August 8, 2008
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| Volunteer wheat plant showing leaf curling caused by the wheat curl mite and leaf yellowing typical of wheat streak mosaic virus. |
The Three-Virus Complex
We have known since the mid 90s that a second virus, the High Plains virus, is often found with wheat streak. Last year, researchers in Kansas found a third virus in this complex that impacts wheat. While researchers don’t know the full impact of all three viruses, they do know the cause and how to control it. All three viruses are transmitted by the tiny wheat curl mite and can be controlled by managing its summer host plants.
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| The tiny wheat curl mite on a wheat leaf. |
The wheat curl mite can only survive on green plant material. Its most important summer host is volunteer wheat, but other grass hosts, including corn, also can serve as over-summering hosts for the mite and virus. In the fall, mites move from these summer hosts to the emerging winter wheat and begin the disease cycle. By far, the greatest risk for developing serious wheat streak is from volunteer wheat that results from hail in the last three weeks before harvest. The resulting volunteer wheat allows for a continuous "green bridge" to carry mites and virus to the next wheat crop. Uncontrolled pre-harvest volunteer wheat virtually assures a serious infection of wheat streak in surrounding fields the following year.
Risk Factors and Their Management
| Uncontrolled pre-harvest volunteer wheat virtually assures a serious infection of wheat streak in surrounding fields the next year. |
Volunteer Wheat. Generally, the risk of developing wheat streak from volunteer wheat emerging after harvest is much lower than the risk from pre-harvest volunteer; however, this depends on the length of time between volunteer emergence and when wheat emerges in the fall. In western Nebraska, the virus risk from this post-harvest volunteer is relatively low because few mites will infest the volunteer after harvest unless a significant mite source, such as pre-harvest volunteer, is nearby. In central and eastern Nebraska, the wheat streak risk from post-harvest volunteer is greater because wheat is harvested earlier and wheat planting occurs later in the fall, allowing much more time for mite populations to infest and increase in this volunteer and infect the new crop wheat in the fall.
Nearby Corn Fields. Another important risk factor is the presence of corn growing adjacent to the wheat field. Mite activity around corn fields increases during ear development and peaks when the ear is beginning to dry down. The level of mite buildup and mite activity around corn fields is nowhere near the level it will be in an infested field with volunteer wheat, but it can be significant. We have seen situations where significant virus infections have resulted from nearby corn. Severe infections resulting from these situations are more likely to occur with extended warm fall weather conditions. Wheat fields adjacent to corn fields should be planted as late as practical, preferably after the corn has dried substantially.
| If hailstorms lead to widespread volunteer wheat, community-wide control efforts are warranted. |
Neighboring Fields. The most likely source of mites carrying wheat streak mosaic virus are the fields right next to a wheat field. Therefore, controlling volunteer wheat will reduce the risk of developing problems in adjacent fields. However, when large hailstorms result in widespread volunteer and a number of uncontrolled volunteer fields are present throughout a community, the risk of developing serious problems is increased for the entire community. A community-wide effort is necessary to control volunteer in those areas where widespread pre-harvest hail resulted in extensive volunteer.
Breaking the Disease Bridge
The most effective way to manage this disease is to break the over-summering >green bridge=, and thus, avoid the buildup of mites and virus before winter wheat is planted in the fall. To be effective at breaking the green bridge, all pre-harvest volunteer plants must be dead for at least a minimum of two weeks before emergence of fall-seeded wheat.
Volunteer can be effectively controlled by tillage or chemical means. Weather conditions will influence the effectiveness of the method used. If conditions following harvest are warm and dry, shallow tillage can provide rapid and highly successful control of volunteer wheat. Tillage is less effective when soils are wet or cool conditions exist. When plants are growing well, glyphosate will provide excellent control of volunteer. If plants are stressed, glyphosate efficacy is reduced and growers should consider shallow tillage or a herbicide containing paraquat, for example, Gramoxone Inteon.
Summary
Numerous factors influence the occurrence of wheat streak mosaic and its associated viruses, many of which cannot be controlled by the grower (e.g., weather conditions). But the grower can control the factor having the greatest impact on reducing wheat streak mosaic by controlling volunteer wheat, especially that resulting from pre-harvest hail.
Gary Hein, Extension Entomologist
Drew Lyon, Dryland Cropping Systems Specialist
Both at the Panhandle REC, Scottsbluff
