Description of the Pest
Plant-parasitic nematodes are microscopic thread-like worms that live in soil and plant tissues and feed on plants by puncturing and sucking the cell contents with a needlelike mouthpart called a stylet. The alfalfa stem nematode feeds in the stems and crowns of the alfalfa plant, while the other nematodes listed above feed on roots.
The nematode life cycle typically includes an egg stage, four larval stages, and an adult stage. The life cycle, from egg hatching to egg production, usually requires 3 to 6 weeks under optimal conditions to complete. Environmental factors, such as soil temperature, soil moisture, host status, and time of infection, can influence the number of nematode generations completed within a year. Nematodes move relatively short distances on their own (a few inches per year), but they are easily spread long distances by soil movement (wind, farm equipment, etc.), irrigation water, nursery stock, seed, and debris in seed and hay.
Both stem nematode and root-knot nematodes cause substantial damage and are of major concern in California. In general, the symptoms and damage described below are characteristic of nematode problems but not diagnostic because they could result from other causes as well. Nematode injury typically occurs in areas or pockets of the field and is not evenly distributed throughout the field.
Alfalfa Stem Nematode
Stem nematodes enter bud tissue and migrate into developing buds. Infected stems become enlarged and discolored, nodes swell, and internodes become shorter than those on healthy plants. Alfalfa plants infected with the alfalfa stem nematode have stunted growth, fewer shoots, and deformed buds. Another typical sign of a stem nematode infection is the presence of "white flags", which are individual alfalfa stems and leaves that turn white, especially during spring months (typically after the first cutting). White flags are caused when nematodes move to leaf tissue and destroy chloroplasts, leaving pale leaf tissue. Stem nematode feeding may also injure the alfalfa crowns, creating entry wounds for fungal and bacterial pathogens (such as Phytophthora or Fusarium) that can cause crown rot and reduced yields. As alfalfa plant dieback occurs, weeds often invade the open areas. Stem nematodes are most prevalent during late winter (January—March) when cooler temperatures favor their development (optimum 59-68oF).
Alfalfa infection caused by Meloidogyne species may be confined to localized areas of a field or extend throughout an entire field. The extent of the damage in the field depends on several factors, including initial nematode numbers, alfalfa variety, and soil temperature at planting time. High initial numbers and relatively warm soil temperatures may cause serious injury to seedlings, resulting in stunting.
Root knot nematode (Meloidognye spp) infects and parasitizes roots of alfalfa plants and causes the plant cells to enlarge into small oval galls on the roots that can be seen with the naked eye. Galls caused by root-knot nematodes are accompanied by lateral root growth, unlike galls caused by the beneficial nitrogen-fixing bacteria, which are easily rubbed off the root with your thumb. In a heavily infested field, young seedlings may be killed by this nematode, even though roots may not display galls.
The Columbia root-knot nematode (M. chitwoodi) produces symptoms similar to other root-knot nematodes, but it is less pathogenic to alfalfa. This nematode causes tiny galls that can easily be missed if roots are not examined carefully.
Root-knot nematodes, like stem nematodes, may enhance the development of diseases such as bacterial wilt, Phytophthora root rot, and Fusarium wilt. In addition, damage by the alfalfa stem nematode may be more severe when root-knot nematodes are also present.
Root Lesion Nematodes
Of the two species of root lesion nematode listed, P. penetrans is more pathogenic. However, it only occurs in localized areas of the state.
Plants infected with root lesion nematodes exhibit aboveground symptoms such as stunting and nutrient deficiencies. Impact on the root system includes reduced root growth and black or brown lesions on the root surface. Lesions may fuse to cause the entire roots to appear brown. Secondary infections of roots by bacterial and fungal pathogens commonly occur with a root lesion nematode infestation; feeding by root lesion nematodes may overcome the resistance of the alfalfa varieties to these pathogens. Damage caused by lesion nematode depends on the alfalfa variety and the species of lesion nematode present in the field. Under severe infestation, young plants often die, resulting in yield reductions.
It is critical to know the nematode species present and the density of their populations to make management decisions. If a previous field or crop had problems caused by nematodes that are listed as pests of alfalfa, numbers may be high enough to cause damage to seedlings. If nematode species have not previously been identified, take soil samples and send them to a diagnostic laboratory for identification.
How to Sample Nematodes in the Soil
- Divide the field into sampling blocks of not more than twenty acres each that represent cropping history, crop injury, or soil texture.
- Take several subsamples randomly from a block, mix them thoroughly and make a composite sample of about 1 quart (1 liter) for each block. If root-feeding nematodes are suspected, take soil samples from within the root zone (6 to 18 inches deep).
- Place the samples in separate plastic bags, seal them, and place a label on the outside with your name, address, location, the current and previous crop, and the crop you intend to grow.
- Keep samples cool (do not freeze), and transport as soon as possible to a diagnostic laboratory.
How to Sample for Stem Nematode
Stem nematode is easily identifiable under a dissecting scope. If symptoms of stem nematode are evident, such as stunted growth and open patches in the field
- Cut several stems with symptoms from several different plants.
- Cut stems into half-inch pieces into a petri dish using scissors or a razor blade.
- Add water until the stem pieces are submerged.
- Let the sample sit for 15 minutes.
- Look for large, clear worms that appear to be swimming.
Contact your farm advisor for more details about sampling, to help you find a laboratory for extracting and identifying nematodes, and for help in interpreting sample results.
Management of Nematodes in Alfalfa
Once the species of nematodes has been identified from the field, management strategies need to be tailored to the species that are present. Because resistant alfalfa varieties are resistant to specific species and not to all pest nematodes of alfalfa, accurate identification is important.
Alfalfa Stem Nematodes
Use of highly resistant stem nematode alfalfa varieties is recommended for production areas where stem nematodes are problematic, to help prevent yield and stand losses. Development of alfalfa varieties with >70% resistance to stem nematodes is ongoing (current levels are mostly >50%). For more information on current nematode-resistant varieties, see the list of alfalfa varieties provided by the National Alfalfa & Forage Alliance.
Prevention (Sanitation and Exclusion)
- Use certified clean, nematode-free seed. Stem nematode is not seedborne, but can live on alfalfa chaff. Certified seed is well cleaned to remove alfalfa debris and stem nematodes and then tested for the presence of stem nematode before seed sales.
- Avoid moving contaminated farm machinery or livestock from an infested field to a clean field.
- Avoid using contaminated wastewater or tail water.
- Manure from feedlots where cattle have been fed infected hay needs to be composted before use and kept out of clean fields.
- Clean equipment when moving from a stem nematode infested field to a clean field. This can be done using a high-pressure washer or blower, or by cutting grass hay prior to moving back into alfalfa.
Fall burning (in alfalfa seed-production systems) decreases nematode infection and plant mortality, while spring burning appears to enhance infection and increase plant mortality.
The alfalfa stem nematode has a very limited host range including alfalfa and, to a lesser extent, sainfoin and potatoes. Consequently, rotation with nonhost crops such as tomatoes, small grains, beans, and corn on a 2- to 4-year basis should reduce alfalfa stem nematode numbers (longer is better for heavily infested fields). Control volunteer alfalfa plants in subsequent crop rotations. Note: Overseeding older alfalfa stands with grasses is not a rotation, since alfalfa hosts remain in the field.
No nematicides are registered for use against the alfalfa stem nematode. Fumigation before planting may be too costly relative to potential economic benefits.
The use of resistant alfalfa varieties is probably the most practical means of managing root-knot nematodes. There are a number of different commercially available alfalfa varieties that are with resistance to the northern and southern root-knot nematodes. Unlike other crops such as tomatoes, planting resistant varieties of alfalfa does not help reduce nematode numbers because there will always be some alfalfa plants in the field that have no resistance to root-knot nematodes, based on alfalfa resistant ratings.
Depending on the root-knot nematode species present in the field, crop rotation can be a useful management strategy. It is important to have the species correctly identified. For Meloidogyne incognita, the following are good rotation crops: barley, oats, wheat, cole crops, corn, cotton, hops, sudangrass, cowpea, and watermelon. For M. hapla, cotton serves as a good rotation crop.
Soil fumigation before planting can be effective against the northern root-knot nematode, but fumigants are expensive and generally not economically feasible on alfalfa. No nonfumigant nematicides are currently registered on alfalfa.
Root Lesion Nematodes
Screening is ongoing to identify alfalfa varieties with resistance to lesion nematodes, but currently there are no commercially certified varieties with lesion nematode resistance.
Lesion nematodes have a very wide host range, and more than one species may occur in a field, making crop rotation ineffective for lesion nematode management. Leaving a field fallow and weed free, can reduce lesion nematode numbers, but not enough to prevent crop damage to new alfalfa plantings. Nematicides are not cost effective for alfalfa production. If lesion nematodes are present in the field, it is important to maintain good plant health. Plants that are stressed (for example from too little water) will be more susceptible to lesion nematode damage.
For more information on parasitic nematodes in alfalfa, see Irrigated Alfalfa Management.