Wheat streak mosaic virus hasn’t been seen in the province for more than a decade, but it’s making a comeback this year.
“Last year we had a dozen or so fields in southern Alberta that tested positive for or were assumed to be wheat streak mosaic, based on the symptoms,” said Mike Harding, research scientist and plant pathologist with Alberta Agriculture in Brooks. “We had been on the lookout for it in case it overwintered, and then we had a very mild winter.”
All positive fields have been south of the Trans-Canada Highway.
A number of factors need to come together for wheat streak mosaic virus to develop. The virus is carried by an insect known as the wheat curl mite, which can migrate from field to field via the wind.
“Wheat streak mosaic virus doesn’t overwinter here,” said Harding. “So if the mite doesn’t overwinter, or isn’t infectious, then we don’t see the disease.”
Symptoms present themselves differently depending on both the weather and the cultivars, and the disease can be difficult to diagnose. Infected plants will display light green, yellow or white streaks on the leaves, parallel to the veins. As the crop matures, some plants will be stunted.
“The most conspicuous symptoms are the ones that are present in younger plants with light green or pale green or whitish streaking along the leaf blade,” said Harding.
The disease can also look similar to stripe rust. When stripe rust occurs, it also manifests as yellow streaks, with orange spores eventually appearing on the leaf.
“If you go back the next day or two and you don’t see orange stripes, it’s probably not stripe rust,” said Harding. “You can protect against stripe rust infection with fungicides, but wheat streak mosaic can’t be managed with fungicides because it is a virus.”
The wheat curl mite can damage the wheat crop by feeding on the plants. But the virus itself reduces the plant’s ability to photosynthesize and reduces its ability to fill grain heads.
“In many cases, you’re losing the green tissue that you need to make plump kernels.”
Wheat streak mosaic virus hasn’t been an issue in Alberta for a long time.
“This is probably the most significant virus that we have had in the last 10 to 20 years,” said Harding. “It’s just a matter of all those planets aligning to get us to that spot. We have to have the mite, it has to be infectious, and it has to spread from field to field.”
Alberta winters typically kill off the wheat streak mosaic virus. But the virus can survive when there is a “green bridge” — such as winter wheat that stays green through the winter.
“If you have a wheat streak mosaic virus in the area and it comes up at the same time that there are wheat curl mite issues in the spring cereals that haven’t been harvested yet, the vector moves into the cereals and survives in the green tissues through the winter and then green bridges through a season when it would normally die,” said Harding.
The disease is rare, so there are no treatments, and no resistant or tolerant varieties.
While there are lots of good reasons to seed winter wheat early, wheat streak mosaic virus is one reason to seed winter wheat at a later date.
“We don’t want to see this overwinter and be a problem again next year,” he said.
Community effort is what really will cut down the disease, and so everyone in the area needs to take action.
“If eight out of 10 avoid green bridging, but two don’t, everyone could have the disease,” said Harding.
Anyone who has a field with wheat streak mosaic in it will want to control their volunteers. Last year, when the disease showed up late in the season, some producers cut the crop and baled it for green feed. Other who found the virus early enough took off the infected crop, and seeded another, non-cereal crop.