Wheat Streak Mosaic Virus
Wheat Streak Mosaic (WSM) on the Rise in Alberta
Alberta Agriculture & Irrigation’s crop health team is seeing a rise in Wheat Streak Mosaic (WSM), a viral disease that primarily affects wheat, causing stunted growth and reduced seed production. The earlier a plant is infected, the greater the impact—ranging from mild symptoms to complete crop loss.
Current Situation
WSM is typically rare in Alberta as both the virus and its vector struggle to survive winter. However, outbreaks have increased in recent years:
- 2017: Last major epidemic in Alberta
- 2022: Over 10 confirmed cases in southern Alberta
- June 2023: Nearly 20 confirmed cases, all reported south of the Red Deer River
Producers, agronomists, and crop scouts are encouraged to monitor symptoms in wheat (winter, spring, and durum), barley, corn, and other cereals and grasses.
Symptoms to Watch For
Early signs of infection include:
- Light green streaks along leaf veins that elongate into a mosaic pattern
- Color variation from chlorosis (yellowing) to pale green and dark green
- Severe cases: Leaves may turn completely chlorotic, scorch, or desiccate
- Purpling on some hosts
- Symptoms often first appear at field edges, on volunteer wheat, or in winter wheat crops
As the disease progresses, infected plants may become:
- Stunted and discolored (lemon yellow)
- Rosetted in growth
- Severely affected before jointing, leading to little or no grain production
Management Challenges
WSM is not a regulated pest, but management options are limited:
- No effective in-season treatments – fungicides do not kill viruses, and insecticides or miticides do not work against the microscopic, reclusive mite vector
- Severely infected fields should be harvested early for silage or green feed
- In extreme cases, the best option may be to terminate the crop and reseed with a short-season broadleaf crop
Preventing WSM: The Importance of Breaking the "Green Bridge"
The most effective control method is crop rotation and eliminating the "green bridge"—continuous green plant material that allows the virus to persist between growing seasons.
To break the disease cycle:
- Control cereal volunteers
- Delay winter cereal seeding by at least two weeks after harvesting spring crops
- Adopt a two-year or longer break between cereals
By creating a two-week gap without a green host, both the virus and the mite vector die, preventing WSM from carrying over to the next season.
Laboratory Testing for WSMV
Diagnosing WSM based on leaf symptoms alone is unreliable, as symptoms can overlap with other infections or deficiencies. To confirm the presence of WSMV, molecular testing is recommended.
The Alberta Plant Health Lab is temporarily accepting sample submissions for testing. Contact for information > planthealthlab@gov.ab.ca
By staying vigilant and implementing preventative measures, producers can help reduce the spread of WSM and protect their crops.