The wet weather has resulted in the emergence of volunteer wheat over the last few weeks in some parts of Kansas (Figure 1). There have been questions about the risk that this wheat poses for wheat streak mosaic virus (WSMV) to the surrounding emerged crop (see Figure 2 for a photo of WSMV symptoms). Here we walk through some considerations, as well as termination strategies.
Figure 1. Volunteer wheat that has emerged in wheat residue. Photo by Sarah Lancaster, K-State Research and Extension.
What is the green bridge?
As a reminder, the term ”green bridge” is used to describe the volunteer wheat that emerges in the summer after wheat harvest. That is because at harvest time wheat curl mites are abandoning mature wheat in search of green tissue to survive on. If there is volunteer wheat around, and that wheat is not terminated, the curl mites can hitch a ride on that wheat until the crop emerges after planting in the fall. In the fall, those mites will migrate from volunteer (and other weedy hosts) to the new wheat crop. This cycle completes the green bridge
Volunteer wheat that emerges very close to planting, or in the spring, is technically not considered part of the ‘green bridge’. This is because the fall wheat crop has already emerged on a much higher area than volunteer wheat. The fall crop itself can serve as a sufficient host for curl mites that have survived the summer.
Figure 2. Typical symptoms of wheat streak mosaic virus. Photo by Kelsey Andersen Onofre, K-State Research and Extension.
Is spring-emerged volunteer wheat as risky as summer-emerged volunteer wheat for WSMV spread?
No. For the reasons we mentioned above, spring-emerged volunteer wheat is less risky than summer-emerged volunteer wheat, because it essentially acts as a (much smaller) neighboring wheat crop. That being said, if curl mites have survived locally, they can still reproduce on this volunteer wheat, just as they would in wheat production fields. If WSMV is a concern, terminating this volunteer crop can avoid successive cycles of mite reproduction. Volunteer from fields with high WSMV levels in 2020 would be of highest concern. When making the decision to terminate this spring-emerged volunteer, the desire to control curl mite populations should be balanced with other agronomic factors.
Are there other agronomic considerations for volunteer wheat that has emerged in the spring?
What are the best termination strategies for spring-emerged volunteer, prior to summer crop planting?
Glyphosate is an inexpensive option to control volunteer winter wheat that will have little to no impact on the following summer crop. Applications of a formulation that contains 4.5 pounds per gallon at a rate of 24 to 44 fl oz/A will be effective, assuming weather conditions are appropriate. No herbicides will work well when temperatures are below 60°F during the day and/or 40°F overnight.
Other herbicides that will control volunteer winter wheat include Group 1 herbicides such as Assure II (fluazifop) or Select (clethodim). Group 1 herbicides do have rotation restrictions when applied before corn or grain sorghum. Residual herbicides such as atrazine, Canopy (chlorimuron + metribuzin), or Sharpen (saflufenacil) can also be included. However, these products also have rotation restrictions to various crops. It is important to consult product labels to determine application rates and rotation restrictions for your specific situation.
What should I do if I think I have WSMV?
Contact your local K-State Extension Office. They will work with you to send photos of the problem (close-up, whole plant, field shot) and plant tissue samples to the K-State Plant Disease Diagnostic Lab.
Use this link for the sample submission form:
Here are guidelines that can help get a good sample to the lab:
K-State Plant Disease Diagnostic Lab
4032 Throckmorton PSC
1712 Claflin Road
Manhattan, KS 66506
Contact information for K-State Plant Disease Diagnostic Lab:
Kelsey Andersen Onofre, Extension Wheat Pathologist
Sarah Lancaster, Extension Weed Science Specialist
Romulo Lollato, Extension Wheat and Forages Specialist