Seeing Yellowing Winter Wheat? Test For Wheat Streak Mosaic Virus Before Applying Nitrogen

May 10, 2017

Severe wheat streak mosaic disease was discovered in several winter wheat fields scouted in Central South Dakota. Due to this discovery, SDSU Extension advises that growers test for the disease, caused by Wheat streak mosaic virus, before applying nitrogen.

“This disease causes wheat leaves to yellow and plants to be stunted (Figure 1),” said Emmanuel Byamukama, Assistant Professor & SDSU Extension Plant Pathologist. “Producers may mistakenly attribute the yellowing to a nitrogen deficiency.”

Byamukama explained that yellowing of lower or older leaves in wheat can be an indication of nitrogen deficiency. However, if the yellowing is the result of Wheat streak mosaic virus, the yellow leaves will have a streaked mottled look to them.

To clearly know what is causing the yellowing, Byamukama encouraged growers to submit a sample to the diagnostic clinic to confirm Wheat streak mosaic virus before adding inputs such as fertilizer or fungicide in yellowing wheat fields.

Test plants here

Plant samples can be mailed or dropped off at the SDSU-Plant Diagnostic Clinic, SPSB 153, Box 2108, Jack Rabbit Lane, Brookings, SD 57007-1090.

Below Byamukama answers some frequently asked questions about Wheat streak mosaic virus.

How can one tell whether yellowing is due to Wheat streak mosaic virus?

The best way to know if a plant has Wheat streak mosaic virus is to submit a sample to the SDSU diagnostic lab. Wheat streak mosaic virus is systemic, meaning that all leaves have symptoms. Younger leaves may appear healthy but over time, these also show symptoms.

Older leaves show severe yellowing and streaking starting from the leaf tips (Figure 2a).

Some wheat fields scouted, however, showed just older lower leaves with yellow blotches (Figure 2b). This is not caused by Wheat streak mosaic virus; it may be some infection initiation that was halted by unfavorable environmental conditions or due to unknown environmental stress.

Producers may wonder why this disease is so prevalent this year? Wheat streak mosaic virus is transmitted by the wheat curl mite. Wheat curl mites are microscopic (0.3 mm long) and can only be seen under magnification (such as 20x hand lens).

They do not move on their own, they depend on wind to move between fields and within a field. Wheat curl mites survive from one crop to another on volunteer wheat and grassy weeds. The pattern of infection within fields can be an indication of where the disease came from. For instance, if the entire field is infected, one can assume the Wheat streak mosaic virus came from within the field. In situations where the infection is occurring along the field edge, one would assume that the mites were blown in from the adjacent field. Mites are estimated to travel up to 2 miles.

All fields found with Wheat streak mosaic virus were winter wheat planted into wheat stubble (Figure 3). It is possible that these fields were planted before all grassy weeds and volunteer wheat had completely dried up after herbicide application last fall. It is recommended that burn-down of grassy weeds and volunteer wheat is done at least two weeks before planting. The long warm fall that we had in 2016 in South Dakota and other winter wheat production areas in the central US may have allowed mite populations to build and spread the virus to more areas.

Management of Wheat Streak Mosaic

Wheat streak mosaic disease can be best managed through cultural practices. Unlike fungal diseases, nothing can be sprayed on virus-infected plants to prevent or cure virus infection. However, a few practices can be used to prevent or reduce the chances of winter wheat getting infected by Wheat streak mosaic virus before planting:

Destroy volunteer wheat and grassy weeds at least two weeks before planting in the fall. Volunteer wheat and grassy weeds are the most important risk factor for the wheat streak mosaic disease. Volunteer wheat and grassy weeds can be destroyed through tillage or herbicide application. This will reduce the ability of the wheat curl mites to use these plants as a green-bridge to the newly emerging wheat.

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Video: Why Your Food Future Could be Trapped in a Seed Morgue

In a world of PowerPoint overload, Rex Bernardo stands out. No bullet points. No charts. No jargon. Just stories and photographs. At this year’s National Association for Plant Breeding conference on the Big Island of Hawaii, he stood before a room of peers — all experts in the science of seeds — and did something radical: he showed them images. He told them stories. And he asked them to remember not what they saw, but how they felt.

Bernardo, recipient of the 2025 Lifetime Achievement Award, has spent his career searching for the genetic treasures tucked inside what plant breeders call exotic germplasm — ancient, often wild genetic lines that hold secrets to resilience, taste, and traits we've forgotten to value.

But Bernardo didn’t always think this way.

“I worked in private industry for nearly a decade,” he recalls. “I remember one breeder saying, ‘We’re making new hybrids, but they’re basically the same genetics.’ That stuck with me. Where is the new diversity going to come from?”

For Bernardo, part of the answer lies in the world’s gene banks — vast vaults of seed samples collected from every corner of the globe. Yet, he says, many of these vaults have quietly become “seed morgues.” “Something goes in, but it never comes out,” he explains. “We need to start treating these collections like living investments, not museums of dead potential.”

That potential — and the barriers to unlocking it — are deeply personal for Bernardo. He’s wrestled with international policies that prevent access to valuable lines (like North Korean corn) and with the slow, painstaking science of transferring useful traits from wild relatives into elite lines that farmers can actually grow. Sometimes it works. Sometimes it doesn’t. But he’s convinced that success starts not in the lab, but in the way we communicate.

“The fact sheet model isn’t cutting it anymore,” he says. “We hand out a paper about a new variety and think that’s enough. But stories? Plants you can see and touch? That’s what stays with people.”

Bernardo practices what he preaches. At the University of Minnesota, he helped launch a student-led breeding program that’s working to adapt leafy African vegetables for the Twin Cities’ African diaspora. The goal? Culturally relevant crops that mature in Minnesota’s shorter growing season — and can be regrown year after year.

“That’s real impact,” he says. “Helping people grow food that’s meaningful to them, not just what's commercially viable.”

He’s also brewed plant breeding into something more relatable — literally. Coffee and beer have become unexpected tools in his mission to make science accessible. His undergraduate course on coffee, for instance, connects the dots between genetics, geography, and culture. “Everyone drinks coffee,” he says. “It’s a conversation starter. It’s a gateway into plant science.”