Wheat Streak Mosaic Appearing In Winter Wheat Fields

By Emmanuel Byamukama

Assistant Professor & SDSU Extension Plant Pathologist

Wheat Streak Mosaic Virus

A few winter wheat fields in Central South Dakota have been found with wheat streak mosaic disease. Incidence of this disease varies from a few plants to large portions of the field with yellowing leaves. This disease can be confused with herbicide injury, water lodging or nitrogen deficiency. Wheat streak mosaic is caused by wheat streak mosaic virus (WSMV).

Symptoms

Symptoms observed in wheat at this time are a result from infections that occurred during the fall. Symptoms start as small chlorotic lines, which expand to form pale green and yellow stripes. The green and yellow patterns form the typical mosaic symptom. Symptoms are more obvious on older leaves where severe yellowing starts from the leaf tips (Figure1). Depending on the time of infection and the cultivar planted, infected plants may be stunted, and symptoms are exacerbated by stress that is caused by dry and hot weather conditions. Though this virus is more common and severe in winter wheat, the virus can also infect spring wheat at times.

Transmission

Wheat streak mosaic virus is transmitted by the wheat curl mite (WCM). Wheat curl mites are microscopic (0.3 mm long) and can only be seen under magnification (such as 20x hand lens). When temperature and natural enemies are not limiting, mites develop from egg to adult in 8 to 10 days, and can increase to a high population density in a short period. Wheat curl mites are not capable of moving from plant to plant or from field to field. These pests are moved within and between fields when they are blown by the wind. However, WCM are capable of crawling over short distances between tillers/or leaves that are in contact with each other. Because deposition of WCM is by wind, this explains why heavy WSMV infections are sometimes found along the field edges. The symptomatic plants along the field edges can sometimes be mistaken for herbicide drift injury.

Management

Wheat streak mosaic disease can be best managed through cultural practices. Unlike fungal diseases, nothing can be sprayed on virus-infected plants to prevent or cure virus infection. Once plants are infected, they do not recover, but symptoms may be mild when temperatures remain below 70°F. Several practices can be used to prevent or reduce the chances of winter wheat getting infected by WSMV before planting:

• Destroy volunteer wheat and grassy weeds at least two weeks before planting in the fall. Volunteer wheat and grassy weeds are the most important risk factor for the wheat streak mosaic disease. Volunteer wheat and grassy weeds can be destroyed through tillage or herbicide application. This will reduce the ability of the WCM to use these plants as a green-bridge to the newly emerging wheat.
•  For areas prone to WSMV infection, delay winter wheat planting in fall. Planting early in the fall especially when temperatures are mild increases the risk of WCM landing and transmitting viruses in emerging winter wheat.
• Plant wheat varieties which are resistant/tolerant to WSMV. The SDSU Crop Performance Test program provides ratings of wheat cultivars against WSMV. A few cultivars are rated moderately resistant (Table 1)
• Include a broad-leaf crop in the rotation. Wheat curl mites can survive on other cereal crops including corn, millet, barley, and sorghum. Therefore, for areas with frequent WSMV epidemics, planting non-host broadleaf crops like field peas, lentils, sunflower etc. will help keep WSMV pressure low.