Wisconsin growers confront purple seed stain after 2026 harvest

By Rebecca Stone

Purple seed stain, caused by Cercospora fungi, appeared across Wisconsin soybean fields in 2026, leaving growers with purple-discolored grain as above-average temperatures, frequent rainfall and high humidity favored infection during seed fill (the period when seeds develop).

University of Wisconsin-Madison Extension soybean specialist Shawn Conley characterized the spread as “pretty much scattered across the entire north-central region.” Southern soybean-growing regions typically experience these weather patterns, and purple seed stain is a recurring concern there.

The 2026 findings turn an appearance problem into a broader marketing issue, even as farmers reported good yields.

What does purple seed stain mean for farm margins?

Conley said the disease generally affects grain quality rather than yield. The discoloration ranges from small purple spots to staining across the entire seed coat (the seed’s outer protective layer).

Grain buyers may give affected soybeans a special grading designation, discount their price or reject them entirely, especially in markets where seed appearance matters. That places the largest risk in the price received for harvested beans rather than lost production.

Many farmers reported good yields, but elevator discounts created frustration. Inconsistent grading among buyers has made the problem worse. Conley said some elevators were docking heavily while others assessed no discount.

Farmers also reported diaporthe seed decay, green soybeans and other seed discoloration, which created more frustration this year.

Can growers challenge elevator discounts?

Conley’s first recommendation for a grower facing a discount is to verify the grain grade. He also encouraged farmers to question unexpected discounts and request a second opinion when necessary.

Because purple seed stain rarely occurs on this scale in the Upper Midwest, some elevators may be unfamiliar with distinguishing it from other seed-quality problems. That check can clarify whether a deduction reflects purple stain or another reported defect and how the load was classified.

Conley also recommended contacting crop insurance agents to determine whether quality losses qualify for coverage. Strong yields may prevent some producers from meeting the thresholds needed to trigger payments. That leaves accurate grading and knowledge of potential discounts central to the immediate financial response.

What should growers plan for 2027?

Purple stain prevention is not on the near horizon, Conley said. Genetic resistance may offer some protection, but soybean varieties in the north-central region have not been extensively screened for purple seed stain resistance.

Growers can ask seed suppliers about varietal susceptibility, or a variety’s likelihood of infection, when selecting seed for the next season.

Conley discouraged a specific investment: fungicide applications targeted at purple seed stain. “Definitely don’t spend money on a fungicide next summer,” he said. Typical R3 fungicide applications offer little protection against seed staining, and even later R5 applications are unlikely to provide enough benefit to justify treatment costs.

Weather remains the biggest factor in whether purple seed stain returns in 2027, leaving accurate grain grading, a clear understanding of potential discounts and informed seed purchasing as the immediate priorities.