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In one case I observed a loss of more than $60,000 for just one field of winter wheat infected with wheat streak mosaic virus when the neighbor did not control his volunteer wheat after a hail storm because he wanted to pasture the volunteer wheat. This figure represents just part of the total loss. If the farmer wanted to plant no-till corn or sorghum (ecofallow) the next year, chances are that his corn or sorghum yields would be reduced due to the reduced amount of crop residue present. In a winter wheat rotation, the reduced amount of crop residue makes the fallow period more of a challenge and reduces the amount of soil water that can accumulate during the fallow period.
In another situation a farmer who hadn’t had a good wheat crop since 1999 was looking at a great crop in 2005. Unfortunately his neighbor didn’t control his volunteer wheat and yields were reduced due to wheat streak mosaic.
These are just two of the many similar incidents I have observed.
Wheat streak mosaic
The biggest threat from volunteer wheat is wheat streak mosaic virus. It’s been estimated that Kansas loses 10 million bushels of wheat each year from this disease. Volunteer wheat in a nearby field is usually the culprit. It serves as a reservoir for the wheat streak mosaic virus and the wheat curl mite that carries the virus from volunteer to newly planted wheat.
The biggest losses from wheat streak mosaic occur when you have a hail storm which causes early volunteer wheat since the most important summer host is volunteer wheat that emerges before harvest that serves as an insect host.
Even without a hail storm, producers can still have a problem with wheat streak mosaic. Wheat is the preferred host for mites and wheat streak mosaic virus, although mites also feed and reproduce on a few native grasses and certain grassy weeds.
Certain wild grasses probably are a source of sufficient virus-carrying mites to infest a small percentage of wheat plants. A few corn hybrids are susceptible to the virus, and many hybrids support high populations of mites under husks until early fall.
Some native grasses and certain grassy weeds are hosts to both the mite and wheat streak mosaic virus, but only a few permit increase of both virus and mites sufficient for epidemics.
In some years and in some locations, the native grasses and grassy weeds may supply enough virus-carrying mites to heavily infest adjacent wheat fields. Mostly, the native grasses and grassy weeds are reservoirs for long-term survival of mites and virus. Jointed goatgrass, downy brome and Japanese brome, all winter annual grassy weeds, can support both the virus and mites but have a life cycle similar to winter wheat. They mature before winter wheat, however, so they do not bridge the gap between winter wheat crops unless they are not controlled.
Of the summer annual grassy weeds, wheat streak mosaic virus routinely is found on barnyardgrass, stinkgrass, witchgrass and green foxtail; and, to a lesser extent, on sandbur, crabgrass and cheat.
The perennial grasses, western wheatgrass, smooth brome and buffalograss, are immune to the virus but are hosts to the mite. Hairy grama, Canada wildrye and Virginia wildrye support populations of both the virus and mite.
Theoretically, some of these grasses could have a significant role in widespread outbreaks of wheat streak mosaic. This rarely happens, however, and the grasses probably play a more important role in local outbreaks within a small geographic area where cropping or other conditions have created weedy fields.
In the Panhandle and southwest Nebraska, lack of soil water causes the perennial grasses to go dormant most years during late July and early August. Localized outbreaks in Nebraska have been traced occasionally to oats, foxtail millet or green foxtatil growing adjacent to winter wheat.
Life cycle of wheat streak mosaic
The wheat streak disease cycle begins in the fall with the movement of mites from volunteer wheat and other grass hosts to emerged fall-planted winter wheat plants. The mite is wingless and webless and depends on the wind for dispersal. In the fall as plants begin to dry out, mites leave the protected parts of plants, i.e. rolled leaves and whorls, and crawl to leaf tips or other exposed areas where they become wind-borne. If mites are carrying the virus, the young winter wheat plants become infected. The earlier winter wheat is planted and the longer mild weather extends into October and November, the greater the spread and development of wheat streak mosaic. Under these conditions there is plenty of time for extensive secondary spread of mites and virus, resulting in a high incidence of infection. Reproduction of the mite stops with temperatures near freezing.
Control volunteer wheat and wheat streak mosaic
Much of the control for wheat streak mosaic hinges on successful control of volunteer winter wheat and weeds in stubble fields and other areas. These weeds use valuable plant resources, such as soil moisture and nutrients. If soil water is available, they may use as much as three inches after harvest. Timely tillage and/or a herbicide application will keep these weeds from producing seed. Moisture conservation and weed seed production are especially important if you plan to plant a crop in that field the next spring.
In addition to the cultural practices (see box) that can be used to prevent volunteer wheat and wheat streak mosaic, mechancial and chemical measures can be used to control volunteer wheat. While these measures can be effective, soil water and nutrients will already have been lost and their use will likely incur an added production cost. Using good cultural practices and timely weed control can limit these losses.
Mechanical control
Several options are available. Most years sweep blades can be used immediately after harvest. The weather must be dry and hot to successfully use a sweep blade, as weeds must wilt within 30 minutes for good control. Usually, if you do not blade immediately after harvest, it’s impossible to get the blade in the ground later unless there’s good moisture. Two bladings usually are necessary in the fall for good weed control, since the first usually plants weed seeds.
Chemical control
Another, and possibly the best, option is to use herbicides, especially given current prices. If no weeds are present until late summer, the best option is to wait until late August or early September and apply atrazine, unless there is a problem with atrazine on the soil type or atrazine is not labeled for the following crop(s). A contact herbicide should be added if weeds and/or volunteer crops are present. If volunteer wheat and other weeds are present after harvest or soon after, apply glyphosate plus NIS and spray grade ammonium sulfate. Some years two applications may be needed in addition to the fall atrazine application.
Controlling volunteer wheat also can help reduce other pest problems including, the Hessian fly, Russian wheat aphid, Take-all, leaf rust, stem rust, barley yellow dwarf virus, and banks grass mite.
So be a good neighbor and control volunteer wheat. You'll be helping your neighbor and in return, helping yourself.
Bob Klein
Extension Cropping System Specialist
West Central REC, North Platte
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