Scouting time
Initial spring symptoms of wheat streak mosaic virus usually show up in April on edges of fields near volunteer wheat. Look for yellow streaking or mosaic patterns on young leaves. Infected plants are stunted and tiller poorly. Tillers of infected plants are sometimes prostrate on the ground. As the weather warms, symptoms become more severe. Leaves on infected plants turn yellow from the tip down, but usually the leaf veins remain green longest. This gives the leaves a yellow and green striped appearance. Often, leaves fail to unfurl completely. This is due to the microscopic wheat curl mite which carries the virus. You can look for curl mites with a 10X magnifying lens under the curled portion of the leaf. Curl mites look like tiny white rice grains. If curl mites are found, odds are very high that the plant is infected with wheat streak mosaic virus. Serological tests for wheat streak mosaic are available from the KSU Plant Diagnostic Lab.
Sampling method
None listed.
Thresholds
Threshold is unknown.
Chemical control
There are no chemicals which are effective or labeled for curl mite control. TAM 107 and TAM 200 were resistant to the curl mites, but the mite populations adapted, and now the varieties are susceptible. [R.L. Bowden]
Non-chemical controls
Losses due to wheat streak mosaic depend on the variety, the weather, the percent of plants infected, and the time of infection. Fall infection is the most severe and can cause 50% loss or more. Spring infections may cause losses closer to the 20% range. the first and most important control is to break the bridge created by volunteer wheat. See "Be a Good Neighbor: Control Your Volunteer Wheat," KSU Extension publication number MF-1004. Volunteer should be killed at least 2-3 weeks prior to the emergence of the new crop. It may be killed by cultivation or by herbicides, but it must be a thorough job because just a few plants can harbor a lot of mites. Since mites travel in the wind, volunteer must be killed within 1/4 to 1/2 mile of the new field. Coordination of volunteer control is a community effort which requires good cooperation between neighbors.
The second control is to avoid early planting. Early planting allows the mites plenty of time to move into the field, reproduce, and spread. Waiting until after the "fly-free" date is recommended for wheat streak mosaic control. Ask your local county extension agent about recommended planting dates for your area.
The third control is to plant a variety with resistance to the virus or the curl mite. Current resistance information is contained in Wheat Variety and Insect Ratings, publication MF- 991. Although many varieties have partial resistance, none are highly resistant to wheat streak mosaic. Researchers are attempting to find improved sources of resistance and incorporate them into future varieties. Find performance ratings for Kansas varieties rated for WSMV close to you.
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