Alfalfa stem nematodes are hitting California alfalfa fields again this year – even harder than they did last year, says Dan Putnam, University of California Extension forage agronomist. Areas reporting damage include ­­­­­­­­­­­the Sacramento Valley and the mid-to-northern San Joaquin Valley, which is the heart of the state’s alfalfa production area.

“We had some fields last year that had some occasional infestations, but this year there are fields that were absolutely devastated,” he says. Some fields that look badly damaged will lose their entire first cuttings but may be back in production by second cutting. In other fields the plants have been completely killed.

Growers, in the meantime, are waiting to see how much of the crop will recover before deciding whether the damage is bad enough to consider interseeding or plowing under, Putnam says.

“It's worse this year,” says Don Miller, Producer's Choice Seed forage breeder, of the microscopic pest. Miller traveled through California to inspect alfalfa fields last week. “Some of those fields are 50% dead,” he predicts.

“Farmers I talked to yesterday said they usually keep their fields four to five years, but these (infested) fields are going out in two,” Miller says.

Alfalfa experts speculate that the loss of the pesticide Furadan may be one of the reasons stem nematodes are becoming an increasing problem. “The occasional use of Furadan may have suppressed them in the past,” Putnam says.

In California field trials, other insecticides have shown some effect on nematode populations, but don’t seem to impact crop health, so are not recommended at this time, he adds. Yet researchers are exploring ideas for spot chemical treatments using GPS to prevent small, infested areas from becoming larger.

Growers mainly have to rely on alfalfa varieties highly resistant to stem nematodes as a control measure. Putnam hopes to persuade seed companies to hike stem nematode resistance from the current 50% and above level to 65% or 70% and above.

Even in highly resistant alfalfa lines, up to 50% of plants may be susceptible to disease. “A lot of people are puzzled by that,” Putnam says. The 50% or higher resistance figure usually works in most cases because, with seedling stands, half the plants can be lost to pest damage or other reasons and still not affect yield. Susceptible plants die and resistant ones survive; planting rates are usually high enough that, even with a 50% loss, a good stand of alfalfa is produced.

“However, 50% resistance may not be adequate with stem nematode, since even resistant lines appear to have been affected,” he says.

“With the severity of the recent outbreak, we are selecting for even higher resistance levels in our breeding program,” says Miller. “It is my feeling that stem nematode is somewhat in a different class when compared to other pests, and that the resistance levels needed for optimal field performance should be set higher. Possibly in that 65-75% range.”

Stem nematodes are probably being spread through irrigation systems and equipment. “The microscopic worms don't move very far on their own, but they can be very easily picked up in water – in situations where you have flood irrigation moving down the field. It's certainly a worse situation than with sprinklers,” Putnam says. Harvesting infested fields last and cleaning equipment afterward may help keep the pests from spreading.

The nematodes don't function well in warmer weather, he adds. Probably in May, when temperatures get to be 85-95° at the soil surface and alfalfa is growing well, infested fields will begin to come back from the injury.

A full review of nematodes in alfalfa is in a newly minted chapter on alfalfa nematodes in the Irrigated Alfalfa Management Book.

For further information, visit UC IPM Online or read Background Information on Alfalfa Stem Nematodes.