Additional resistance is needed to manage the alfalfa-feasting pest
California growers lost up to 50% of yield to stem nematode in portions of many first-cut alfalfa fields early this cool, wet spring, the second year in a row. Pacific Northwestern alfalfa fields suffered severe nematode damage as well, says Saad Hafez, University of Idaho nematologist.
In the past, as the weather warmed, the pest disappeared. Cool, wet conditions continuing into this summer, however, kept it viable enough to feed on second-cut fields, too, says Don Miller, director of development and an alfalfa breeder at Producer's Choice Seed and Cal/West Seeds.
And the microscopic pest could hit yet again this fall, he says.
“In these cool weather conditions, the nematodes are reproducing more cycles, more generations, so you get a lingering effect in the second cut. As we approach fall, it will happen all over again as temperatures start cooling off,” Miller predicts.
And there's little alfalfa growers can do to control the pest in those fields, says Jerry Schmierer. He's the agronomy farm advisor for Colusa, Sutter and Yuba counties in the Sacramento Valley, where much of the California nematode damage has occurred.
Stem nematode attacks alfalfa through the soil and the crop can't be fumigated economically. Current nematicides won't control it, either. It has feasted on fields in Idaho, Washington, Oregon, Nevada, Wyoming, Montana and Colorado as well as in California, says Hafez.
“We don't know enough about this critter that we can tell you anything,” Schmierer says, “other than, if you've got it, you're going to lose your first cutting, especially if you're in the Sacramento Valley. And the nematode won't stop feeding until it gets hot.”
In the patchy, nematode-infested parts of alfalfa fields, “it looks just like you took a cutting — only you didn't haul anything off the field,” he adds.
The best way to combat the pest is to plant alfalfa varieties resistant to it. But Hafez, Schmierer and Miller say that many of the current alfalfas labeled as having high resistance to stem nematode aren't at sufficient levels.
“Some of the problem in California is that some varieties don't have enough resistance and cannot survive the pressure of the nematode they have gotten in the last two or three years,” Hafez says.
Miller agrees. Any alfalfa labeled for high resistance to a pest actually contains nearly 50% of plants that may be susceptible to the insect or disease. In the past, those varieties usually had enough resistance to keep pests at bay.
A number of varieties labeled as highly resistant to stem nematode show in the low 50% levels of resistance now — a few even have resistance in the low 60s, he says.
“But maybe we need more varieties to be in the 60% or low 70% level. Maybe an extra 10% might give us enough field tolerance under these severe conditions. We can take a variety that has 50% resistance and put it through one more selection cycle and pop it up another 10-20%,” he says.
Additional stem nematode-resistant varieties with 60% or more resistance could be in the marketplace within a year and a half to two years, Miller estimates.
He says Producer's Choice and Cal/West are “gearing up” to add more stem nematode resistance in their nurseries this fall. Schmierer says several alfalfa seed companies are seeing the need for added resistance.
Miller's companies are also hosting an Aug. 25 stem nematode seminar with Hafez, University of California forage experts and Cal/West's California breeder, Lei E., at the Cal/West Seeds Research Center in Woodland. A few hay groups have asked that similar seminars be held at their annual winter meetings, Miller says.
To keep stem nematode from infecting clean fields, Hafez says prevention and sanitation are vital.
“The main thing is to be sure you have a good resistance level in the variety you're planting in new fields. Then, also, sample before you plant the field to see if you have nematodes in it or not.”
Also check the water source for contamination. The pest is an aquatic creature that easily moves by irrigation water, especially water that's been recycled from infected fields.
“If you have channel water or wastewater from a ditch, you can get the nematode from somebody else's field. Grazing animals can also spread it, because nematodes can survive in digestive systems,” Hafez reports.
Moving equipment from a nematode-infested field to a clean field will move the pest as well, so Hafez recommends cleaning machines before entering new fields.
“We also try to tell the grower, when he does the cutting, especially the first cutting, he's going to have a lot of nematodes loose in the field. He's going to need to delay the water after the first cutting as long as he can.
“If he waters right away, he's going to spread more nematodes,” says Hafez.
Try to avoid any other stresses on plants by keeping fertilizer and insecticide management up to date. Also, if fields are damaged enough, growers will want to rotate them to crops not susceptible to stem nematode, he says.
White flagging is a classic symptom usually seen before first cutting, the nematologist adds. That's where one plant in thousands will spring up completely white in fields.
But flagging was seen in second-cut fields as well this year, Miller says.
Growers should scout for nematodes in spring and between cuttings, watching mostly the crown for feeding damage.
“You can see a lot of swelling on the new buds and deformation on the leaves. And you will see a lot of dead tissue — a lot of dead buds around the crown,” Hafez says.
He's hopeful that viable nematicides will soon be on the market. “We started testing those products and they look promising. They may be available next year or the year after if the company can get registration.”