Alfalfa stem nematodes may be causing reduced growth and alfalfa crop losses in many parts of California, according to University of California Cooperative Extension (UCCE) experts.

“This infestation will be devastating economically, since it will affect the first cutting, and perhaps the second in our region,” says Yolo County pest management farm advisor Rachael Long. “This new infestation, on top of the severe water limitations, is bad news for alfalfa growers.”

Stem nematode symptoms in a two-year-old Yolo County alfalfa field.

There have been alfalfa stem nematode outbreaks in the past, “but never as severe as this,” adds Jerry Schmierer, UCCE farm advisor in Colusa, Sutter, Yuba and Glenn counties. Mick Canevari, San Joaquin County farm advisor for more than 30 years, has never seen such extensive alfalfa injury and believes that stem nematode infestation is a contributing factor

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, UC-Davis nematologist Becky Westerdahl and forage specialist Dan Putnam are working closely with farmers, seed companies and pest control advisors to identify the problem.

Researchers suspect that warm January weather and abundant February rainfall 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.

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,” says Putnam.

Farmers could mistakenly think alfalfa plants are dying from water stress and irrigate unnecessarily, causing additional problems, Long says. Check leaves and stems of affected plants for the nematode, she advises. Infested plants typically have swollen nodes and shortened internodes. If in doubt, collect leaves and stems from several plants showing symptoms of nematode infestation, seal samples in plastic bags and deliver them to the county UCCE office.

If nematode infestation is confirmed, Westerdahl suggests that 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 second cutting. 

Further information on stem nematodes in alfalfa is available at alfalfa.ucdavis.edu, in a chapter titled “Parasitic Nematodes In Alfalfa,” written by Westerdahl, and in “Background Information On Alfalfa Stem Nematodes,” 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