Stop the Streak

“We will be at a bit more elevated risk in western Kansas this year,” she notes. Plentiful rain throughout the summer allowed volunteer wheat to germinate and emerge continuously through the summer months and September, she says.

Volunteer wheat serves as a refuge during the summer for the wheat curl mite, a tiny insect that carries three serious wheat viruses: High Plains mosaic, Triticum mosaic and wheat streak mosaic. As wheat is planted this fall, mites can migrate from those volunteer plants into the newly emerged crop, infecting it. Because there is no treatment for the viruses, yield losses often follow.

Wheat streak mosaic, in particular, has been a problem for Kansas farmers. They saw outbreaks of the disease in 2015 and, again, in 2017. Last year, experts estimate the virus cost wheat growers at least 19.2 million bushels of wheat, about $76.8 million.

BE A GOOD NEIGHBOR. Like the summer preceding that 2017 outbreak, this season has been challenging for volunteer wheat control, Falk Jones says. Summer hail storms shattered many wheat heads, and timely rains helped those fallen wheat seeds germinate.

Max Engler, who grows wheat and milo near Deerfield, Kansas, sprayed his wheat fields twice since the June harvest only to watch another flush of volunteer wheat emerge in September.

Falk Jones says volunteers should be dead a full two weeks before growers start drilling wheat to ensure mites will not infect the new crop.

Some growers who own cattle let volunteers grow as a late-fall feed option, but this is a dangerous practice for neighboring wheat fields, Jones says. Although they prefer the newest, most tender plant tissue, wheat curl mites can live on any part of the plant that is green.

“If you’re grazing the top of the wheat, they can still survive near the base of the plant. So, while you might be decreasing populations of those mites, there are still ones out there that can move and cause problems,” she cautions. The mites can also survive on certain grassy weed species, particularly jointed goatgrass, field sandbur and many foxtail species.

When a host plant dies and dries out, the mites move to the top of the plant and use the ever-reliable Kansas winds to drift to new plants and, eventually, new fields.

“One way to think about how they move is like smoke fanning out from a single source,” Jones explains. “They can easily move a quarter of a mile but can move [even] farther on wind currents.”

This mobility has turned the mite into a community crisis, with every farmer’s management affecting his or her neighbor’s fields.

Engler has seen this dynamic up close. Absentee landowners and the sudden turnover of rental land can leave fields without good weed and volunteer control, essentially dooming their neighbors’ wheat fields.

In 2017, one untended field spread wheat streak mosaic to others around it for miles. It cost Engler and his neighbors a lot of yield.

“It’s really out of your hands,” he says. “There’s nothing to do once it gets started.”

Jones agrees, and adds, “There has to be a neighborhood effort to control this.”

VARIETY SELECTION HELPS. Since 2015, Engler sees wheat varieties differently.

“We certainly look at wheat streak mosaic resistance more than we used to,” he says. “I probably wouldn’t pick a variety unless it had some decent tolerance to it now.”

Growers have a smattering of options here, Jones notes. Wheat varieties Joe, Clara CL and Oakley CL show good resistance to the virus, but that protection starts to break down when temperatures climb above 75ËšF. TAM 112, Byrd and Avery varieties can offer some virus protection by slowing mite reproduction.

Planting wheat a little later in the fall can reduce the risk that the crop will emerge when wheat curl mite populations are large and looking to move, Jones adds.

Kansas State University researchers are actively breeding wheat with a new resistance gene, WSM3, which will protect wheat from all three mosaic viruses. And, the resistance will persist even in higher temperatures. However, varieties with this new gene are still several years from commercialization.

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