Viral Wheat Disease Lands One-Two Punch

(AG Journal) – Late April’s unexpectedly destructive blizzard is the obvious villain when it comes to the uncertain health of the western Kansas wheat crop, but it’s not the only one.

A severe infestation of wheat streak mosaic also has tongues wagging and tempers flaring across the region.

“It’s a problem across the whole western third of Kansas, and it hasn’t been limited to just patches of volunteer wheat,” said Erick DeWolf, K-State extension plant pathologist. “This year we are seeing entire fields and even quarter-sections consumed by the virus.”

“The patches of wheat streak mosaic are continuing to expand,” he added. “They are still spreading.”

Unlike fungal diseases, which can be controlled with fungicide, there’s no effective treatment for the virus, which causes yellow streaks to appear on leaves in a mosaic pattern. It can reduce yields dramatically depending on the timing of the infestation and is linked with other viral diseases like high plains virus and barley yellow dwarf virus that also chip away at the crop’s potential.

The complex of viruses is carried and spread by wheat curl mites, a tiny bug that rides the wind from field to field.

“Last year was a really great year for mites. We were finding them on every crop it can live on,” noted Sarah Zukoff, a K-State entomologist with the Southwest Research and Extension Center in Garden City.

That said, volunteer wheat is largely blamed for providing a “green bridge” that allowed large populations of the mites to carry over from one wheat crop to the next.

“It can only live for a few days without green tissue,” Zukoff said. “And the hotter it is the faster it dies.”

In addition to volunteer wheat, other cereal grains, grasses and even corn can harbor the mite, but to a lesser degree.

The unusual virulence of the outbreak was discussed in Garden City during a research field day held just prior to the Memorial Day weekend.

“We’ve had to field a lot of calls,” said Rick Horton, owner of Horton Seed Services, based at Leoti, Kansas. “We have to stop sugar-coating this. It’s caused by volunteer wheat left from July on. Some of that ground hasn’t been touched.”

A few critical patches can spread the infestation across neighboring fields, he said, leaving mass destruction behind. He supports finding a way to force farmers to manage their volunteer wheat or face legal action.

“In my area you can drive for 15 miles and see fields that are 75 to 100 percent lost,” he said.

Last year’s unusually wet summer brought out multiple flushes of volunteer wheat and made it harder for farmers to get into their fields. In addition, exceptionally low grain prices discouraged farmers from conducting multiple field operations. Also notable was the exceptionally mild fall, with the first freeze delayed until mid-to-late November.

“Freezes don’t affect them too much, since they can migrate down into the plant,” Zukoff said. “It knocks them down, but it doesn’t eliminate them.”

John Fenderson, a commercial manager for WestBred, based out of Stillwater, Oklahoma, said wheat streak mosaic had appeared this spring in parts of Texas where it had never been seen before.

“It wasn’t a disaster but it’s definitely there,” he said.

Its presence was traced to messy fields that were allowed to go to seed after harvest.

“We did enough chasing to find the culprit, and it was volunteer wheat,” Fenderson said.

He suggested high “load factors” simply overwhelmed the environment, spilling over to create a viral epidemic. He also wondered if the lack of the disease in recent years had shifted the focus of variety selection away from lines with adequate resistance.

Oakley CL, Ron L and white wheat Joe are popular western Kansas wheat varieties that offer resistance to the virus, according to DeWolf. Colorado State University’s Snowmass also carries a gene that confers wheat streak mosaic resistance. Wheat breeders are hoping to introduce new forms of resistance into future varieties.

With harvest approaching, a vigorous educational campaign is under way to remind farmers to destroy all volunteer wheat at least two weeks prior to planting their next crop to ensure a clean break in the pest’s reproductive cycle.

“It will take a community-wide effort to do a good job of controlling volunteer wheat,” DeWolf told growers. “What you do this summer will set the stage for what happens next fall.”

Curl mite infestations occur in the fall; viral disease symptoms typically show up the following spring.