As your crop matures into R5 and R6 reproductive stages, soybean disease identification is critical, particularly if you experience warm temperatures and high humidity. These conditions can play host to soybean fungal diseases, including those that cause purple seed stain.
Purple seed stain means your soybean seeds look purple or light pink. This discoloration normally starts at the hilum, or the dimple where the bean was attached to the pod. It doesn’t usually hurt yield, but at the elevator it can cost you real money if your seed ends up being labeled as damaged. And what’s more, if stained seeds are used in future crops, the fungus’ effects can spread, threatening germination and causing other problems.
Here’s what you need to know about purple seed stain and how to mitigate risk factors that can hurt your wallet.
How weather affects soybean disease identification
Pay attention to your soybean crop during its reproductive growth stages, particularly if temperatures hover between 75 F and 80 F and relative humidity is 75% or greater.¹
That’s because the fungus Cercospora kikuchii (C. kikuchii), the lead culprit in purple seed stain, thrives in those conditions. Research indicates at least three other pathogens (C. cf. flagellaris, C. nicotianae and C. cf. sigesbeckia) can also create this type of problem for your crop.
Also watch for cloud cover and heavy dews that linger throughout the morning, as these can create a welcoming environment for purple seed stain development.
What’s the difference between Cercospora leaf blight and purple seed stain?
C. kikuchii causes both purple seed stain and Cercospora leaf blight, a foliar soybean fungal disease. If you have fields with leaf blight, purple seed stain might be more common in that location, though it’s not a certainty. If you happen to have both problems, it doesn’t mean both will be especially severe.²
You can use fungicides such as Revytek® and Revysol® from BASF to treat Cercospora leaf blight.
Variety selection and purple seed stain management
Early maturing soybean varieties tend to have more seed diseases than those that mature later. The reasons for this might be twofold. First, weather conditions might favor disease as the crop matures. Second, varieties that mature early and then grow for a relatively long time before harvest might provide a greater window of opportunity for purple seed stain to develop.
Keep this in mind when selecting varieties to mitigate pathogen threats. As an added benefit, a mix of varieties can enable you to harvest your crop in stages rather than struggling to get everything to the bin or the elevator all at once.
Does purple seed stain affect soybean yield?
Purple seed stain typically doesn’t hurt yield, so you might not feel the pinch at harvest. That said, you might find it’s a different story at the elevator, where your payout can be docked because of poor bean quality since purple seed stain affects seed composition, including protein, sugars and select amino acids.
How purple seed stain can affect future growing seasons
Avoid planting soybean seeds from diseased plants. That’s because research has found nearly 31% higher germination rates and more than 58% greater seedling vigor for seeds from healthy plants compared to seeds from plants with purple seed stain.³
Future seedlings are especially at risk for germination problems when more than 50% of the seeds are covered in purple stain.⁴
Experts are available to help you make your management decisions to mitigate the risk of purple seed stain. Reach out to your seed retailer, a nearby extension office agent or a seed company professional like your regional BASF representative.
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Endnotes
- Webster, Richard Wade, Hope Becton, and Febina Mathew. “Cercospora Leaf Blight and Purple Seed Stain of Soybean.” North Dakota State University Extension, Jan. 2025,https://www.ndsu.edu/agriculture/extension/publications/cercospora-leaf-blight-and-purple-seed-stain-soybean. Accessed 21 May 2026.
- Spurlock, Terry, and Nick Bateman. “Identification and Management of Seed Disease on Soybeans.” Arkansas Row Crops Blog, University of Arkansas System Division of Agriculture, 4 Oct. 2021,https://arkansascrops.uada.edu/posts/crops/soybean/identification-management-soybean-seed-disease.aspx. Accessed 21 May 2026.
- Turner, Richard E., et al. “Effects of Purple Seed Stain on Seed Quality and Composition in Soybean.” Plants, vol. 9, no. 8, 5 Aug. 2020, p. 993, https://doi.org/10.3390/plants9080993. Accessed 21 May 2026.
- Mangel, Dylan. “Purple Seed Stain.” CropWatch, University of Nebraska–Lincoln,https://cropwatch.unl.edu/plant-disease/soybean/purple-seed-stain/. Accessed 21 May 2026.
