Did the Drought Stop Wheat Virus?

Persistent drought after harvest and delayed planting could indicate a lower risk of Wheat Streak Mosiac Virus this year. Still, experts caution producers to continue monitoring and managing volunteer wheat as they plant this fall.

“Right now, we’re thinking that there is a slightly lower risk of wheat streak heading into fall planting. This is largely due to extended drought that has limited the emergence of volunteer, especially out west,” said Kelsey Andersen Onofre, K-State wheat and forage extension specialist. “But the caveat is that there are other hosts for curl mites out there in the landscape. Those mites find a way and they can surprise us.”

In 2021, WSMV was the third most significant disease to strike the Kansas wheat crop loss, according to the Kansas Cooperative Plant Disease Survey Report released in December 2021. Statewide, the estimated yield loss was 3.1%, equivalent to 11.6 million bushels — well above the average for the last five, 10 and 20 years. According to the report, “This was the highest yield loss due to this disease since 2017, when there was an estimated 5.6% loss, and 2006 prior to that (7% loss).” Losses were reported in six of the nine crop reporting districts but had high variability from the eastern to western sides of the state, with the highest losses in the central corridor at 8.3%.

The spike in disease pressure resulted from heavy germination of volunteer wheat in late summer and early fall 2020. The report noted, “This would have created a ‘green bridge’ upon which the wheat curl mite (the vector that transmits the virus) was able to survive between 2020 harvest and 2021 planting. It would have then been able to wreak havoc on the planted winter wheat crop.”

This year’s harvest had the opposite weather — it has been, and continues to be, very dry. As a result, Andersen Onofre explained she’s received mixed reports from across the state — it was too dry for volunteer wheat to emerge in northwest Kansas, some volunteer wheat in western Kansas is being fenced off to graze cattle, and volunteer wheat is only starting to emerge in other areas as producers are starting to plant.

Does that mean producers are off the hook for WSMV this year? Andersen Onofre warns growers not to be complacent in controlling volunteer wheat. Even in areas that did not have volunteer wheat emerge immediately after harvest, several other grassy species can play host to the wheat curl mites — Bermuda grass, barnyard grass, green foxtail, downy brome, rye brome, just to name a few.

“Curl mites can survive in grasses in ditches and along waterways, so we can’t say for sure there’s no risk,” said Andersen Onofre. “But volunteer wheat is the highest risk reservoir. Usually when we see big outbreaks of wheat streak, there’s some volunteer wheat nearby.”

Andersen Onofre also cautioned producers grazing cattle on volunteer wheat or who have fields neighboring grazing pastures to balance short-term forage needs with potential long-term impacts on next year’s harvest. For these producers, her first suggestion is to make sure to destroy the remaining volunteer ahead of planting wheat or entering a fallow period.

“The full life cycle of the mite takes about 10 days,” said Andersen Onofre. “Generally, we say you need at least two weeks of no host within two miles of where you’re going to be planting fall wheat. This two-week break in host is necessary for the curl mites to die off.”

Another suggestion for high-risk fields is to plant WSMV-resistant varieties like KS Dallas or PlainsGold Guardian. Still, she reminded producers that these varieties only protect against one of the viruses that cause WSMV and may not provide protection against high plains virus or Triticum mosaic virus. Additionally, these varieties have a resistance gene that is temperature sensitive, and “turns off” at high temperatures.

“We don’t have perfect resistance, but we do have varieties that can do better and make yield even if they’re in one of those high-risk situations,” she said.

For producers who have only recently had volunteer show up in fields within the past couple of weeks, Andersen Onofre is less worried. If the “green bridge” was disrupted in the drought-stricken period after harvest, there might not be enough mites to cause a severe infection via this fresh flush of volunteer. Additionally, later planting dates in cooler weather are less favorable for infection as mite populations are less active and reproducing less aggressively.

“There are just fewer mites in the landscape, so the cooler it gets at planting, the better off we are,” Andersen Onofre said. “But that still doesn’t stop things. If you plant right into a field of volunteer or right next to an infested field, there’s still a risk.”

Managing for WSMV doesn’t stop at planting. Producers should monitor for WSMV throughout the growing season. Sometimes producers can see fall symptoms of WSMV if the weather stays warm, but typically symptoms won’t show until the green-up period next April or May. Symptomatic leaves will display dark green coloring and light-yellow discolorations. Infected fields often will not head out well or will have low grain fill. Symptoms typically are more severe on the edge of a field and fan inward, indicating the source of mites and direction of wind. Andersen Onofre reminded producers that K-State’s diagnostic lab can test for WSMV and other wheat viruses in the fall or spring to determine the presence of the virus.

“If you are suspicious of some wheat and you’re seeing symptoms in the fall or early in the spring, you can always send us a sample,” she said. “We can verify for sure if it’s WSMV or not.”

Whether or not the drought and late planting dates will break the “green bridge,” the impact of WSMV won’t be fully realized until next summer. But even if the indicators point to lower risk this growing season, the recommended strategies to combat this virus stay the same.

“There are some mixed signals, so stay vigilant,” said Andersen Onofre said. “The best thing we can do is to make those decisions before planting — control our volunteer wheat, watch our planting dates, and choose varieties that have some type of resistance. But none of those tools are perfect on their own; they all have some limitations.”

Learn more about how to stop the streak of WSMV, even in a drought year, at https://kswheat.com/wheatstreak.