Davis, California
March 18, 2009
Alfalfa fields should be a lush
green carpet this time of year, but in many parts of California,
fields are scarred with patches of stunted, dead-looking,
twisted and deformed plants.
University of
California scientists have determined a voracious
microscopic pest known as the alfalfa stem nematode may be
causing the reduced growth and crop losses.
"This infestation will be devastating economically, since it
will affect the first cutting, and perhaps the second in our
region," said Yolo County pest management farm advisor Rachael
Long. "This new infestation, on top of the severe water
limitations, is bad news for alfalfa growers."
Jerry Schmierer, the agronomy farm advisor for UC Cooperative
Extension in Colusa, Sutter, Yuba and Glenn counties, said there
have been alfalfa stem nematode outbreaks in the past, "but
never as severe as this."
Mick Canevari, who has served as the agronomy farm advisor for
San Joaquin County UCCE for more than 30 years, said he has
never seen such extensive alfalfa injury and believes that stem
nematode infestation is a contributing factor. "Many fields are
just horrible," he said.
While some plants may survive stem nematode damage and recover
to produce adequate yields, a stand of alfalfa can decline
rapidly after stem nematodes become established, considerably
lowering productivity.
Schmierer, Canevari, Long and UC Davis specialists Becky
Westerdahl and Dan Putnam are working closely with farmers, seed
companies and pest control advisors to identify the problem.
No one knows for certain what triggered the alfalfa stem
nematode infestation to be so severe this year, but researchers
suspect that warm weather in January combined with abundant
rainfall in February provided perfect conditions for the stem
nematode to develop and infect alfalfa stands. Changes in
cultural practices or pesticide use patterns may also be
contributing factors. The UC team is studying the exact causes
for this year's outbreak to better understand how it may be
prevented in the future.
UC quickly implemented field trials in Glenn and Yolo counties
when the stem nematode outbreak became apparent in early March.
Typically farmers fight the pest by planting resistant
varieties, rotating alfalfa plantings with crops not susceptible
to the pest, and reducing the spread by cleaning equipment and
preventing water transfer from infected fields to clean fields.
Once outbreaks occur, options are limited, but researchers are
trying several potential pesticide treatments in their field
trials.
"We need to find out whether there are things that growers can
do when they are faced with this problem," said alfalfa
specialist Putnam.
Long said farmers could mistakenly think alfalfa plants are
dying from water stress and irrigate unnecessarily, causing
additional problems. To be certain what factors are behind plant
dieback in alfalfa fields, leaves and stems of affected plants
should be checked for the nematode. Infested plants typically
have swollen nodes and shortened internodes. If in doubt,
farmers may collect leaves and stems from several plants
exhibiting symptoms of nematode infestation and deliver the
sample sealed in a plastic bag to the county UC Cooperative
Extension office.
If nematode infestation is confirmed, nematologist Westerdahl
suggests farmers wait for warmer weather, which will favor plant
growth and cause the nematodes to die. Although scientists have
found stem nematode in currently affected fields, the widespread
damage is not typical for this nematode. If stem nematode is the
primary problem, the dead-looking crowns will regenerate and
grow new buds and stems with increased yields by the second
cutting.
Further information on stem nematodes in alfalfa is available at
http://alfalfa.ucdavis.edu/IrrigatedAlfalfa/pdfs/UCAlfalfa8297Nematodes_free.pdf
in a chapter titled "Parasitic Nematodes in Alfalfa," written by
Westerdahl, and in "Background information on alfalfa stem
nematodes,"
http://ucanr.org/alfalfa, by Long.
For more information about the alfalfa stem nematode, contact:
• Rachael Long, UCCE Yolo
County, (530) 666-8143,
rflong@ucdavis.edu
• Jerry Schmierer, UCCE Colusa County, (530) 458-0575,
jlschmierer@ucdavis.edu
• Mick Canevari, UCCE San Joaquin County, (209) 953-6100,
wmcanevari@ucdavis.edu
• Becky Westerdahl, UC Davis, (530) 752-1405,
bbwesterdahl@ucdavis.edu
• Dan Putnam, UC Davis, (530) 752-8982,
dhputnam@ucdavis.edu
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