A crop consultant brought in some stunted, seedling alfalfa plants and asked us to help identify the problem. The plants came from a field in the southwest part of San Joaquin County that was only planted about three months ago. After looking at them under the dissecting microscope, we came to the conclusion that the problem was stem nematode. Identification of stem nematode can be easily done under a dissecting microscope by cutting pieces of stems and the crown in a petri dish with water. The picture above was taken a few years ago and illustrates them well. The blue arrow points at some translucent wiggly lines emerging from a piece of stem, which are the stem nematodes.
Stem nematodes live and feed in the stems and crown of alfalfa plants. Symptoms of infection include plant stunting, shortened internodes, swollen buds, and white flagging where stems lack chlorophyll. They are a particular problem in the spring when the weather is cooler. As the temperature warms, the nematodes recede deeper into the soil and go dormant until cool weather returns. At this time of year, they can keep otherwise healthy fields of alfalfa stunted and delay the first cutting. In some cases, they may cause complete plant die-back, leaving open areas in the field where weeds can become a problem.
This particular seedling field was a bit of a “head scratcher” for a couple of reasons. First, we don’t tend to observe stem nematode in seedling fields; they’re more typical in older stands. Second, crop rotation to grass crops is a recommended practice where stem nematode has been a problem. This is because stem nematode has a limited host range, including alfalfa, sainfoin, and potatoes. While the field previously had grass hay, the grass was overseeded into an older alfalfa stand. In recent years, the grass grew vigorously, especially in the spring. What we suspect is that last spring when the grass was vigorous, the alfalfa plants were stunted and not visible. The intermittent alfalfa plants were a host for the stem nematodes, which impacted the seedlings this spring.
Stem nematodes can be a perennial problem in California alfalfa, and we have seen evidence of them in different parts of the county with different soil types. Unfortunately, management is challenging. In areas where stem nematodes are a problem, growers should plant nothing less than a highly-resistant variety. The National Alfalfa and Forage Alliance produces a pamphlet that shows varietal resistance to various pathogens, including stem nematode. It is important to remember that even with a highly-resistant variety, it is typical to have susceptible plants next to healthy plants within a stand. Other cultural control practices include planting clean seed and using proper sanitation methods. This might include not moving equipment and wastewater from infected to clean fields and also not applying manure from cattle fed with infected stem nematode hay to clean fields. Since stem nematodes have a limited host range, rotating out of alfalfa for at least two years can help to reduce the problem, but longer rotations (four years) might be advised under severe infestations. Rotation crops could include small grains, corn, or dry beans. There are no registered nematicides for stem nematode.
If you suspect stem nematode to be a problem in your field, please contact your local UCCE Advisor, who can help you with diagnosis.