for July-September (National Climate Report), it is thought that the fungus had good opportunities to establish on soybean seeds. Additionally, late season rains and warm weather prior to harvest likely encouraged rapid development of the seed stain in plants that had low disease pressure prior to harvest.
What to do about infected seeds?
With purple seed stained soybeans, yields are not reduced. However, reduced seed prices may result from dockages associated with this disease. The USDA has found that while affected seeds may have germination reduction if saved, the staining does not reduce protein or oil value/uses of soybeans.
Diaporthe Seed Decay of Soybean
Diaporthe Seed Decay of Soybean is also a fungal disease that infects the pods, stems, and seeds of soybeans. The pathogen responsible for plant and seed symptoms is Diaporthe longicolla. The rain and mild temperatures experienced in South Dakota in the month prior to harvest are likely to blame for seed decay. The pods were infected first, which then led to seed infection and decay. The fungus is common in our state but typically leads to little or no yield loss. However, in 2026, heavily affected soybeans fields are likely to see reduced yields associated with this disease. For fields with high levels of disease, growers should avoid saving seed and consider residue management strategies. Since infection occurs before seed formation (Figure 4), crop residues may contain overwintering spores that can survive between growing seasons and serve as an inoculum source the following year. This is especially important for those who may plant continuous soybeans, plant other host crops such as sunflowers, or have heavy presence of broadleaf weeds.
Source : sdstate.edu