Stop the Streak: Do Your Part

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When it comes to growing wheat, the hardest challenges are the ones a producer cannot address - a hailstorm that knocks down the best-looking field, storm clouds that pass overhead without ever letting a drop loose or a deer antler shed that takes out a combine wheel. It is equally frustrating to feel as if there is nothing you can do to save a crop when a disease like Wheat Streak Mosaic Virus (WSMV) shows up in the spring and there is no chemical to stop the spread.

Blake Connelly with Alliance Ag & Grain in Ford County reported in a Kansas Wheat Harvest Report earlier this week that WSMV significantly impacted yields and test weights. Similarly, a central Kansas producer noted that WSMV "took over half of what the wheat could’ve potentially made."

The elevated incidence of WSMV this year is not attributable to a single cause but rather a "perfect storm" of environmental conditions and management challenges, dating back to last summer and fall. An extended warm autumn in fall 2024 proved particularly detrimental, providing ideal conditions for the rapid proliferation of wheat curl mite populations—the primary vector for WSMV, Triticum mosaic virus (TriMV), and High Plains Mosaic Virus. The wheat curl mite is a small but mighty adversary that flourishes around 70 degrees Fahrenheit, meaning the prolonged mild temperatures allowed them to thrive.

Allan Fritz, K-State wheat breeder, emphasized that "volunteer wheat is still the primary issue," serving as a crucial "green bridge" for mites and the virus to survive between cropping seasons. He noted that while many producers maintained excellent volunteer control, economic pressures or the sheer volume of volunteer flushes in some areas led to less diligent management, inadvertently creating reservoirs for mites and the virus.

Even hidden volunteer wheat within double-cropped fields or mixed in summer cover crops can harbor mites, becoming sources of inoculum for new wheat crops. A mild winter further exacerbated the problem by allowing more mites to overwinter successfully, leading to high populations capable of dispersing long distances—up to one or two miles—and resulting in concerning levels of viral diseases across the state.

The K-State wheat breeding programs are continually working to increase both resistance and tolerance to these viruses in up-and-coming wheat varieties in the pipeline. These genetic improvements are critical because there is no effective treatment for WSMV once an infection occurs. As a result, K-State's breeding programs are dedicated to developing varieties that can either resist infection or tolerate its presence with minimal yield impact.

The Wsm2 gene has long been a cornerstone of WSMV resistance, present in varieties like KS Territory, KS Dallas, KS Bill Snyder, KS Big Bow and KS Mako. While offering good WSMV protection, Fritz noted a crucial limitation: Wsm2 does not protect against TriMV, which is increasing in frequency and often occurs in mixed infections, leading to more severe yield losses and highlighting the need for broader spectrum resistance.

Beyond outright resistance, K-State breeders are integrating and enhancing tolerance in new varieties. Tolerance provides a baseline of protection, but it does not mean producers will not have infections or yield loss. 

"Tolerance means we still see symptoms, they’re just not as bad," Fritz explained. "We think that’s why it’s really important for producers to continue to do their part to control volunteer wheat.”

A major breakthrough in wheat breeding is the combined utilization of Cmc4, a wheat curl mite resistance gene, and Wsm1. Fritz explained when mites feed on varieties expressing the Cmc4 gene, they can still transmit the virus, but they will die before reproducing, effectively limiting the mite population spread within the field.

"It isn’t like a silver bullet," Fritz said. "Those mites can blow in. They’re going to feed, they’re going to transmit the virus, then they’ll die. It's a good thing to keep the mite populations down, but it needs to be integrated with good genetic resistance to the virus."

Wsm1 is another gene in the wheat breeding toolbox, providing resistance to both WSMV and TriMV, but the gene had traditionally had a large yield drag. By teaming up with K-State's Wheat Genetics Resource Center and the USDA wheat genetics program under Dr. Mary Guttieri, researchers think they may have solved the riddle of how to incorporate Wsm1 from intermediate wheatgrass without that drag on final harvest yields. 

"We have evidence now that we’ve lost that yield drag,” Fritz said. “So, we can actually use this gene for WSMV protection and not lose yield potential."

The K-State wheat breeding program is now backcrossing both the Cmc4 and Wsm1 genes into adapted, high-performing varieties, including KS Providence and KS Mako. This strategy aims to create "super packages" with multiple resistance genes, offering comprehensive and durable protection. For instance, backcrossing into KS Mako will result in a variety possessing Wsm1, Wsm2 and Cmc4, providing a multi-layered defense.

While still a few years out from commercial availability, these breeding advancements represent a remarkably rapid delivery of solutions, considering the typical 11-year timeline for variety release. K-State is also routinely incorporating these resistance genes into many new breeding lines.

Fritz consistently stresses, however, that genetic resistance, no matter how advanced, is not a silver bullet. 

"I don't want people to lean solely on the genetics," he cautioned. "This isn't like, oh, we can go to the grocery store and buy another one of these. These genes are hard to find. They're hard to transfer. They are not frequent in the gene pool."

Therefore, integrated disease management remains absolutely crucial. Specifically, producers should look towards aggressive volunteer management and variety selection for fall 2025 due to the severity of WSMV infections this summer. Producers should also carefully consider their cover crop choices, so that cereal species or pockets of hidden volunteer do not inadvertently become hosts for wheat curl mites, even without showing symptoms.

"We know we can actually lose these resistance genes," Fritz warned, "so we need to be really mindful about putting together management practices with the resistance. We still need to take care of our volunteer. We still need to do all those things that we need to do to manage this as if we didn't have resistance and then use the resistance as the insurance policy."

Learn more about how to stop the streak of WSMV at kswheat.com/wheatstreak.