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Mite transmits viruses damaging to wheat


Looking closely at unhealthy, discolored plants in Texas Panhandle wheat fields is part of Dr. Charles Rush’s job. He is a plant pathologist with Texas Agricultural Experiment Station. But the scientist knows the damage isn’t drought-induced at all.

The damage is caused by two different viruses, the wheat streak mosaic and the High Plains, he said. Both are transmitted by the wheat curl mite (Aceria tosichella), common to the central plains of the United States.

"In the past few weeks, farmers, Texas Cooperative Extension specialists, county agents, crop consultants and insurance agents have reported some wheat fields were all but dead," said Rush. Diagnosis can be tricky, he added, since one or more viruses might be present. Wheat streak mosaic can occur any place wheat grows.

Mark Harrison, independent crop consultant and agronomist working in Dallam and Hartley counties and Union County, N.M., agreed.

"In the last month or so, we have seen WSMV mostly with some HPV but not much. Usually the wheat streak will show up in the spring as farmers start fertilizing and irrigating, which increases the chances for infection," Harrison said

"One grower reported most of his irrigated crop circle was dying down rapidly, and that’s a red flag the mite has been spreading the viruses in early planted irrigated fields."

In addition, several client farmers were just plowing up damaged fields of 200 to 700 acres. "With fuel costs what they are right now, trying to salvage severely damaged crops would not be cost effective," Harrison said.

This year, Rush tested samples from enough locations to verify his suspicions.

"Wheat streak is wide spread now, but the really bad fields, the ones that are dying, have usually been infected with both viruses that multiply only on living plants," he said.

Wheat is an excellent host for wheat streak mosaic virus; other grass species can host both wheat streak and High Plains, Rush said. Often the worst-looking fields are neighbors of conservation reserve program land. Neither virus can travel without the help of a carrier. Research has shown their only transmitter is the wheat curl mite, the pathologist said.

The tiny eight-legged creature is white and cigar-shaped. It belongs to a group of microscopic plant-feeding pests, of the order, Acarina. The mite crawls slowly and depends almost entirely on wind for movement.

Mature wheat is no longer suitable as a food source, so the mite stands on its tail end to catch an air current and ride to a new, fledgling host.

Dr. Jerry Michels, Experiment Station entomologist at Bushland, said mites are common in most years, but this year is extremely bad for the two viruses.

"Unfortunately, chemical control of the mite will not prevent onset of either virus, so treatment is not warranted," Michels said. The only way to lessen the mite’s impact is to break the link between late-summer grasses and winter wheat.

"Since many producers plant as early as possible to gain cattle forage, this will be a continuing problem," Michels said.

In late summer and early fall, mites easily move from maturing grasses to newly-sown wheat. The best course for the future will be varieties resistant to both the mite and the viruses, he said. Plants infected by either virus will show similar symptoms with subtle differences, Rush said. High Plains virus causes a yellowing in appearance. By contrast, wheat streak mosaic virus stunts growth and causes mottled yellow streaks with on leaves.

Yield losses can be severe, if immature sprigs harbor wheat streak infection in the fall. Proof won’t show up until harvest time, but by then, farmers might not have enough crop to justify cutting. Stress from drought, high temperatures, in addition to attacks by other insects or diseases, can accentuate the damage inflicted by these two viruses.

Scientists in the Texas and Oklahoma panhandles have confirmed both viruses with some Barley Yellow Dwarf disease across the region.

"But that’s what we usually see, just not the way we are seeing it this time. Once in a while, a sample will have stripe rust, too," Rush said. Farther south or east of the area, more foliar diseases are common.

Many initial samples had double infections and started dying early. Because the two viruses are often seen together, Rush was not alarmed by this fact. But to the untrained eye, the damage mimicked drought. Running enzyme linked immunosorbent assays, Rush saw continually high levels of wheat streak mosaic virus. In more recent samples, High Plains virus has gone down slightly, he said.

With drought and high temperatures thrown into the mix, injured plants cannot handle more stress and die. But is the recent decline in High Plains virus a trend? Rush cannot say, but as fields are dying, surviving plants show only wheat streak mosaic virus.

Near Guymon, High Plains virus is still prevalent and fields also test positive for both viruses. The crop is a week or two behind plants maturing in the Amarillo area.

Rush’s diagnostic lab works closely with Extension personnel who guide producers in managing production year round.

Much of the work by county agents and area specialists brings in the information Rush needs from fields and farmers.

His four-year survey is determining the incidence and severity of these viruses. Over the last three years, rates of High Plains virus have been low. But now, it’s all over the map. Why is this year different? The mystery so intrigues Rush that he will be addressing a series of new questions.

For example, what if the problem cannot be tied completely to the mite, but rests with an abundance of over-summering grasses that might be harboring the viruses?

"Again, we just don’t know until more work is completed," said Rush.

Rush and Michels are delving deeper into the environmental conditions that favor epidemics of mite-vectored infections. Could CRP hold the key to the entire puzzle? For now, they can only speculate.

"What we know is that hundreds of thousands of dollars have been lost this year. The cause in many cases will go unrecognized because what’s left so closely resembles drought," Rush said.

The damage has been done to many fields even with a rebound after spring rains and dry winter. Early in the season, even the wheat board warned harvest might fall to just over 50 percent of the normal crop. But the rains were timely, and farmers hopeful. Then, suddenly, before its time, wheat in some fields turned yellow.

Now instead of 60 bushel yields, they will get zero -- neither forage nor grain because the wheat just up and died. Little, if any remaining nutrient value would help cattle.

Growing a dual-purpose crop brings risk each year to High Plains farmers, but despite such pitfalls, raising forage for grazing and targeting a grain crop makes good sense.

"Years ago, an area farmer offered good advice after I pointed out the advantages of planting in late fall to reduce certain disease problems," Rush said, "The farmer politely said he could not do that and stay in business. He quoted the farmer as saying: "Your job is to allow me to plant the wheat in early September in order to get some forage as well as grain."

Experiment Station scientists are working on new varieties of wheat that may allow early planting for optimal forage. Specific lines with virus and aphid resistance designed for early planting and grazing also need to be researched, Rush said.

"I believe such varieties would be extremely useful and widely adapted in the Panhandle," he said. But new and focused research projects will need to be initiated. Rush also envisions efforts to help identify germplasm with resistance to High Plains virus.

"The good thing to remember is the virus outbreaks may not be the same next year," he said.

Writer: Pam Dillard (806) 677-5600,
Contact: Charles Rush (806) 354-5804,

Pam Dillard | EurekAlert!
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