Wheat Journal

March 14, 2009 12:24 PM

Wheat Gene With Resistance to Stripe Rust

An international team of researchers has discovered a gene that will make bread wheat capable of resisting stripe rust, a fungus that causes crop losses in many states.

Scientists transferred a resistant gene, known as Yr36, from a race of wild wheat into a handful of domesticated pasta and bread wheat varieties. The wild wheat was collected in Israel at the Fertile Crescent where ancient varieties of wheat have grown for centuries, says Ann Blechl, a geneticist at the USDA–Agricultural Research Service (ARS) Western Regional Research Center in Albany, Calif. Other scientists involved include Xianming Chen, a USDA–ARS plant pathologist in Pullman, Wash., and researchers from the University of California–Davis and the University of Haifa in Israel.

The researchers used a detailed map of a region of one wheat chromosome to isolate a candidate gene sequence into a susceptible bread wheat variety. Subsequent tests showed the transformed plants were resistant to at least eight races of stripe rust. Now breeders can use sequence-based DNA markers to incorporate the resistance into new and existing wheat varieties.

Wheat producers have been battling stripe rust in the Pacific Northwest since the 1950s, Chen says. Severe outbreaks occurred in the South and Midwest in 2000, and three years later the disease wiped out 25% of the wheat crop in California.

Caused by the fungus Puccinia striiformis, stripe rust is spread by the wind and is most likely to ruin crops in mild winters, wet springs and wet summers.

The fungus evolves rapidly, developing new races that overcome various race-specific seedling resistances. While Yr36 provides only partial resistance to adult plants at high temperatures, it is useful because it protects against all known strains of stripe rust.

Watch Wheat This Spring

Wheat insects are common, but they don't routinely cause economic disasters. That's both a blessing and a curse, says Phillip Sloderbeck, Kansas State University entomologist.

"Because outbreaks are random, growers don't always scout fields as they should,” Sloderbeck says. "When we do have a wheat insect outbreak it often seems more pronounced than in other crops, but it's usually because the population wasn't detected in time to get good control.”

Sloderbeck is noticing a disturbing production tactic that may actually lead to increased insect pressures: Some growers are adding a low rate of insecticide when topdressing nitrogen in the spring.

"They do this without knowing what pests are out there. Plus there's a good chance they are killing off any beneficial insects and allowing pests in the field to build up to levels that require additional sprays,” he observes. This is also a good way to increase the chances of insects developing resistance.

"When market prices are high, the tendency is to add inputs to save every last kernel,” Sloderbeck says. "But it's a much better bet to scout and make spray applications based on established thresholds.”

Control Options for Wheat Insects

"New” isn't exactly the right term for the changes in the wheat insecticide arsenal for the 2009 growing season—it's more like "improved” options.

Lorsban Advanced is a water-based formulation of chlorphyrifos from Dow AgroSciences. The use rate remains the same as Lorsban 4E, but the amount of active ingredient per gallon is slightly lower. All of that leads to the product's reduced odor and slightly higher flash point, which allows for fewer restrictions on transportation and storage.

Warrior II with Zeon Technology is a more concentrated formulation of lambda-cyhalothrin, available from Syngenta. The 2.08-lb.-per-gallon use rate represents nearly twice the amount of product per gallon.

Syngenta crop-protection insecticide brand manager Jeff Cecil explains that this product is different because it is water-based and represents a reduction in solvent load compared with the previous product. Zeon Technology refers to the insecticide's patented capsule suspension formulation, which places a barrier around the active ingredient, decreasing handler and applicant exposure. At the same time, insecticidal and residual activity is enhanced through built-in UV protection and improved rainfastness.

Fungicide Use and Application

Research conducted by Kansas State University indicates that a single fungicide application made to susceptible wheat varieties when the risk of disease is high will often result in a 4% to 13% yield increase relative to wheat that remained untreated, with an average increase of approximately 10%. A lower yield response is likely if the disease remains at low levels or is absent. The following guidelines will help maximize the potential for effective disease management and a positive yield response.

¡Ensure proper application timing. The timing of fungicide applications between the full extension of the flag leaves and anthesis increases yield or grain quality and reduces disease severity. Fungicides that are used to manage glume blotch or head scab should be applied between the beginning of anthesis and 50% flowering.

¡Pay attention to disease scouting reports. The risk of severe disease and yield loss is greatest when foliar diseases show up early and result in consistent disease pressure throughout the growing season. It is important to be aware of outbreaks and disease levels found locally.

¡Know the vulnerabilities of your varieties. Knowing the susceptibility of wheat varieties to leaf rust, stripe rust, tan spot or powdery mildew can help improve disease prevention and yield loss. Fungicides are more beneficial to overall yields when they are applied to the varieties that are susceptible to those diseases. Varieties with high to moderate
resistance to those diseases will not gain as much from fungicides.

Additional results from the study can be found at www.oznet.ksu.edu/library/plant2/EP130.pdf.

Genotypic Research

USDA–Agricultural Research Service (ARS) has four regional small-grains genotyping centers to help develop new wheat varieties with improved disease resistance, stress tolerance and other traits. The USDA–ARS Western Regional Small Grains Genotyping Lab, located in Pullman, Wash., provides wheat and barley breeders with genetic profiles of germplasm materials. Its technology can identify resistant germplasm within a few days.

The USDA–ARS Cereal Crops Research Unit in Fargo, N.D., is creating a database to store genotypic information from all locations. The other two centers are the USDA–ARS Plant Science Research Unit in Raleigh, N.C., and the USDA–ARS Plant Science and Entomol-ogy Research Unit in Manhattan, Kan.

To keep up with USDA–ARS research and other happenings, visit www.ars.usda.gov/is/pr/


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