By Scout Nelson
Wheat streak mosaic virus (WSMV) is a serious disease that affects winter wheat across the Great Plains. The disease can damage crops and lower grain yields. There is no in-season treatment that can cure wheat plants once they become infected. One of the best ways to reduce the risk is to control volunteer wheat and grassy weeds before and after planting.
Connie Strunk, SDSU Extension Plant Pathology Field Specialist, along with Madalyn Shires and Anna Lagerhausen, explained the importance of weed control in limiting the spread of WSMV. Their guidance focuses on removing plants that support wheat curl mites, planning planting dates carefully, and continuing weed management after winter wheat is planted. These steps are especially important when disease pressure is high, as seen in 2025 and 2026.
Understanding the Green Bridge
Wheat streak mosaic virus needs living plant tissue to survive. It cannot remain active in dead crop residue or soil. The wheat curl mite is a tiny pest that carries the virus from infected plants to healthy wheat.
Volunteer wheat is often the main source of the virus after harvest. These plants grow from grain left in the field during harvest or from grain that falls to the ground. Hail damage, harvest losses, and rain after harvest can lead to more volunteer wheat growth. Grassy weeds, volunteer corn, and sorghum can also provide living hosts for the mites and virus.
When infected volunteer plants remain green until new winter wheat emerges, mites can move from those plants into the new crop. This link between old and new plants is known as the green bridge. Removing these living hosts helps break the cycle and lowers the chance of infection.
Why Weed Control Matters
Grassy weeds and volunteer cereal plants can support wheat curl mite populations in fields and nearby areas. Field edges, roadside ditches, and other areas with unmanaged grasses may also increase disease risk.
Weed control helps by removing plants that mites use for food and shelter. It also removes the living tissue the virus needs to survive between growing seasons. If these plants remain in place, mites and the virus may continue moving from one crop to the next.
Farmers should check fields and nearby areas for volunteer wheat and grassy weeds. Special attention is needed after hailstorms, harvest losses, or rainfall that encourages new plant growth.
Control Weeds Before Planting
Volunteer wheat and grassy weeds should be controlled at least two weeks before winter wheat planting. This timing gives the green bridge time to break and helps interrupt the wheat curl mite life cycle.
When herbicides are used, farmers may need to allow additional time for plants to die completely. Mites can remain on green plant tissue for a short time while treated plants are dying. A well-timed weed control plan can reduce the chance that mites move from dying plants to newly emerged wheat.
Farmers should follow herbicide label directions and local recommendations when planning field treatments.
Continue Management After Planting
Pre-plant weed control is important, but management should not stop once winter wheat is in the ground. Volunteer wheat and grassy weeds can continue to support mites after planting. In areas with high disease pressure, farmers should keep checking fields and removing unwanted green plants until a killing frost ends their growth.
In October 2026, symptomatic volunteer wheat was found in South Dakota, showing that infected plants can remain a concern well after winter wheat planting. Early research also suggests that winter wheat is a main place where the virus survives through winter. Reducing fall infections may help slow disease spread in the 2027 growing season.
Planting Date Affects Disease Risk
Early-planted winter wheat may face a higher risk of WSMV infection. Warm fall weather can keep wheat curl mites active, while newly emerged wheat provides a fresh host. The risk increases when volunteer wheat and grassy weeds remain nearby.
Farmers can reduce this risk by planting within recommended dates for their area and avoiding planting too early. Local planting guidance can help producers balance disease prevention with other crop needs.
Other Ways to Reduce Risk
Farmers can use several practices alongside weed control to protect winter wheat:
- Choose suitable varieties: Plant wheat varieties with tolerance or resistance to WSMV when available.
- Rotate crops: Use broadleaf crops in the rotation when practical to reduce the supply of suitable host plants.
- Check fields regularly: Look for volunteer wheat after hailstorms, harvest losses, or rain.
- Manage field borders: Control grassy weeds in nearby areas when possible, especially grasses that remain from the previous season.
- Remove green bridges: Eliminate volunteer wheat and other living plants that allow mites and the virus to survive between crops.
- Confirm suspected disease: Contact the SDSU Plant Diagnostic Clinic to help identify the disease and develop a stronger management plan.
Protect Winter Wheat Through Prevention
Preventing wheat streak mosaic virus is more effective than trying to manage it after infection. No fungicide or other in-season treatment can cure wheat plants infected with WSMV. Timely weed control, careful planting dates, regular field checks, and suitable wheat varieties can help reduce disease risk. By combining these practices, farmers can protect winter wheat health and support better yield potential.
Photo Credit: South Dakota State University
Categories: South Dakota, Crops, Wheat