Figure 1. Volunteer wheat growing in previous winter wheat field. All volunteer wheat in this field had Wheat streak mosaic virus symptoms. Volunteer wheat and grassy weeds need to be controlled with an herbicide at least two weeks before winter wheat planting. Credit: E. Byamukama
Many wheat fields in South Dakota were cut for hay this year due to drought conditions and the need for hay. In some fields where moisture was adequate or rainfall has occurred, wheat and weeds are growing back. Deciding what to do with this growth presents complex problems. The wheat and weeds are providing valuable groundcover and vertical architecture to partially replace that lost in the haying process. At the same time, they are using valuable moisture and can cause insect, disease, and weed problems if they are not properly managed. Deciding on whether to remove this growth, or remove and replace it, or simply leave it alone for a while will depend on what is planned for the field next year and what conditions exist in the field along with probable weather trends.
Wheat Streak Mosaic Virus
Numerous fields of winter wheat and some spring wheat fields were seriously affected by Wheat streak mosaic virus (WSMV) this spring. WSMV is spread by wheat curl mites. According to researchers from the University of Nebraska-Lincoln, wheat curl mites live on many species of grasses but particularly favor barnyard grass, green foxtail, and wheat.
Prevent the Green Bridge
Volunteer Wheat & Grassy Weed Control
The grassy weeds and volunteer wheat provides the wheat curl mites with a place to live (called the “green bridge”) until the new winter wheat fields begin to grow in the fall (Figure 1). “Green bridge” can happen with fungal root and foliar diseases but it is most commonly thought of in terms of insects and insect transmitted diseases. Volunteer wheat and other grasses ideally would be dead two weeks prior to planting the fall crop. This provides time for the wheat curl mite population to die along with the WSMV that they spread. It also allows time for the cereal aphids that transmit Barley yellow dwarf virus to die off as well. Delaying seeding in the fall until after the weather cools (date varies with location and climate) can help to minimize the chance of these diseases spreading into the field from uncontrolled infestations in neighboring areas. Insecticide seed treatments do not reduce wheat curl mite populations. There may be some benefit in their use for the reduction of aphid populations during the fall in winter wheat, which may also reduce transmission of Barley yellow dwarf virus. Control of volunteer wheat and grassy weeds, and delayed planting is the best way to prevent a green bridge.
Adjacent Field Sanitation
The second most challenging scenario would be a field that is not going to wheat but is bordering a field going to winter wheat this fall or spring wheat next spring. Good sanitation steps are required here also. Wheat curl mites and the cereal aphids can transmit diseases in both the spring and fall from adjacent fields that have hosts and the virus present. Wind often aids this movement. This commonly makes it easy to confuse WSMV infestation with herbicide spray drift damage. Preventing spread to adjacent fields requires taking many of the same steps as those taken for fields going to wheat. Plants that host the virus need to be controlled early, especially in areas on the upwind side of the planned wheat field. Appropriate planting date guidelines still pertain.
Cover Crop Cleanup
Lastly, a few areas have received some good rain recently. Producers in these areas may be considering a cover crop to produce some feed or cover this fall. These are also areas that could harbor volunteer wheat. These areas should also be cleaned up two weeks before planting a cover crop. Volunteer wheat growing in a cover crop is another area that can harbor the wheat curl mite, and aphids until they can move to newly planted winter wheat fields this fall.
The Bottom Line
Surface residue is very important to winter wheat success but so is soil moisture and good sanitation that prevents diseases and insects from bridging to the subsequent crop. Controlling volunteer winter wheat and breaking the green bridge, along with conserving moisture through post-harvest sanitation and retaining residue, are all tactics that will help next years’ wheat crop.
Figure 2. Field of winter wheat that greened up after being hayed.
Credit: R. Beck