By Andy Porter
In a field on South Fork Russell Creek Road, about 25 researchers,
wheat growers, extension agents and others studied patches of yellowed
plants amongst the green sprouts. Although conversation was friendly, the
subject was serious.
The focus of the field trip was soil-borne wheat mosaic virus.
Although the yellow patches had been seen in isolated fields for several
years, the virus was only recently identified as the cause. This year
outbreaks appear to have spread.

The discovery was announced Monday by Matt Weber and Marshal McKinley,
agronomists with the McGregor Company.

Among the group was Tim Murray, chair of the Washington State
University Plant Pathology Department, who had traveled from Pullman to get
a first-hand look at the problem.

Along with Richard Smiley, plant pathologist with the Oregon State
University Pendleton Research Station, they headed up a roundtable
discussion of what is happening and what could be done to control the
problem.

?This is an unusual disease, transmitted by a soil-borne fungus,?
Murray said. While not new to the world of plant pathogens, the virus has
been rarely identified in the Pacific Northwest. Previously it was confirmed
only in one irrigated field in Umatilla County in 2005 and in the Willamette
Valley in 1994.

Murray said the main control used elsewhere is disease-resistant
varieties of wheat. A priority will be identifying which strains are most
resistant and finding a place to test those varieties.

?It?s not difficult (to implement), but it will take some time to get
up to speed,? he said.

Apart from disease-resisting strains, there are few other ways to
control the disease, Smiley said.

Sanitizing equipment by cleaning off soil is one method, ?but there?s
not much else,? he said.

Murray and Smiley said the virus is easy to confuse with another
disease, wheat streak mosaic virus.

?There?s probably about 30 viruses that affect wheat, but it takes a
biochemical analysis to differentiate them,? Smiley said.

The cool, relatively damp conditions this spring appear to have
triggered more outbreaks. Warmer weather, generally above 70 degrees, slows
and eventually stops the disease development, confining symptoms to lower
leaves.

?What?s seen here is going to fade away as the weather warms up.
Depending on what happens, there may not be an appreciable loss,? Smiley
said about the Russell Creek Road field, with the major losses in the
infected areas.

Murray and Smiley also said it is possible the infection has been in
Walla Walla County for some time, but that the right conditions hadn?t
occurred until recently to bring the virus out.

?It?s going to vary from year to year,? Murray said. ?We?re in a year
now where we?re seeing a good expression of the disease.?

Wheat mosaic facts


a.. Soil-borne wheat mosiac virus first attracted attention in 1919 in
the central United States. The virus was identified as the cause in 1923,
making it one of the earliest known wheat viruses and the first to be
characterized as soil-borne.


a.. The disease now occurs throughout the United States, China, Japan,
France, Egypt, Italy, Argentina, Brazil and Zambia.


a.. Normally only autumn-sown (winter) wheat develops symptoms,
although spring wheat is also susceptible.


a.. Symptoms range from mild green to prominent yellow leaf mosaics
(mottling). Stunting can be moderate to severe. Rising spring temperatures
slow and eventually stop disease development.


a.. The disease is spread by cultivation (such as when machinery
carrying infected soil is moved to another location), wind, water and other
factors. For reasons not understood, the virus sometimes spreads more
rapidly and over longer distances than can be explained by soil or water
movement.



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