Amarillo, Texas
May 21, 2004
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. |