Control Volunteer Wheat; Limit Risk of Wheat Streak Mosaic
August 10, 2007
In recent years, wheat streak mosaic occurrences have increased and significantly affected wheat production across Nebraska. This increase is largely due to warm extended falls which allow a longer window for the virus vector to survive and transmit the virus to new crop winter wheat. While another mild fall will increase potential wheat streak problems, growers can reduce the risk to them and their neighbors by managing the summer hosts for the virus vector.
Cycle of Disease Transmission
Wheat streak mosaic virus is transmitted by the tiny wheat curl mite. This 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 host plants 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 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 next year.
Uncontrolled pre-harvest volunteer wheat virtually assures a serious infection of wheat streak in that field and surrounding fields the following year.
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 around an infested field with volunteer wheat, but it can be significant. Wheat fields adjacent to corn fields should be planted as late as practical, preferably after the corn has dried substantially.
The most likely sources of mites are the fields next to a wheat field. Controlling volunteer wheat will reduce the risk of developing problems in that field as well as 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 wheat.
The goal must be to completely eliminate pre-harvest volunteer for at least two weeks between harvest and emergence of fall-seeded wheat.
The most effective way to manage this disease is to break the over-summering "green bridge" to avoid the buildup of mites and virus before winter wheat is planted. The most effective way to do this is to ensure that all volunteer plants are killed. 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 surviving on the volunteer even though it did not look good. The goal must be to completely eliminate pre-harvest volunteer for at least two weeks between harvest and emergence of fall-seeded wheat.
Volunteer can be effectively controlled with tillage or herbicides. Weather conditions will influence the effectiveness of the method 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 when soils are wet or cool conditions exist.
Herbicides also offer the benefit of conserve crop residue while controlling volunteer. If volunteer wheat is growing well and not stressed, glyphosate can provide excellent control. Be sure to add ammonium sulfate to the spray mixture for maximum effectiveness. If volunteer is stressed by dry conditions, however, other herbicides or tillage may be better options.
It may take up to 14 days to completely kill all volunteer plants with glyphosate products. If fall-seeded wheat is expected to emerge before complete plant death, growers are advised to use tillage or Gramoxone Inteon, with or without atrazine, rather than glyphosate. Plant coverage is important for good control with Gramoxone Inteon so it should be applied in at least 10 gallons/acre of spray solution. See the label for more details. With moisture, atrazine will provide some control of emerged plants and provide residual control of later emerging plants. Atrazine does persist in soil, so growers should be aware of the crop rotation restrictions for their soils and location.
Numerous factors influence the occurrence of wheat streak mosaic. While many of these factors, such as weather conditions, can not be controlled by the grower, one significant can be. Adequate control of volunteer wheat, especially that resulting from pre-harvest hailstorms, will have the greatest impact on reducing the risk of developing wheat streak mosaic.
Gary Hein, Extension Entomologist
Drew Lyon, Dryland Cropping Systems Specialist
Both at the Panhandle REC, Scottsbluff