AgriLife Research maps wheat curl mite resistance genes

The Texas A&M AgriLife Research wheat genetic program at Amarillo has mapped the resistance genes to the wheat curl mite in TAM 112, according to Shuyu Liu, Ph.D., AgriLife Research small grains geneticist in Amarillo, US.

Wheat curl mite resistance is important because it is the vector for several viruses, including wheat streak mosaic virus, Liu said.

The study was completed by AgriLife Research scientists in Amarillo, including Liu; Smit Dhakal,a master’s degree student at West Texas A&M University; Dr. Jackie Rudd, wheat breeder; Dr. Qingwu Xue, crop stress physiologist; and Dr. Charlie Rush, plant pathologist.

Hard red winter wheat cultivars released by AgriLife Research are widely used in the Southern High Plains of the U.S., Liu said. TAM 111 and TAM 112, both wheat varieties developed in the AgriLife Research wheat breeding program in Amarillo, are very popular now.

TAM 112 has better bread-making quality, drought tolerance and resistance to greenbug. TAM 112 was released in 2005 and has risen in planted acres since then, he said. Based on breeders’ observations and virus testing by the pathologist, it is known that both TAM 111 and TAM 112 have moderate tolerance to wheat streak mosaic virus. Only recently, Rush and his research associate Jacob Price tested TAM 112 for resistance to the wheat curl mite in a series of greenhouse studies.

TAM 112 has a resistance that reduces the mite populations after infestation, which means less symptoms caused by wheat streak mosaic virus and other mite-transmitted virus, Liu said.

Dhakal is working with the AgriLife Research wheat genetics program at Amarillo for his thesis research under the supervision of Liu. Liu said they have developed a new protocol and screened a mapping population, segregating for wheat curl mite resistance, using a wheat curl mite collection maintained by Rush’s plant pathology program.

“We found that there are at least two resistance genes to wheat curl mite in TAM 112,” Dhakal said. “One is on a wheat-rye translocation that has been used by the Texas A&M wheat breeding program for many years. An additional, newly discovered gene was found and is being studied further. Either one of these two genes slows the development of wheat curl mite population after infestation and reduces the disease symptoms.”

He said the new gene may condition resistance to a wider range of wheat curl mite populations. The Texas A&M wheat breeding program has used TAM 112 as a parent to develop new cultivars for many years, and most of the breeding lines from these crosses, some with only the single gene, are resistant to wheat curl mite.

“Wheat curl mite resistance in a popular cultivar like TAM 112 is very good for farmers and breeders because it can be easily transferred into other germplasm lines or cultivars,” Liu said.

A survey of wheat curl mite resistance in advanced lines from many wheat breeding programs, including the Texas Elite yield trial; the Southern Regional Performance Nurserycontaininglines from Texas, Oklahoma, Kansas, Colorado and Nebraska; and the Northern Regional Performance Nursery, containing lines from Nebraska, South Dakota, Montana and Canada, showed that several had resistance to wheat curl mite.

Many of the resistant lines were derived from TAM 112, having either one or both of the two genes in TAM 112, Liu said.

Field trials from the pathology program at the Texas A&M AgriLife Research and Extension Center in Amarillo showed TAM 112 had significantly less severe disease symptoms than Karl 92, which is susceptible to wheat curl mite and wheat streak mosaic virus.

Liu pointed out another advantage is that these mite resistance genes do not seem to be temperature dependent, while the known resistance genes to wheat streak mosaic virus in released cultivars are only effective at relatively low temperatures, under 64.5 degrees.

The temperature sensitivity in the current wheat streak mosaic resistances makes them of less value to farmers in the Southern High Plains who often plant winter wheat early, while temperatures are still high, in order to maximize forage production for winter grazing.

“When farmers plant early to get enough forage production, this promotes epidemics of wheat streak mosaic virus,” he said. “The wheat curl mite resistance in TAM 112 should reduce incidence and severity of several mite-vectored viral diseases, and it is not restricted to conditions under 64.5 degrees.

“Therefore, wheat curl mite resistance in TAM 112 can play an effective role in reducing losses to mite-transmitted viral diseases that are extremely common and widespread throughout the western Great Plains, from the Texas Panhandle all the way to the Canadian border.”