Yellow dwarf a common sight in many Indiana wheat fields

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 two 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 two 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.

To estimate yield potential for a field with yellow dwarf, start by counting the heads in several arbitrarily selected 19-inch row sections for wheat planted at the standard row spacing. The population of wheat heads for a high yield should range from 60-70 plants per square foot, and for an adequate yield 55 heads per square foot. These densities assume each head has 16-18 spikelets and produces 35 kernels. Other variables that must be taken into account when determining yield include reduced head size, spikelet number and kernel size.

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.

Infection can occur in both the fall and spring, but fall infection is more damaging. Stunting, reduced tillering and predisposition to winter killing are results of fall infection. Spring infection causes noticeable yellowing and purpling of the flag leaf, but not severe stunting.

Source: Purdue University

Related stories:

New virus threatens High Plains wheat crop
Triticum mosaic virus poses a new threat to Texas wheat, according to Texas AgriLife Research scientists in Amarillo.
Building disease-beating wheat
Pioneered by CSIRO researchers, in collaboration with the International Maize and Wheat Improvement Center (CIMMYT) and Sydney University, the research illustrates the major genetic improvements possible without genetic modification (GM) technology.
First all-African produced genetically engineered maize is resistant to maize streak virus
Maize streak viruses (MSV), geminiviruses that can destroy most of a maize crop, are endemic to sub-Saharan Africa and adjacent Indian Ocean islands where they are transmitted by leafhoppers in the genus Cicadulina. Maize can supply 50% of the caloric intake in sub-Saharan Africa but, in certain years, a farmer’s entire crop can be wiped out. Now, scientists at the University of Cape Town, South Africa, along with colleagues at the South African seed company, PANNAR Pty Ltd, have developed a resistant variety of maize that they hope will help alleviate food shortages as well as promote the reputation of genetically engineered (GE) foods in Africa.
Researchers demonstrate way to control tree height
Forest scientists at Oregon State University have used genetic modification to successfully manipulate the growth in height of trees, showing that it’s possible to create miniature trees that look similar to normal trees – but after several years of growth may range anywhere from 50 feet tall to a few inches.
Researchers developing better wheat
Eighteen universities across the United States are combining desirable genes from different varieties of wheat to make better and more competitive varieties.
Which grass is greener to power the bioenergy era?
(PhysOrg.com) -- Talk about a field of dreams. Cornell bioenergy plant experts are learning which field grasses are the best candidates for "dedicated energy" crops in the Northeast, considering the region's climate and soil conditions.
Scientists behind 'doomsday seed vault' ready the world's crops for climate change
As climate change is credited as one of the main drivers behind soaring food prices, the Global Crop Diversity Trust is undertaking a major effort to search crop collections—from Azerbaijan to Nigeria—for the traits that could arm agriculture against the impact of future changes. Traits, such as drought resistance in wheat, or salinity tolerance in potato, will become essential as crops around the world have to adapt to new climate conditions.
Researcher says: No-till practices show extended benefits on wheat and forage
With more than 3 million acres of wheat in north Texas, 50 percent or more of which is grazed by 1 to 2 million head of cattle, it is important to look at tillage practices and their effect on forage production, said a Texas AgriLife Research expert.

News discussion:

Space & Earth science news