It’s time, once again, to “stop the streak,” in Kansas fields. As farmers start to line up their wheat seed purchases and plan their 2022 crops, now is also the time to get out there and control volunteer wheat — or face potential disaster come next summer in the form of yield-robbing wheat streak mosaic virus.

Wheat streak mosaic virus is just one of many viruses that are vectored by the wheat curl mite in Kansas. Volunteer wheat and other grassy weeds can play host to the wheat curl mite, allowing its populations to flourish until the next wheat crop is up and growing in your field. At which time, like an unwanted houseguest, the mite packs its bags to move from the now-dead host plants to your green and growing wheat field, bringing the wheat streak mosaic virus and others with them. This is called a “green bridge” by experts, and controlling it at least two weeks prior to planting the next wheat crop is one top way to protect your fields. That two-week window is enough time to let the wheat curl mite population die, experts say.

2020-21 season

Aaron Harries, Kansas Wheat vice president of research and operations, says 2020 saw localized troubling outbreaks of wheat streak mosaic virus across the state.

“Some farmers had fields that lost 60% of their yield,” Harries says. “In a high-yielding year like this, that’s substantial. And if you calculate that with the current price of wheat, that’s pretty big.”

In their Kansas State University Agronomy eUpdate of July 21, K-State Extension specialists echoed that in their call for farmers to manage their volunteer wheat populations.

They say farmers faced environmental challenges to control the volunteer wheat in some areas due to late-summer rainfall in 2020. That rain delayed farmers getting into fields to spray for volunteer wheat last fall, and caused flushes of volunteer wheat to grow and serve as that green bridge for the wheat curl mite.

But the challenges didn’t stop last fall. Kansas saw conditions ripe for a larger-than-usual wheat seed bank left behind after harvest, which would lead to a larger-than-usual volunteer wheat flush.

These problems included:

1. excessive rainfall delaying wheat harvest in many parts of the state

2. freeze damage during late boot and early heading causing many delayed wheat heads to emerge in parts of south-central and southwest Kansas

3. hailed-out wheat

4. a considerable amount of fusarium head blight, even in western Kansas where this disease usually isn’t a problem

5. waterlogged wheat in parts of central Kansas

6. drought-stressed wheat in southwest Kansas

Prepping for 2022 crop

Kelsey Andersen Onofre, K-State assistant professor of plant pathology, says that larger-than-expected crop of volunteer wheat will harbor a larger-than-expected crop of wheat curl mites.

“This year producers will need to be extra-vigilant about volunteer management, as we will have higher-than-average numbers of mites in the landscape moving into the 2021-22 season,” she says. Producers often like to wait several weeks after harvest before making their first herbicide application to control volunteer wheat, in order to have as much as possible emerged before spraying or tilling the first time.

Glyphosate and atrazine are two herbicides often used to chemically control volunteer wheat. It’s not uncommon for a second application or a tillage pass to be needed later in the summer, just to make sure the green bridge is eliminated within a half-mile of wheat planted in the fall.

But, with the later 2021 harvest, farmers are going to need to be extra-mindful of their timing before harvest to meet the two-week window. And a late summer that’s also wet can bring multiple flushes of volunteer wheat and other grassy weeds that can support moderate populations of the wheat curl mite.

Other hosts

“There are many grassy weed species, as well as crops [corn, sorghum, etc.], that can host the curl mite,” Onofre says.

There is considerable range in the ability of a grassy weed species to host the mite and the virus, according to experts. Barnyardgrass is among the more suitable hosts for both virus and mites, but fortunately it is not that common in wheat fields. However, various foxtails are more abundant — and even if they’re a poor host, they could be a disease reservoir. These grasses may play an important role in allowing the mites and virus to survive during the summer months, particularly in the absence of volunteer wheat.

“In most of the severe cases that we saw in the 2020-21 season, however, nearby uncontrolled volunteer wheat was still the cause,” Onofre says. “Management of volunteer wheat should still be the main focus of a management campaign.”

Farmers may do a good job in their own fields, but what to do if a neighbor has a volunteer wheat flush close to your field?

Harries says the first step is to choose a wheat variety that has built-in resistance to wheat streak mosaic, and therefore can offer you some protection. But communication is also key.

“Try to open a line of communication with your neighbor, and try to convey to them the need to control their volunteer,” he says. “We say it needs to be two weeks dead prior to planting new wheat, and that’s the critical point. But I think it’s also communication with your neighbor. Farmer helping farmer. Farmers look out for each other.”

Genetic tools

Wheat farmers have a few genetic tools in their fight against wheat streak mosaic. There are a few varieties adapted to Kansas that have wheat streak mosaic resistance through the WSM2 gene, including: KS Dallas, KS Hamilton, Guardian, Oakley, Joe and Clara. An Oklahoma State University variety, Breakthrough, has the WSM1 resistance gene.

However, as Onofre reminds growers, wheat streak is just one of a complex of viruses that include Triticum mosaic virus, wheat mosaic virus (or High Plains disease) and others.

“This is important, because some of the resistance genes that are available only work for one of these viruses [such as the resistance gene WSM2],” she explains. “WSM2 can help slow infections, but we will still see symptoms if other viruses are present. Additionally, WSM2 is temperature-sensitive and does not work above 65 to 70 degrees F.

“These limitations can sometimes lead producers to believe that the gene itself is not working, which is not the case,” Onofre adds. “We just cannot expect this gene alone to provide perfect control.”

Some wheat varieties have a resistance gene to the wheat curl mite, Onofre adds, which can also slow infections — but it’s not perfect under high pressure. They include: TAM 112, Byrd, Avery, Langin, KS Western Star, Whistler, Canvas, Guardian, Crescent AX, Incline AX, Fortify SF, TAM 115, TAM 204, and T158. These varieties are susceptible to the viral diseases, but they generally slow the development of the mite populations in the fall.

Set for success

“The best way to get the most out of this genetic resistance is to place them in a situation for success,” Onofre says. “We can do this using the 2-by-2 rule, i.e., by making sure all volunteer wheat within 2 miles is dead within two weeks of planting. We can also lower our chances of infection by planting late, when the resistance genes have the highest chance of working [due to cool temperatures] and mites are less active.”

That could affect farmers planting wheat early for grazing, or if we continue to see high temperatures into October.

Fortunately, Harries says, Kansas wheat breeders have more varieties coming in the pipeline to address the wheat streak mosaic challenge. But until those are available, it’s up to every wheat grower to stop the streak any way they can.

Read more in the K-State Research and Extension publication MF3383 Wheat Streak Mosaic. Onofre, Extension wheat pathologist, can be reached at [email protected].