Background information on alfalfa stem nematodes

By Rachael Long, UC Cooperative Extension pest management farm advisor, Yolo County

Stem nematodes survive in the crown of infected plants, infested hay and crop debris, seed and in soil. Survival in soil probably does not exceed two years under normal rainfall or irrigation. Long-term survival is possible under very dry conditions in hay, seed and soil. The nematodes are spread to new areas by surface water runoff, irrigation, wind-blown crop debris, soil and crop debris clinging to equipment, humans and livestock, and with seed. Runoff water is very important in the spread of stem nematodes within a field and to adjacent fields.

When free moisture is present on stems and leaves, the nematode moves rapidly up the outside of a plant and begins feeding in the buds. The nematode reproduces to high levels during periods of favorable temperature and rapidly spreads when plants are wet.   A complete life cycle from egg to adult takes between 19 and 23 days at optimum temperatures of 59-70° F.  Because these conditions usually occur during early spring or mild winters, damage is most severe in the first cutting. When moisture is limited and temperatures are high, stem nematode spreads through internal tissues, but at a much slower rate. This slower movement may permit infected plants to outgrow the infestation as the summer progresses. However, damage and resulting spread reappears when conditions again become favorable.

Normally stem nematode is not a widespread problem for alfalfa production. However, the number of fields infested with stem nematode varies from year to year is based on the weather conditions that occur during late winter and early spring. To be certain that stem nematodes are causing plant dieback in your field, leaves and stems of suspected plants should be checked for the nematode. If in doubt, collect leaves and stems from several plants exhibiting symptoms of nematode infestation. The sample should be sealed in a plastic bag and dropped off at the county UC Cooperation Extension office.

To manage the alfalfa stem nematodes, efforts should be made to prevent the introduction and limit the spread of this pest into new areas.  When working in an infested field, clean crop debris and soil from equipment before entering a clean field (which also includes equipment used for weevil sprays). After harvest, remove all dry hay and dust clinging to balers or other equipment because stem nematodes have the ability to withstand very dry conditions. Where possible, situate new fields upwind of infested fields and where runoff is not received from an infested field. Because stem nematode is most severe in the first cutting, harvest when the top two to three inches of the soil is dry to reduce infection of new growth. Rainfall or irrigation before or soon after cutting promotes invasion of new buds.

Rotation with crops such as small grains or corn for two to four years will reduce stem nematode populations.  The greatest benefits can be expected from longer rotations.  However, low numbers of the nematode may survive during the rotation period and begin to increase when alfalfa is reintroduced. Volunteer weeds must be controlled during the rotation period. In addition, there are several alfalfa varieties reported to have some level of resistance to stem nematode, which should be used where stem nematodes persist as a problem. Unfortunately, the level of resistance to stem nematode in currently available varieties is not very high. Therefore, even the best varieties may become infected and develop symptoms during years with extended periods of wet, cool conditions.

Current research is focusing on whether foliar applied pesticides will control stem nematodes in alfalfa. In the meantime, be sure to control alfalfa weevils at this time of year if they are causing damage to developing crowns. Before you decide what to do with your alfalfa stand, wait for warmer weather that will favor plant growth and cause the nematodes to dieback.  Those dead looking crowns will probably regenerate and grow new buds and stems with increased yields by the second cutting.