We usually want to root for the little guy.

But when the “little guy” is the wheat curl mite and he’s helping viruses like wheat streak mosaic get a toehold in your wheat field, that sense of fair play goes out the window.

Take one look at wheat curl mites and they don’t look that menacing. But, when these mites take on wheat plants, they can stress the plants—opening the doors to viruses like wheat streak mosaic that can devastate a wheat field. Wheat yield loss caused by wheat curl mites and the viral diseases they spread can take up to 100 percent of a field, and they’ve been tallied at 7 percent of Texas’ statewide wheat yield losses, explained Shuyu Liu, a Texas AgriLife Research small grains geneticist in Amarillo, Texas.

“We can estimate the viral disease incidence and severity visually, but the wheat curl mite is invisible by the naked eye,” Liu said. “Wheat curl mite carrying viruses can be spread by wind and temperature also affects the mite movement and the viral diseases infection process.”

Mites affect wheat plants by laying their eggs along the leaf’s veins. Most can be found on the terminal leaves and move to new leaves as they emerge. When populations get heavy, the leaves curl inward. They will survive nearly freezing temperatures and can be picked by the wind to survive on summer grasses and volunteer wheat.

Farmers have used chemical methods and cultivation to handle wheat curl mites. But since the mite can hide in curled leaves, chemical applications often are ineffective, Liu explained.

“Late planting and removing volunteer grass are somewhat effective, but not very effective in the High Plains because farmers want to plant wheat earlier for grazing,” he said.

Researchers have thought that the key to slowing down wheat streak mosaic and other mite-transmitted viruses would be to stop the wheat curl mites from infesting wheat in the first place. And now that the Texas A&M AgriLife Research wheat genetic program in Amarillo has mapped genes in the TAM 112 variety that stop the wheat curl mite, that might be more easily accomplished.

Recently, Liu led a team of scientists in mapping the wheat curl mite resistance genes in TAM 112. The team consisted of Smit Dhakal, a master’s degree student at West Texas A&M University; Jackie Rudd, wheat breeder; Qingwu Xue, crop stress physiologist; and Charlie Rush, plant pathologist.

They started with TAM 112, a hard red wheat cultivar that was released to the public in 2005 and that has grown in popularity up and down the Southern High Plains. Rudd observed through many field trials that it and TAM 111 both seemed to show moderate tolerance to wheat curl mites and thus wheat streak mosaic virus.

That got the team to wondering if there was a gene for resistance either to the virus or to the wheat curl mite that could be identified and used for further wheat breeding.

So, Rush, the pathologist in Amarillo, conducted greenhouse experiments and field experiments to estimate the severity of the wheat streak mosaic virus vectored by the wheat curl mite, Liu explained. In side-by-side comparisons TAM 112 showed that its resistance genes reduce mite populations following infestation. The fewer mites on the plate means the less time wheat streak mosaic and other viruses have to cause damaging symptoms.

“Based on the available information, the wheat genetic group then used a breeding population developed by the breeding group and identified the genes controlling resistance to the wheat curl mite,” he said. Liu said there’s now been a protocol developed to screen mapping populations for this wheat curl mite resistance.

There are actually two resistance genes to wheat curl mite in TAM 112. The first is found on a wheat-rye translocation that the Texas A&M wheat breeding program has used for many years, explained Dhakal. Liu explained that it is present in many released wheat cultivars and also provides resistance to rust and powdery mildew.

The second is newer and requires further study. Either one will slow the wheat curl mites and the progression of diseases in wheat.

“These two resistance genes have been used in many breeding programs including the Texas A&M AgriLife Research breeding program,” Liu said. “There are many advanced lines derived from TAM 112 in the breeding program. The next step is to identify genetic markers linked to the gene so wheat breeders can more precisely and quickly find which line has the resistance gene.”

Fortunately, so far researchers haven’t found any negative effects on grain and forage quality from this resistance gene.

“Due to the resistance to the wheat curl mite, mite-vectored viral disease symptoms are reduced,” Liu said. “Therefore, the grain and forage yield and quality should be increased.”

Perhaps most important to Southern High Plains farmers is that the wheat curl mite resistance gene in TAM 112 and other wheats that have the rye translocation are effective even at temperatures greater than 64.5 degrees Fahrenheit. That means that farmers who plant wheat for use as forage have a way to control mites as well.

Prior to this new finding, known resistance genes to wheat streak mosaic only turn on at lower than 64.5 degrees. But wheat farmers in the Southern High Plains have to plant their winter wheat early when temperatures are higher. Earlier planting maximizes forage production for winter grazing. Identifying this wheat curl mite resistance gene can help reduce the severity of mite-vectored viruses in wheat.

He said that more and more breeding programs—such as the Southern Regional Performance Nursery and the Northern Regional Performance Nursery—are using sources of the mite resistance gene from TAM 112 in their lines.

“In the Texas Elite wheat yield trial, about half of the tested lines have resistance to wheat curl mite,” he added. “Farmers will have more choices for mite resistant cultivars.”

It’s just one way they can strike one for the big guy.

Jennifer M. Latzke can be reached by phone at 620-227-1807 or by email at jlatzke@hpj.com.

Date: 5/26/2014

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