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[1] Yellow dwarf virus: Indiana wheat fields
[2] Barley yellow dwarf virus: Nebraska wheat fields

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[1] Yellow dwarf virus: Indiana wheat fields
Date: 28 May 2007
Source: PhysOrg [edited]
<http://www.physorg.com/news99587069.html>


Warm temperatures late into last fall are partially to blame for damaged wheat showing symptoms of yellow dwarf, said a Purdue University expert. "We were having daily highs in the low 60s and nighttime temperatures were not freezing," said Greg Shaner, Purdue Extension crop disease specialist. "This gave aphids (the primary
vector) more time to reproduce and transmit the virus that causes yellow dwarf."

During the past 2 weeks, symptoms of yellow dwarf have become evident in many wheat fields. Fields are now showing irregular plant height and erect flag leaves with purple tips, both symptoms of yellow dwarf. Another symptom is a general pale green color, which could be mistaken for nitrogen deficiency.

Yellow dwarf not only cosmetically damages the plant, but also stunts its growth. Infected plants have a stunted root system as well as short stalks and smaller leaves. The wheat heads also are smaller, and there are less of them, resulting in yield loss.

On a healthy well-formed wheat head, there should be roughly 16-18 spikelets, and each spikelet should have 2 kernels. A good wheat crop would produce 35 kernels per head, Shaner said. "Right now, there is absolutely nothing farmers can do," he said. "The infection is there, the damage is done, and there is no real additional infection taking place."

However, if damage is severe enough, growers can still make the decision to tear up a field and replant, but the window of opportunity is closing. It is becoming late for planting corn, but soybeans are still an option. That decision for much of the state should have been made a week or 10 days ago after the heads emerged.

Yellow dwarf is caused by a virus and spread by aphids. When an infected aphid feeds on a wheat plant, it transmits the virus to the plant, Shaner said. The offspring of an infected aphid are not carriers of the virus but, typically, not for long. Shortly after the new aphids start to feed on an infected plant, they acquire the virus and spread the infection as they move to healthy plants, he said.

Yellow dwarf is a sporadic disease. For Indiana, it's a problem roughly one out of every 10 years, Shaner said. Growers could use an insecticide on the wheat seed, but that may not be economically justified as a routine practice, Shaner said. For aphid control, spraying a foliar insecticide in the fall would be more worthwhile, he said. It also is important to plant after the fly-free date, because temperatures are normally low enough to ensure the insects are less active.

[Byline: Source: Purdue University]

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Communicated by:
J. Allan Dodds
Former ProMED-mail plant disease moderator <dodds@ucr.edu>

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[2] Barley yellow dwarf virus: Nebraska wheat fields
Date: 30 May 2007
Source: Agriculture Online [edited]
www.agriculture.com/ag/story.jhtmlstoryid=/templatedata/ag/story/data/1180550447229.xml&catref=ag1001


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 carriers of the virus are 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.
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.

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.

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.

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 2 aphids generally have a lower damage potential than greenbugs, another aphid that may be seen in Nebraska wheat.

"However, all 3 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 [2007] largely contributed to this widespread epidemic of barley yellow dwarf virus we are seeing," Wright 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. To determine whether aphid treatment is warranted, it is necessary to determine the identity and number of aphids present. A variety of effective insecticides is available for aphid control on wheat.

[Byline: Sandi Alswager Karstens, University of Nebraska]

--
Communicated by:
J. Allan Dodds
Former ProMED-mail plant disease moderator <dodds@ucr.edu>

[_Barley yellow dwarf virus_ (BYDV) (_Luteovirus_, _Luteoviridae_) is distributed worldwide and the most destructive of the virus diseases that affect wheat. Grain yields may be reduced by 1/3rd. It attacks a wide range of grass species (_Gramineae_) including wheat, oats, rice, maize, rye and barley. Oats are usually considered more susceptible than wheat; tolerant host cultivars are available.
Symptoms include leaf discoloration from yellow to purple, stunting, reduced tillering and empty seed heads. Plants infected in autumn may not survive the winter or show severe symptoms when growth resumes.
Diseased plants often occur in circular patches within the field, which are associated with colonization by the aphid vectors. The virus can be transmitted by more than 20 species of aphids and is retained when the insect moults. It cannot be transmitted by seed, pollen or mechanical means.

BYDV exists as several different strains, which are differentiated by their ability to be transmitted by various aphid species and their virulence on a selected variety of oats. Some strains of BYDV are transmitted equally well by several aphids, whereas other strains can be transmitted by only one or 2 aphid species.

Map of US states:
<http://www.census.gov/geo/www/us_regdiv.pdf>

Pictures:
Healthy and diseased wheat plants:
<http://www.ent.iastate.edu/imagegal/plantpath/wheat/ydwarf/0093.04ydwarfmv.html>
BYDV affected wheat field:
<http://www.ent.iastate.edu/images/plantpath/wheat/ydwarf/0093.10ydwarfwheat.jpg>
Red leaf symptoms on oats:
<http://www.rothamsted.ac.uk/ppi/pics/virus_big.jpg>
_Luteovirus_ particles, electron micrograph:
<http://www.ncbi.nlm.nih.gov/ICTVdb/WIntkey/Images/c2.gif>

Links:
BYDV information and pictures of symptoms:
<http://www.agric.wa.gov.au/pls/portal30/docs/FOLDER/IKMP/PW/PH/DIS/CER/FN019_1993.HTM>
BYDV fact sheets:
<http://ohioline.osu.edu/ac-fact/0005.html>  and <http://ipm.ppws.vt.edu/stromberg/smallgrain/biology/wydwarf.html>
CIMMYT (International Maize and Wheat Improvement Center) BYDV newsletter:
<http://www.cimmyt.org/research/wheat/bydvnews/htm/BYDVNEWS.htm>
BYDV taxonomy:
<http://image.fs.uidaho.edu/vide/descr062.htm>.
- Mod.DHA]

[see also in the archive:
2003
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Cereal viruses, oat, barley - USA (Alaska) 20030405.0832
2001
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Cereal viruses, wheat - Uzbekistan 20011103.2723 Cereal viruses, barley, wheat - Tunisia 20010829.2049
1999
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Crop diseases - Canada (Manitoba) (02) 19990825.1484 Crop diseases - Canada (Manitoba) 19990816.1417 Barley Yellow Dwarf virus alert - New Zealand 19990708.1140]

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