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Beware pre-harvest volunteer wheat
Nebraska In recent years, wheat streak mosaic has had a significant impact on wheat growers in Nebraska. A major factor in the increase in wheat streak mosaic presence is the occurrence of warm, extended falls to allow the mites to infest and the virus to infect the new-crop winter wheat. Continuation of this mild fall weather elevates the risk of developing serious wheat streak problems. Wheat streak mosaic virus is transmitted by the tiny wheat curl mite. This mite can only survive on green plant material, and its most important summer host is volunteer wheat. But other grass hosts, including corn, can also serve as over-summering hosts for the mite and virus. In the fall, mites move from these over-summering 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 occurring within about three weeks prior to harvest. This volunteer 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. The most likely source of mites carrying wheat streak mosaic virus is 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. 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 volunteer plants must be completely dead. Last summer volunteer wheat did poorly in many locations and fooled some growers who had pre-harvest volunteer into thinking that the volunteer was dead--only to produce significant problems in adjoining wheat fields this spring from wheat streak mosaic. Obviously, in these situations, mites were still surviving on the volunteer even though it did not look good. The goal must be to completely eliminate pre-harvest volunteer for two weeks between harvest and emergence of fall-seeded wheat. Volunteer can be effectively controlled by tillage or chemical means. Weather conditions will influence the effectiveness of the method that is 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 at providing adequate volunteer control when soils are wet or cool conditions exist. An option that conserves crop residue is to use herbicides to control volunteer. If the volunteer wheat is growing well and not stressed, glyphosate can provide excellent control. However, if volunteer is stressed by dry conditions, alternate herbicides or tillage may be the best option for control. Numerous factors influence the occurrence of wheat streak mosaic, but many of these factors cannot be controlled by the grower (e.g., weather conditions). Adequate control of volunteer wheat, especially that resulting from pre-harvest hailstorms, will have the greatest impact at reducing the risk of developing wheat streak mosaic. Date: 7/23/07
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