In the Nebraska Panhandle, major losses were caused by wheat streak mosaic virus this year. One wheat grower said his winter wheat yields averaged 8-22 bushels/acre. His test weight averaged 54 lb/bushel. A number of growers had their fields released so they could seed another crop. Many times this is not successful because of limited soil water. (For more information watch for an August 4 article by Nebraska Extension Dryland Cropping Systems Specialist Cody Creech.)

By far, the greatest risk of losses from mite-vectored viruses occurs when there is a summer "green bridge" of volunteer wheat emerging before harvest. This almost always occurs as a result of wheat seed head shatter from hail storms (Figure 1).

The mites and viruses survive the summer on this "green bridge," then the mites move from volunteer wheat to newly emerged winter wheat plants in the fall, transmitting the viruses. Mites can transmit wheat streak mosaic virus, High Plains virus, and Triticum mosaic virus, creating the likelihood of severe mixtures of viral infections.

Mites depend on wind for dispersal. In the fall, as mite populations increase, they leave the protected areas of volunteer wheat plants (rolled leaves and whorls) and crawl to leaf tips or other exposed areas where they become airborne. After landing on a new host, the mites crawl to the youngest leaf and begin to feed and reproduce.

In heavily infested volunteer wheat, most mites will carry the virus. Transmission to the young winter wheat plants requires only a few infected individuals. Immature mites acquire the virus in as little as 15 minutes as they feed on infected leaves. Mites remain active for most of their lives (two to four weeks or longer with cool temperatures), but the transmission efficiency of adult mites decreases with age.

Research has found that mites can only survive a day or so under hot temperatures, but two to four days under lower temperatures. Thus, a significant number of mites may be transported by the wind from greater distances than we originally thought; however, the greatest risk is to the field closest to the volunteer wheat.

Volunteer wheat is not the only host for the wheat curl mite (Figure 2). Recent research has evaluated the suitability of weedy grasses as hosts for both the curl mite and the wheat streak virus. Barnyardgrass topped the list in terms of suitability for both virus and mites, but fortunately it is not a common grass in wheat except in low areas, including terraces. In contrast, green foxtail is a rather poor host but could be an important disease reservoir simply because of its abundance. In one example, the source of a winter wheat field infected with wheat streak mosaic virus was a grain sorghum field with grassy weeds adjacent to the winter wheat field. Take note of significant stands of these grasses where they can be treated and control them as you would volunteer wheat.

Other over-summering hosts include corn or foxtail millet. An important consideration for these crops is to try to minimize the overlap of the green and growing summer crop and the emerged new crop of wheat. The greater the overlap the greater the risk from these hosts.

Timely Weed Control Helps Manage Yield Loss Risk

Risks from not controlling volunteer wheat and weeds:

• Pre-harvest volunteer wheat can cost you or your neighbor the value of next year's winter wheat crop due to loss from wheat viruses. In addition, loss of the winter wheat residue can lead to much lower yields in the following corn or grain sorghum crop. While both pre- and post-harvest control of the “green bridge” are important, timely control of pre-harvest volunteer wheat will have a greater impact on ultimately reducing virus transmission and yield loss (Figure 3).
• Any volunteer wheat that has emerged around wheat harvest even in a summer crop like sunflower or corn can also be a significant risk. Figure 4 shows considerable volunteer wheat in a sunflower field in the fall that was heavily infested with mites. This field resulted in total loss to adjoining wheat fields.
• Uncontrolled volunteer wheat and weeds can cost you 30 bu/acre or more of corn or sorghum the next year due to soil moisture loss.
• Weed control may be more difficult and expensive in next year's crop.
• Planting the next crop may be more difficult in last year's stubble.

Figure 3. Comparison of the increased disease risk of pre-harvest volunteer wheat (above, usually from hail storms) versus post-harvest volunteer wheat control in the development of wheat streak mosaic. (Source: Justin McMechan)

The earlier the winter wheat is seeded and the longer mild weather extends through the fall, the greater the risk of spreading wheat streak mosaic and other viral infections. For instance, near Ogallala, the winter wheat yield doubled with a week delay in seeding date. Under warm fall conditions, the probability of secondary spread of mites and viruses increases, resulting in greater incidence of infection.

In another field near Ogallala, wheat seeded on September 8 was lost to the wheat streak mosaic virus while a field across the road seeded September 19 to the same winter wheat variety was not affected by the virus.

Impact of Wheat Seeding Time

Reproduction and spread of mites stop with cool temperatures in the fall; however, mites can survive cold winter temperatures. The virus survives the winter within the plant, and the mites survive as eggs, nymphs, or adults protected in the crown of the wheat plant. As winter wheat greens up in the spring, mites become active and the virus may be spread to healthy winter wheat plants or to emerging spring wheat, although this is much less of a threat than fall transmission.

One example of the impact of wheat streak was when volunteer wheat from one neighbor's hailed winter wheat crop served as a host for the virus vector and the other neighbor’s wheat crop across the road averaged just 5 to 6 bushels per acre on 320 acres. Yields in nearby areas which had not been hailed and didn't have pre-harvest volunteer wheat yielded 60 bu/ac.

Some summer annual grassy weeds also can act as mite and virus hosts. The greater the density of these grasses, the greater the risk. Controlling them will reduce the risk.

Additional Losses Associated with Volunteer Wheat

Volunteer wheat can provide habitat for many other pathogens that may later be a problem if continuous wheat is seeded in the fall. Root rots as well as seedling blights caused by Fusarium species are specific examples.

Aphids. Volunteer wheat also attracts the Russian wheat aphid and other cereal aphids. If allowed to remain until the new wheat crop emerges, the risk of aphid infestation and barley yellow dwarf increases.

Hessian Flies. Volunteer wheat allowed to survive through late summer and fall also dramatically increases the risk from Hessian fly.

Wheat Stem Sawflies. Some weedy grasses can serve as a host for sawflies. If these weeds are allowed to persist in the environment, their seed production will increase the available hosts for sawflies the following year. This could compound the challenge of managing a pest that is already very difficult to manage.

Moisture Loss. Volunteer winter wheat and weeds also use soil water which would otherwise be used by the following crop. The average soil water loss due to volunteer wheat is 3 inches which can result in yield losses of 30 bushels or more in corn or sorghum. Occasionally the loss can be as much as 100 bushels. How does this happen when we only save 3 inches of soil water? With the additional 3 inches of soil water the corn or sorghum crop will survive up to three weeks longer without rain before being lost to drought. Hence, if enough rain is received in time, we have observed yields of 100 bushels or more of corn or sorghum where volunteer wheat and other weeds were controlled after wheat harvest.

Increased Weed Seed. Letting weeds produce seed increases the weed seed bank and makes weed control more difficult in the succeeding crops. The weed residue also makes planting more difficult in the following crop.

Summary

Volunteer wheat surviving through the summer and fall creates numerous risks for the subsequent wheat crop as well as other rotated crops. Controlling volunteer wheat will reduce the risks from these threats and ultimately improve the bottom line.

For herbicide options to control volunteer wheat and annual grasses, check the 2017 Guide for Weed, Disease, and Insect Management in Nebraska, EC130, (http://marketplace.unl.edu/extension/extpubs/ec130.html).