Five wheat fields across southern Alberta between Pincher Creek and Medicine Hat have been confirmed to have wheat streak mosaic virus (WSMV). Growers are being asked to monitor for the disease, which last hit more than a dozen fields in the province in 2016.
Michael Harding, plant pathologist at the Crop Diversification Centre in Brooks, Alta., said Alberta is the only province to report the virus, which is spread by microscopic wheat curl mites. But fields in the northern United States, including in South Dakota, have reported cases in wheat crops.
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“The good news is you don’t have to do anything,” said Harding of trying to treat the virus. “The bad news is there is nothing you can do.”
But there are ways to prevent further spread of the virus by wheat curl mites, which can overwinter in winter wheat or weeds that retain some green vegetation.
“If you have a really widespread set of symptoms throughout a spring wheat field and the field is at or before the heading stage, you may want to monitor and if the flag leaves are scorched, the heads are not going to fill,” said Harding.
At that point, it’s best to bale the infested portion of the crop and market it as green feed, he said.
“If there is no living host there, the virus and the vector only survive a short time so you won’t spread it to other fields by doing that,” said Harding, adding that terminating the crop if the virus is detected early in the season is also an option.
In addition to swathing and baling, tillage is also recommended followed by glyphosate.
Grazing is not recommended nor is application of nitrogen fertilizer, which will increase mite numbers.
WSMV symptoms include stunted plants with mottled and yellow or green mosaic streaks on the leaves.
Harding said the last Alberta outbreak in 2015-16 saw some damage on several wheat crops but only a couple saw whole-field impact.
“If the crop has already headed out and wheat streak mosaic is introduced, it’s probably not that big of a risk for a catastrophic yield loss,” said Harding. “But if it’s prior to heading and you are seeing it in the field, that could be concerning and you really want to keep your eye on it.”
Cooler, rainy weather can also slow the mite down because it is most active in the mid- to high 20 C range.
Winter wheat growers should be especially cautious of the virus, said Harding, who recommends spring wheat crops be completely taken off the field before planting winter wheat.
“If we can break that green bridge from this season to the next one, then we essentially solve the problem for next year,” he said.
If producers suspect they may have WSMV and need help confirming a diagnosis, they can email Harding at michael.harding@gov.ab.ca or contact him by phone at 403-362-1338 to arrange for a laboratory test.
