Lincoln, Nebraska
May 25, 2007
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."
A variety of effective insecticides is available for aphid
control on wheat.
For more specific information about treatment thresholds and
insecticides visit the May 25 edition of Crop Watch, extension's
crop production newsletter.
For a list of rates and restrictions for suggested insecticides,
visit the UNL Department of Entomology Web site.
Additional information on aphids in small grains, including
photos of common species and a key for their identification, can
be found in UNL Extension NebGuide G1284, Cereal Aphids,
available from local UNL Extension offices or on the Web. |
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