Stem nematodes are causing significant damage to some alfalfa fields in California’s Sacramento and San Joaquin valleys, report Dan Putnam and Rachel Long with University of California (UC) Extension.

The nematodes are microscopic, wormlike pests that primarily move through fields in water and infect plant crowns and stems. Signs of feeding damage include stunted plants with swollen stems and white flagging. Eventually, when nematodes injure crowns and open the way for secondary pathogens to move in and cause crown rot, stand loss occurs.

Growers will also want to carefully monitor nematode-infected stands for alfalfa weevil damage. Treatment thresholds are generally 20 weevil larvae/sweep, but numbers will be far less where alfalfa isn’t growing, because the crop is less able to outgrow the feedings.

Stem nematodes favor temperatures in the 60-70oF range. Generally, only first-cutting hay is affected. When temperatures warm in late spring, nematodes go dormant in the soil. When temperatures cool in the fall, the nematodes re-infect plants.

Since there are no registered or cost-effective sprays to control stem nematodes, growers should plant resistant varieties and take management steps keep the nematode out of fields or from spreading.

A recent post on UC Extension’s Alfalfa And Forage News Blog offers preventative management steps.

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