Wheat streak mosaic virus discovery raises alarms for Alberta growers

Wheat streak mosaic virus has appeared in southern Alberta winter wheat crops, with potential to infect spring wheat.

Spread by the wheat curl mite, the virus initially appears as light green or yellow streaks running parallel to leaf veins, and can limit production, depending on severity.

“We’ve confirmed it in probably a dozen fields and we’ve heard that people have seen symptoms of it, or what looks like it, in dozens more. And in a few of the fields, it’s severe, so it’s definitely around,” said Alberta Agriculture crop pathologist Mike Harding.

“We had maybe a dozen or so fields last year that were confirmed to have wheat streak mosaic but we’ve got many, many more than that this year. It has really kind of had a break-out year.”

There are no in-crop treatments available for wheat streak mosaic virus and no effective insecticide treatment for wheat curl mites. The tiny mites can’t be seen without the aid of a hand lens or microscope and are able to hide and avoid chemical treatments.

Harding said some Alberta farmers who identified the virus early were able to disc down their wheat crops and plant early season canola, which is not a host to the virus.

However, that is not an option at this point in the season.

“If the wheat crop is to be grown for grain and you discover that you have a very severe case of wheat streak mosaic, you could take it off as green feed or silage, so then you could still get something for it rather than run the risk of losing the grain crop to the virus,” Harding said.

Other than those options, the remaining strategy is to remove any green plant tissue so the mites don’t survive to carry the virus into next year.

For that reason, farmers may want to delay planting winter wheat this fall.

“Neither the virus nor the vector would normally survive our winters unless they have a green bridge,” said Harding, “so the best chance for us to manage this would be in the fall, where we avoid seeding winter wheat early, adjacent to wheat fields that are infected with wheat streak mosaic. And also making sure that we control volunteers in fields that had wheat streak mosaic.”

Wheat curl mites do not have wings, so they rely on wind to travel to new feeding areas. They can multiply rapidly in hot, dry weather but dry out quickly when not protected by host plants.

Wheat streak mosaic virus is already causing severe crop loss in Montana and North Dakota.

Mary Burrows, a plant pathologist at Montana State University, said she has seen 100 percent winter wheat crop loss in some fields this year and estimates 55 to 85 percent yield loss in spring wheat crops in some parts of the state.

“Some guys are haying the fields and then spraying glyphosate to control any green material,” said Burrows.

She noted cheat grass and downy brome are also hosts for the mite and the virus, so weed control is an important weapon.

Burrows is telling Montana farmers to delay winter wheat planting this fall until Oct. 1. They would normally plant in early September but they should instead take time to eliminate potential mite and virus hosts.

Conditions last year combined to create ideal conditions for an outbreak this year in Montana, Burrows said.

“We had widespread hail last fall in about three or four counties … in the primary winter wheat production area, and then followed by low wheat prices, causing guys not to apply Roundup to control their green bridge.

“And then a mild fall, early planting because of timely moisture and an open winter, which basically made a big, old mess.”

Burrows and Harding recommend crop testing to properly identify the virus because it shares early symptoms with some other crop problems.

It is sometimes confused with stripe rust but the latter produces orange pustules.

Harding said stripe rust has been reported in southern Alberta this year but so far it is limited.

“That’s good news but it’s important to still be on the lookout for that disease because it can be quite explosive.”

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