Wheat streak identification can be an issue during drought

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Kay LedbetterCharlie Rush, Ph.D., Texas A&M AgriLife Research plant pathologist in Amarillo, stands in front of his wheat streak mosaic study. Wheat streak mosaic, a common disease that occurs in the Texas Panhandle and Great Plains every year, is not easily distinguishable from drought stress, and that could result in wasted inputs by producers. 

Charlie Rush, Ph.D., Texas A&M AgriLife Research plant pathologist in Amarillo, and his senior research associate, Jacob Price, recently conducted a field day to educate farmers about the impact of wheat streak and other mite-vectored viruses on grain yield. 

“We are trying to explain that when plants are infected, they cannot efficiently use the water applied in irrigated situations and they are no longer able to take up nutrients,” Rush said. “We are trying to develop an economic threshold for wheat streak so farmers will know when it is worthwhile for them to try to manage it and get something out of the crop or to make the determination that it is diseased to a point where it is not worth putting on additional inputs.”

Rush said producers sometimes have difficulty differentiating between drought stress and wheat streak mosaic. Because his current AgriLife Research wheat study near Bushland is exhibiting wheat streak symptoms under different irrigation regimes, he utilized them as a teaching grounds for producers interested in learning to identify the virus and its symptoms.

“The wheat streak mosaic virus is transmitted from plant to plant by the microscopic wheat curl mite,” he said. “It doesn’t have wings, but moves across the field by being blown by the wind. This generally makes a gradient of disease severity across the field.”

A trap strip along the border of the study field became extremely infected in the fall and was beyond saving by early May, Rush said, which is similar to what can happen when producers don’t control volunteer wheat.
The study plots of wheat Rush is using were planted early in September and include strips of Karl 92, which is susceptible to wheat streak, and TAM 112, which is less susceptible to wheat streak and has a high level ofresistance to the wheat curl mite. The two cultivars are planted under different irrigation treatments.
“The fields are infected not just by wheat streak mosaic virus, but also triticum mosaic, which is also vectored by the wheat curl mite,” he said. “They look very similar and to determine which one it is, you need to send it to the diagnostic lab. But it doesn’t really matter as far as the producer is concerned – there is nothing you can do about it at that time.”

Kay LedbetterA trap strip along the border of the study field became extremely infected in the fall and was beyond saving. Disease progression typically starts on the edge of the field and over time spreads in a gradient pattern into the field, Rush said.

“When you look at a wheat plant and are trying to decide what you have, if you see yellowing of the leaves and it is uniform and solid yellow, it is probably not a virus disease,” he said. “But if you see streaks of green and yellow and little flecks, that’s a mosaic pattern and you almost certainly have one of the diseases.”

Barley yellow dwarf is another virus that affects wheat, but it can occur anywhere in the field and doesn’t normally start on the edge because it is vectored by aphids, Rush said. It is characterized by irregular circular areas that are yellow and dwarfed, and the leaves can start turning reddish/purplish, which can be also confused with nutrient deficiency or freeze damage. 

Price said one of the main things they try to stress is getting rid of the volunteer wheat, which serves as a reservoir for both the viruses that cause these diseases and the vectors that spread them.

“Plant as late as possible, and make sure all volunteer wheat has been killed at least two weeks before planting,” he said. “As long as there is green tissue, the wheat curl mites can live on it.”

Kay LedbetterStreaks of green and yellow and little flecks on wheat in a mosaic pattern are the symptoms of wheat streak mosaic, a common disease in the High Plains. Price said one of the worst things he hears when talking to producers is that they would think their wheat was suffering a nutrient or water deficiency, so they started pouring on the water, only to have it die.

That prompted their initial study, he said, because, “We really didn’t know if these pathogens had an effect on water use, water uptake, and therefore subsequent grain yield. What we found is that when the plants are infected, there is a reduction in the root systems, up to 50 percent, and that impacts water use efficiency. So all the water these producers were putting on their fields was essentially wasted.”
Price said at that point, the next step of the study was to look at the gradient of disease severity – as the plants become more and more diseased, there is a greater reduction in water use efficiency and root development.
“We began using a spectral reader that picks up color reflectance from the wheat. By looking at the yellow spectrum, we can get an estimate of the disease severity along the gradient. Using this information, we can equate that to expected yield loss. We are hoping to relate that and be able to find out where in this disease gradient it is optimal to continue to water and where it is not.
“We hope that eventually we will be able to go into a field and identify whether you have one of these diseases and assess the severity at a certain date and then relate that severity to what your yield losses will be,” Price said.
Additionally, he said, they are developing an early detection system through a Listserv and website, http://bit.ly/1sVKFfA. Producers can subscribe and will be notified of the name of the county, types of pathogen found and the discovery date. Samples also can be submitted to the Plant Diagnostics Lab at the Texas A&M AgriLife Research Center in Amarillo for testing.
Rush said if a field starts turning yellow early in the fall or spring, “you might as well turn in the cows, but if it doesn’t start turning yellow until May, then it might be worth continuing with it.”
“We hope our studies will allow us to determine how sick the plant is at a particular time and correlate that to yield loss at the end of the season,” he said. “We believe we can start making those correlations by mid-April, which would be plenty of time for the producers to know whether to apply further inputs to that crop or not.”

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