Wheat Streak Mosaic Virus

This past season was a bad one for wheat streak mosaic virus (and other viruses vectored by the wheat curl mite) in parts of Kansas. Late summer rainfall in 2020 was favorable for volunteer wheat establishment in some areas. One of the best preventative measures for wheat streak is the control of volunteer wheat early and often after harvest. If volunteer wheat is allowed to stand, it creates a “green bridge”, allowing wheat streak mosaic and wheat curl mites to survive locally. Volunteer wheat should be terminated at least two weeks prior to planting to allow sufficient time for mites to die off.

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Wheat streak mosaic is one of the most economically devastating wheat diseases in Kansas and the Great Plains. Preliminary estimates for the 2019 crop year indicate that losses were low this year, at only 0.3% statewide. There were a few localized outbreaks of WSMV in the west central and northwest crop reporting districts. Control of volunteer wheat is the best way to prevent wheat streak mosaic virus losses in the wheat crop.

In 2017, this disease caused a conservative $76.8 million in direct losses to wheat farmers, a loss of 19.2 million bushels of wheat. The 2017 loss was 5.6% yield loss, up from an average 1.5% loss.

According to the Kansas Cooperative Plant Disease Survey Report, September 28, 2018, wheat streak mosaic virus complex (WSM) was the third most important disease of 2018, at 0.07% estimated loss statewide, or 194 thousand bushels.

This is an unusually low yield loss due to this disease, much lower than the 5-, 10-, and 20-year averages (1.69%, 1.28%, and 1.32%, respectively). This is in stark contrast to the yield loss in 2017 (5.6%), a year of unusually severe WSM due to decreased control of volunteer wheat in late summer 2016 and weather conditions conducive to disease development.

After such a high loss in 2017, with some observed wheat fields suffering as much as 100% yield loss, farmers may have better controlled the volunteer wheat in post-harvest 2017. This would have decreased the ability of the wheat curl mite to survive between 2017 harvest and planting, which would have better controlled the disease spread. In addition, there was a much more rapid transition to cool temperatures in the fall of 2017. These cold temperatures in October and November likely reduced the activity and movement of the wheat curl mites.

Wheat streak mosaic virus complex is comprised of wheat streak mosaic virus, high plains virus, and Triticum mosaic virus, and is transmitted by wheat curl mites.

In 2018, WSM only caused detectable yield loss in two crop reporting districts, the Northwest and the West Central districts, causing 0.2% and 0.4% loss respectively.

The disease is most common in the western portion of the state, with sporadic outbreaks in central and eastern Kansas.

In 2017, central and western Kansas had above average disease intensity.

Stop the Streak

There are basically only three ways to control the spread of wheat streak mosaic:

Timely removal of volunteer wheat and other grassy weeds. The best way to prevent the spread of the wheat streak mosaic virus is to remove volunteer wheat and other grassy weeds. Volunteer wheat must be completely dead and dry for two weeks before planting a new wheat crop. Volunteer wheat and other grassy weeds can be removed with herbicides or tillage, but it’s absolutely essential to allow time for herbicides to work.
Avoid early planting; plant after the hessian fly-free date. By avoiding early planting, Kansas wheat farmers are able to avoid times when wheat mite populations are the highest in late summer and to decrease the interval between planting and fall freeze events. "When we say avoid early planting, we’re not talking about planting outside of the window for success of your wheat crop,” said KSU Plant Pathologist Erick De Wolf. “We’re encouraging you to plant on the later side of the recommended planting dates."
Plant varieties with moderate or high levels of resistance to WSMV.
- Joe, Clara CL and Oakley CL are current varieties with moderate resistance to WSMV. Unfortunately, the WSM2 resistance genes in these varieties is less effective at temperatures above 75 degrees Fahrenheit.
- TAM 112, Byrd and Avery varieties slow the development of mite populations.
At this point in time, there are no chemical options such as insecticides or pesticides that are effective at controlling the wheat curl mite.

Genetic Resistance

Research is headed down the path of genetic resistance, and Kansas wheat farmers should know that help is on the way. However, there are only a few varieties with moderate resistance at the current time.

The WSM3 gene, which was discovered by Bernd Friebe at the Wheat Genetics Resource Center (WGRC) at Kansas State University, comes from wild relatives of wheat and is resistant to not only Wheat Streak Mosaic Virus, but it also provides protection from Triticum Mosaic Virus and High Plains Mosaic Virus as well. Another benefit of WSM3 is that it is not temperature sensitive, which has been a weakness in the current sources of resistance. This research is being partially funded by Kansas wheat farmers through the Kansas Wheat Commission’s two-penny wheat assessment. The WGRC is housed at the Kansas Wheat Innovation Center in Manhattan.

Through traditional breeding processes, it can take several years before resistance genes show up in varieties available for planting. A new variety with the Wsm3 gene should be released for Foundation seed in about three years, making it available to farmers the following year.