Severe stem nematode infestations in northern San Joaquin Valley and Sacramento Valley alfalfa fields have caused yield losses as high as 100 percent for first cuttings this spring.

“The worst stem nematode damage I’ve seen is in the Orland area of Glenn County,” said Jerry Schmierer, University of California Cooperative Extension (UCCE) agronomy farm advisor in Colusa, Sutter, Yuba, and Glenn counties.

Schmierer has visited numerous fields at the request of puzzled alfalfa growers to determine what is causing bare strips in otherwise lush fields.

“The problem is you can’t see the problem (nematodes); you can only see the effects.” Schmierer said. “The fact is stem nematodes have been in the fields for awhile.”

In a normal field of 6 to 8-inch tall alfalfa, nematode-damaged plants are only about an inch tall, and not growing. The damage has been seen as far south as the Los Banos area.

Stem nematodes enter bud tissue and migrate into developing buds. Infected stems become enlarged and discolored, nodes swell, and internodes become shorter than those on healthy plants, according to the University of California Integrated Pest Management Web site.

Alfalfa plants infected with the alfalfa stem nematode have stunted growth, fewer shoots, and deformed buds. As nematode populations increase, lower stems on infected plants may turn black. Long periods of parasitism during moderate temperatures and high humidity may cause stem blackening for 1 foot or more above the ground.

Another typical sign of a stem nematode infection is the presence of ‘white flags,’ which are branches devoid of chlorophyll. White flags are caused when nematodes start moving to leaf tissue and destroy chloroplasts, leaving pale leaf tissue. As plants die in the field, weeds often invade the open areas.

Sending samples to a UC lab is the only method to positively identify the culprit, Schmierer says.

“When growers contact me I provide the good and bad news,” Schmierer said. “The good news is the alfalfa appears dead, but it’s not. The bad news is the grower can lose up to 100 percent of the first cutting and up to 50 percent of the second cutting. The third cutting should be OK.”

There are no registered nematicides for stem nematodes. The best natural weapon to halt nematode progression is temperature 70-degrees and above, Schmierer says. At that point the alfalfa is growing at a faster pace faster than the nematodes.

“Stem nematodes are not uncommon in California alfalfa fields, but this is the worst outbreak that we’ve ever seen,” says Dan Putnam, University of California Extension forage specialist.

UC researchers are puzzled about why stem nematodes are so severe this spring. They suspect warm January weather combined with abundant February rainfall provided perfect conditions for increased stem nematode development.

“There may be other factors that have contributed to this damage, but we haven’t identified them yet,” Putnam said. “It is possible that shifts in insecticide usage or a new biotype may have led to an increased population, but that’s pretty speculative. We’ve ruled out herbicide interactions. However, it is possible that weather patterns were simply ideal for this pest, leading to a severe outbreak.”

The UC team is working to better understand the stem nematode outbreak so growers can better manage future outbreaks. The group is utilizing several pesticides in field trials in Glenn and Yolo counties.

“Frankly it’s a fishing trip,” Schmierer said. “There is a possibility that some pesticides might work. However, we could be completely off on our timing and pesticide efficacy. There’s so much about this pest that we don’t know so it’s a challenge to address it.”

“This new infestation, on top of the severe water limitations, is bad news for alfalfa growers,” said Rachael Long, Yolo County pest management farm advisor.

Mick Canevari, a UCCE agronomy farm advisor in San Joaquin County for more than 30 years, has never seen such extensive alfalfa injury. He believes the stem nematode infestation is definitely a contributing factor.

“Many fields are just horrible,” Canevari said.

While some plants can survive stem nematode damage and recover to produce adequate yields, a stand of alfalfa can decline quickly once stem nematodes become well established, lowering productivity.

Long says farmers can mistakenly believe the stem nematode plants are dying from water stress and irrigate unnecessarily, causing additional problems.

The stem nematode outbreak was first detected in Glenn, Yolo, and San Joaquin counties.

Growers are cautioned to choose highly resistant varieties; make sure they are rotating out of alfalfa for at least two years between alfalfa crops, and keep equipment clean to prevent the spread of the pest.

“Some growers don’t pay attention to pest resistance ratings until something like this happens,” Putnam said. “They should remember this in the fall when they choose alfalfa varieties for planting; make sure they are highly resistant for stem nematode.”

More information on alfalfa stem nematodes, including photos of the pest, is available at http://alfalfa.ucdavis.edu. A chapter on nematodes is available in the book “Irrigated Alfalfa Management” published in December 2008.

emial: cblake@farmpress.com