By Scout Nelson
Wheat Streak Mosaic Virus (WSMV) continues to spread across South Dakota wheat fields during the 2026 growing season. Disease activity is similar to last year, with the mild winter allowing the virus to survive in winter wheat and become active earlier than normal. Symptoms first appear in late April and continue increasing in both winter and spring wheat as the season progresses.
Madalyn Shires, Assistant Professor & SDSU Extension Plant Pathology Specialist, along with Connie Tande and Connie Strunk, continue helping producers identify Wheat Streak Mosaic Virus and other crop diseases while providing management recommendations through SDSU Extension and the SDSU Plant Diagnostic Clinic.
The virus causes yellow streaks, mottled leaves, reduced tiller growth, and in severe cases, plant death. Dry weather can make symptoms even worse by increasing plant stress. Most reports this year come from eastern South Dakota, although disease levels vary from field to field.
WSMV spreads through the wheat curl mite, a tiny insect carried by the wind. The mites feed on wheat leaves and move easily between fields. Curled or twisted leaves often indicate mite activity before virus symptoms become severe. The mites reproduce best when temperatures range from 75°F to 80°F.
The virus also survives on volunteer wheat and grassy plants, including smooth brome, foxtail grasses, oats, sorghum, and corn. Managing these plants around field edges helps reduce disease pressure. Farmers are encouraged to remove volunteer wheat and grassy weeds during the green bridge period between crops to stop the virus from moving into new wheat fields.
SDSU Extension recommends planting winter wheat no earlier than September 15 to September 20, when wheat curl mite populations begin to decline. After applying herbicides to remove volunteer wheat or grassy weeds, producers should wait at least two weeks before planting new wheat.
Growing resistant winter wheat varieties and rotating with broadleaf crops also help lower disease risk. Miticides have not proven effective against wheat curl mites.
Farmers should also watch for Barley Yellow Dwarf Virus, High Plains Wheat Mosaic Virus, and Triticum Wheat Mosaic Virus, which can appear with similar symptoms.
Producers who notice unusual symptoms should contact SDSU Extension or submit samples to the SDSU Plant Diagnostic Clinic. During the 2026-2027 season, a limited amount of free wheat disease testing is available to assist with disease identification and management.
Photo Credit: gettyimages-milos-cirkovic
Categories: South Dakota, Crops, Wheat