Clean Fields are Nice,
But the Money You'll Keep in Your Pocket is Even Nicer

July 24, 2009

Each year wheat producers lose millions of dollars in potential income by not controlling volunteer winter wheat and weeds in wheat stubble. In the Central Great Plains, the largest losses are due to wheat streak mosaic.

These losses occur when there is a summer "green bridge" of volunteer wheat emerging before harvest, most often as a result of hail storms. The volunteer wheat serves as a "green bridge" from one crop to the next, as wheat curl mites move from it to newly emerged winter wheat plants in the fall, transmitting the wheat streak mosaic virus.

The mite depends entirely on the wind for dispersal. In the fall as mite populations increase, mites 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, and transmission to the young winter wheat plants requires few mites. The eggs hatch and the larvae acquire the virus in as little as 15 minutes as they feed on infected leaves. Mites remain infective for most of their lives (two to four weeks or longer with cool temperatures), but the transmission efficiency of adult mites decreases with age.

	Skip-row ecofallow corn in winter wheat stubble that received a timely herbicide treatment after the winter wheat was harvested with a stripper header.

Factors Affecting Disease Risk

The earlier winter wheat is planted and the longer mild weather extends through October and November, the greater the risk of spread and development of wheat streak mosaic. Under warm fall conditions, the probability of secondary spread of mites and virus increases, resulting in greater incidence of infection. Reproduction and spread of the 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 can be spread to healthy winter wheat plants or to emerging spring wheat.

One example of wheat streak severity resulted in a neighbor across the road from a hailed winter wheat crop averaging 5 to 6 bushels an acre on 320 acres with yields. Yields in nearby areas which hadn't been hailed and didn't have volunteer wheat were 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

• 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 perhaps barley yellow dwarf increases. Volunteer wheat allowed to survive through late summer and fall also dramatically increases the risk from Hessian fly.

   

• Moisture Loss. Volunteer winter wheat and weeds also use soil water which would have been used by the following crop. The average soil water loss is 3 inches which can result in 30 bushels or more of corn or sorghum. Occasionally the loss can be 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 can wait up to 3 weeks longer for 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 weeds were controlled after harvest versus not controlled.

   

• Increased Weed Seeds. 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.

   

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

Robert Klein, Extension Crops Specialist for Western Nebraska
Robert Wright, Extension Entomologist
Stephen Wegulo, Extension Plant Pathologist
Gary Hein, Director, Doctor of Plant Health Professional Program