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Barley yellow dwarf virus widespread in Nebraska wheat fields

Agriculture.com Staff 05/30/2007 @ 12:37pm

Barley yellow dwarf virus is causing many wheat fields in southeast and south central Nebraska to turn yellow, University of Nebraska-Lincoln specialists say.

The carrier of the virus: aphids that are enjoying the cool, wet weather, said Stephen Wegulo, UNL plant pathologist in the university's Institute of Agriculture and Natural Resources.

"Leaves are yellowing starting from the tip, or margins, and progressing toward the base," Wegulo said. "The yellowing is visible on the flag leaf of the wheat plant, giving fields a yellow cast."

Aphids have been reported infesting wheat in southeastern and south central Nebraska the last few weeks, said Bob Wright, UNL entomologist. Although there are more than 20 aphid species, oat-bird cherry aphid and corn leaf aphid were identified in these fields. These two aphids generally have a lower damage potential than greenbugs, another aphid that may be seen in Nebraska wheat.

"However, all three aphids are among the most important vectors of the virus and both the virus and the aphids are favored by cool, wet weather. The rainy and cool temperatures we had in April and May largely contributed to this widespread epidemic of barley yellow dwarf virus we are seeing," Wright said.

The virus has a wide host range including wheat, barley, oats and other wild and cultivated grasses. "Yield losses of up to 25 percent in wheat, 40 percent in barley, and 33 percent in oats have been estimated to be caused by barley yellow dwarf virus," Wegulo said.

After a plant has the virus, dwarfing occurs. This symptom is common to all the cereal crops that can be infected by the virus. Other symptoms are highly variable between the crops and often are mistaken for nutrient deficiency symptoms, Wegulo said.

"Typically leaf discoloration in shades of yellow, red, or purple occurs from the tip to the base and from the margin to the midrib in wheat, barley or oats," he said.

The virus can be minimized by controlling aphids, Wright said.

"However, this may not be economical as the virus may be spread by aphids that escape treatment or migrate from non-treated areas," he said.

In most years, aphids do not overwinter in Nebraska in high numbers, but winged forms may migrate in the spring with southerly winds. If conditions are suitable for their growth, relatively rapid growth is possible. Often in Nebraska, spring aphid populations are controlled by natural enemies such as lady beetles and parasitoid wasps.

Infections of cereal crops by the virus can occur in the fall and continue throughout the following growing season, Wegulo said.

"Infections that occur in the fall are more damaging than those that occur in spring," he said. "Once infection has occurred, nothing can be done to cure the plant."

One way to limit fall infections in winter wheat is to avoid planting too early.

To determine if aphid treatment is warranted, it is necessary to determine the identity and number of aphids present.

"When scouting for aphids in wheat, estimate the number of aphids per stem at several locations in a field," Wright said. "Don't just count aphids in the 'hot spots' because you need to get an estimate of the overall aphid population in the field."

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