California alfalfa growers af-fected by this spring's stem nematode outbreak should be careful to avoid spreading the microscopic worms to non-infested fields and choose highly resistant varieties the next time damaged fields are planted to alfalfa.

That advice is from Dan Putnam, University of California extension forage agronomist, who says future infestations can't be predicted largely because the cause of this year's problem is unknown. The worst stem nematode outbreak in the state's history affected more than 50,000 acres in the Sacramento and northern San Joaquin valleys. Serious first-cutting yield losses were expected, with some recovery anticipated by the second cutting.

“Then we hope there won't be any residual effects by the June harvest,” says Putnam.

He and his colleagues plan to monitor fields season-long to identify any lingering impact. Some plants in badly infested fields died. But, in most cases, remaining plants should produce enough additional growth to compensate. Stand life in those fields probably will be shortened because weakened plants are more susceptible to disease, says Putnam.

Stem nematodes have damaged California alfalfa before, primarily in small pockets in the Sacramento Valley and Intermountain area.

“The concern is, of course, that since we've never seen it as severe as it is this year, it might be a problem that's gaining steam over time,” says Putnam.

Another possibility is that this winter's weather may have created a “perfect storm” for nematode development. The worms thrive in cool, wet conditions, feeding on alfalfa crowns and buds and reproducing rapidly. Most become inactive when temperatures rise above 75°, but become active again when conditions are favorable.

Warm weather in January was followed by a wet period in February. Then temperatures stayed between 40° and 75°, the optimum range for stem nematodes, through much of March, says Putnam.

Ideal weather conditions may have interacted with one or more other factors to exacerbate the problem. But one suspected secondary contributor — plant injury from winter-applied herbicides — was discounted early. University of California researchers had a herbicide trial in an infected field, so they compared nematode damage in sprayed and unsprayed areas.

“We didn't see anything definite to cause us to believe that was a factor,” Putnam reports.

He advises growers with infested fields to harvest them last throughout the growing season and to clean equipment before going back to fields that don't appear infested.

“I can't stand here and tell you that it'll be 100% effective at preventing movement of this pest, but it's probably a very good idea,” he says.

Nematodes also can spread from field to field in runoff water, “but that's more difficult to control.”

His recommendation to plant highly resistant varieties comes despite the fact that some varieties with that rating were seriously damaged in the outbreak. He has two possible explanations:

“One is that, at certain levels of infestation it could be that no resistance level is helpful. The other question is whether or not some of these varieties are as highly resistant as the companies think.”

Varieties are rated highly resistant if at least 51% of the plants resist the pest, but that may not be enough resistance for some fields. Putnam urges seed companies to identify existing varieties with resistance levels significantly higher than 51% in fall dormancy groups 4 through 8.

More research on nematode prevention and control is needed, too. Much is not well-known about the biology of this pest and factors affecting it.

“The alfalfa industry as a whole needs to think about how to address problems like this in the long run,” he says. “That's a key issue.”