In some areas of the state, recent rains have stimulated yet another flush of volunteer wheat (Figure 1). Volunteer wheat is known to be an important reservoir for wheat streak mosaic and wheat curl mites that spread this disease. Many growers are asking if this newly emerged volunteer elevates the risk for problems with wheat streak mosaic becoming established this fall.
Figure 1. Newly emerging volunteer wheat. Photo by Erick DeWolf, Extension Plant Pathology.
While volunteer wheat is known to be an important reservoir for wheat streak mosaic, all volunteer wheat is not equal in its contribution to disease outbreaks. The greatest risk comes from volunteer wheat that emerged shortly after harvest and was left all summer long. This volunteer wheat is rapidly colonized by the curl mites and infected with viral diseases. When this volunteer wheat is removed, the risk is reduced because the virus and curl mites do not survive more than a few hours without a living host. If new volunteer wheat emerges during the summer or fall, the risk of disease returns but not to the same degree. The risk is lower relative to the initial flush because there is less time for it to be colonized by curl mites and infected with virus prior to wheat planting. Volunteer wheat that emerges at planting, or just as the new wheat crop is emerging, is less likely to become a reservoir for disease than volunteer that has been present all summer. If fact, the late emerging volunteer wheat may pose no more risk than early planted wheat fields.
The need to control this volunteer wheat depends on the density of the stand and rotational plans for individual fields. A thin stand of sparse volunteer wheat may be able to wait until other planned herbicide applications are made in the spring. In contrast, a heavy or thick stand of volunteer wheat or other winter annual grasses like jointed goatgrass, downybrome, or other brome species may necessitate some action this fall. Glyphosate is our best herbicide for controlling these winter annual grasses. Glyphosate can be tank-mixed with other residual herbicide programs that often are used to manage winter annual broadleaf weeds such as mustards, henbit, or marestail.
Erick DeWolf, Extension Plant Pathology
Curtis Thompson, Extension Agronomy State Leader and Weed Management Specialist