Wheat streak mosaic virus (WSMV) is the most important viral disease affecting wheat fields in South Dakota. WSMV can cause a yield loss of 10 to 80 %, depending upon the year and growth stage of the wheat plant. The earlier the plant is infected, the greater the yield loss. The wind-blown wheat curl mite (Aceria tosichella Keifer) vectors WSMV from plant to plant. Wheat streak infects wheat, oats, barley, corn, sorghum, millets, and other grass species. WSMV is endemic to South Dakota, meaning it is always here but varies in its amount.

The major symptoms of wheat streak mosaic are stunted plants with mottled and streaked leaves. Light green and dark green or yellow and green mosaics that coalesce into streaks are characteristic of this disease. In severe reactions, streaks may become necrotic as disease progresses. Other symptoms include:

1. Spreading rather than erect growth (prostrating),
2. Delayed heading,
3. Reduced tillering,
4. Sterility or poor seed set,
5. Poor grain fill, and
6. Reduced yields.

If your field is infected with WSMV there is nothing you can do at that time. Please
remember that foliar fungicides do not offer protection against viral diseases. Fungicides only work to control a fungal disease.

Some management tips to avoid having a wheat streak infection include: Use cultural management practices such as appropriate planting dates and controlling volunteer grassy weeds. In order to stop the wheat curl mite’s life cycle a two week break in the green bridge is necessary. Avoid planting winter wheat early in the season. Winter wheat planting should be delayed until after mid to late September for optimum WSMV control. Consider planting spring wheat early as there is less risk of obtaining WSMV infection than late-planted spring wheat. Whenever possible use a wheat cultivar with the most tolerance/resistance that is available for your area. The “2011 Winter Wheat Variety Yield Results” contains disease ratings for WSMV (see table 3 on page 5). WSMV symptoms are often found first along the edge of a field or in patches near wheat volunteers.