ROCKVILLE, Md. (DTN) -- Mother Nature is busy whipping up a recipe for wheat streak mosaic virus in western Kansas this year.
With good soil moisture and plenty of volunteer wheat in the mix already, she only needs a long, warm fall to round out the risk for an outbreak, said Kansas State University Extension agronomist Jeannie Falk Jones.
"We will be at a bit more elevated risk in western Kansas this year," she told DTN. Plentiful rain throughout the summer allowed volunteer wheat to germinate and emerge continuously for the past three months, she said.
Volunteer wheat serves as a refuge during the summer to the wheat curl mite, a tiny insect that carries three serious wheat viruses: High Plains mosaic, triticum mosaic and wheat streak mosaic. When wheat is planted in the fall, the mites can migrate from the volunteer plants into the newly emerged crop and infect 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, when experts estimate the virus cost wheat growers at least 19.2 million bushels of wheat and $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 said. Hail storms shattered many wheat heads, and the timely rains have helped the fallen wheat seeds germinate.
Max Engler, who grows wheat and milo near Deerfield, Kansas, has sprayed his wheat fields twice since harvest, only to watch another flush of volunteer wheat emerge this month.
"We've seen more volunteers coming up from rains a week ago," he said. "But now it's getting to be the time when you plant and want wheat to come up!"
Falk Jones said volunteers should be dead a full two weeks before growers start drilling wheat to ensure the mites will not infect the new crop.
Some growers who own cattle are often tempted to let their volunteers grow and provide late-fall forage for the livestock, but this is a dangerous practice for neighboring wheat fields, Falk Jones added. 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 cautioned.
Although the mites prefer wheat, they can also survive on certain grassy weed species, particularly jointed goat grass, 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," Falk Jones explained. "They can easily move a quarter of a mile, but can move farther on wind currents."
Wheat streak mosaic symptoms in a field often display this same pattern, with symptoms at their most severe near the source of the infection and gradually dissipating farther away, she added.
This mobility has turned the mite into a community crisis, with every farmer's management affecting his or her neighbors.
Engler has seen this dynamic up close in his region. Absentee landowners and the sudden turnover of rental land can leave some fields without good weed and volunteer control -- and doom their neighbors' wheat.
In 2017, one un-tended field spread wheat streak mosaic to other fields around it for miles and cost Engler and his neighbors a lot of yield. "It's really out of your hands -- there's nothing to do once it gets started," he said.
"There has to be a neighborhood effort to control this," said Falk Jones.
PICK VARIETIES CAREFULLY
Since 2015, Engler sees wheat varieties differently.
"We certainly look at wheat streak mosaic resistance more than we used to," he said. "I probably wouldn't pick a variety unless it had some decent tolerance to it now."
Growers have a smattering of options here, Falk Jones said. 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 75 degrees. TAM 112, Byrd and Avery varieties can also offer some virus protection, by slowing mite reproduction, Falk Jones said.
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, Falk Jones added.
Kansas State University researchers are also actively breeding wheat with a new resistance gene, WSM3, which will protect wheat from all three mosaic viruses -- and this type of resistance will persist even in higher temperatures. However, varieties with this new gene are still several years from commercialization.
Read more about wheat streak mosaic virus and its management from Kansas State University here: https://www.bookstore.ksre.ksu.edu/…
Read from Kansas Wheat here: http://kswheat.com/…
Emily Unglesbee can be reached at Emily.firstname.lastname@example.org
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