How to Manage Pests
UC Pest Management Guidelines
Plant parasitic nematodes are microscopic roundworms 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. As nematode populations increase, lower stems on infected plants may turn black. Long periods of parasitism during moderate temperatures and high humidity may cause stem blackening for 1 foot or more above the ground. Another typical sign of a stem nematode infection is the presence of "white flags," which are branches devoid of chlorophyll. White flags are caused when nematodes start moving to leaf tissue and destroy chloroplasts, leaving pale leaf tissue. As plants die in the field, weeds often invade the open areas.
Root Knot Nematodes. Infection of alfalfa 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 population level, alfalfa variety, and soil temperature at planting time. High initial populations and relatively warm soil temperatures may cause serious injury to seedlings, resulting in stunting. The northern root knot nematode (M. hapla) 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. 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 the northern root knot nematode, 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 the northern root knot nematode is also present.
Root Lesion Nematodes. 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, population levels 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.
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, place them in a plastic bag, and send them to a laboratory for positive identification. If root-feeding nematodes are suspected, take soil samples from within the root zone (6 to 18 inches deep). 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. Place the samples in separate plastic bags, seal them, and place a label on the outside with your name, address, location, and the current/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. 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.
Resistant Varieties. Identify the species of nematodes present in the field because resistant alfalfa varieties are resistant to specific species and not all pest nematodes of alfalfa. It has been suggested that without some level of alfalfa stem nematode resistance, alfalfa production would be seriously threatened in many areas. For more information on current nematode-resistant varieties, see the current leaflet Winter Survival Fall Dormancy & Pest Resistance Ratings for Alfalfa Varieties from the National Alfalfa Alliance Web site.
Prevention (Sanitation and Exclusion). Use clean, nematode-free seed. Avoid moving contaminated farm machinery or livestock from an infested field to a clean field. Avoid using contaminated wastewater or tail water. Keep manure from feedlots where cattle have been fed infected hay out of clean fields.
Cultural Practices. Fall burning (in alfalfa seed production systems) decreases nematode infection and mortality, but spring burning appears to enhance infection and increase plant mortality.
The alfalfa stem nematode is a biological "race" of stem nematodes, and its host range is very limited. Consequently, rotation with nonhost crops such as sorghum, small grains, beans, and corn on a 2- to 4-year basis should reduce alfalfa stem nematode populations. Be careful to avoid reinfesting the field with contaminated irrigation water or machinery. Eliminating old and volunteer alfalfa plants through a weed control program is also important to avoid reinfestation. Also, cutting alfalfa fields only when the top 2 to 3 inches of soil are dry should help reduce reinfestation. University of Idaho variety trials demonstrate that frugal applications of irrigation water to keep soil surfaces dry will minimize spread into later cuttings.
Chemical Control. No nematicides are registered for use against the alfalfa stem nematode. Fumigation before planting may be too costly relative to potential economic benefits.
Resistant Varieties. The use of resistant alfalfa varieties is probably the most practical means of managing root knot nematodes. A number of resistant varieties such as Archer II and a new variety, Ameristand 444NT, are now commercially available for the Columbia root knot and northern root knot nematodes. Unlike other crops such as tomatoes, resistant varieties of alfalfa do not help reduce nematode populations.
Crop Rotation. 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 identified. For Meloidogyne incognita, the following are good rotation crops: barley, oats, wheat, cole crops, corn, cotton, hops, sudangrass, cowpea, and watermelon. For M. napla, cotton serves as a good rotation crop.
Chemical Control. 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.
Resistant Varieties. Alfalfa germplasm with resistance to lesion nematodes has been identified and developed. Archer II, Ameristand 403T and Ameristand 444NT are commercial varieties developed for the lesion nematode. These varieties have satisfactory resistance to Pratylenchus species and are the best means of controlling lesion nematodes because the cost of chemical control is prohibitive.
Crop Rotation. Lesion nematodes have a very wide host range and more than one species may occur in a field making crop rotation not effective for lesion nematode management. However, leaving a field fallow, followed by treatment with a nematicide, can reduce lesion nematode populations.
For more Information, see Irrigated Alfalfa Management.
UC IPM Pest Management Guidelines:
UC ANR Publication 3430
B. B. Westerdahl, Nematology, UC Davis
P. B. Goodell, Entomology, Kearney Agricultural Center, Parlier
S. Hafez, Univ. Idaho, Parma, ID.
Acknowledgment for contributions to Nematodes:
U. C. Kodira, Nematology, UC Davis