** IMPACT **
University of California,
Div. of Agriculture & Natural Resources
---University Pest Management Guidelines---
WARNING ON THE USE OF PESTICIDES
Pesticides are poisonous. Follow all precautions and recommendations on the pesticide label. The grower is legally responsible for residues on treated crops. Keep all pesticides out of the reach of children, unauthorized persons, pets and livestock. Consult your County Agricultural Commissioner for correct methods of disposing of unwanted pesticides and containers.
COMMODITY: ALFALFA-HAY
Record number: 1200111
REVIEWED: 9/95 (BBW)
NEMATODES:
Common Name: Alfalfa stem
Scientific Name: Ditylenchus dipsaci
Common Name: Root knot
Scientific Name: Meloidogyne hapla, Meloidogyne incognita, M. javanica, M. thamesi, M. arenaria, and M. chitwoodi
Common Name: Stubby root
Scientific Name:Trichodorus sp. and Paratrichodorus sp.
Common Name: Stunt
Scientific Name:Tylenchorhynchus clarus
Common Name: Lesion
Scientific Name: Pratylenchus penetrans and P. neglectus
DESCRIPTION OF THE PESTS:
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 crowns, stems, and buds of the alfalfa plant, while the other nematodes listed above feed on roots. Root-feeding nematodes are most damaging to crops grown in sandy soils.
DAMAGE:
Both stem nematode and root knot nematodes cause substantial damage and are of major concern in California. The stem nematode infests and weakens stem buds, stunts growth, destroys the crown, and can eventually kill the plant. Damage is often associated with rainy weather and occurs most frequently in spring, but also occasionally in fall. Damage to the crop can be extensive if the fungal pathogen, Phytophthora megasperma, is also present. Root knot nematodes can cause stunting, reduce a stand, and increase the incidence and severity of Fusarium and bacterial wilts.
Stubby root nematodes are important in certain parts of the state and can be especially damaging to seedling alfalfa fields. Stunt nematode and lesion nematodes are common in alfalfa fields in some areas and have been shown to suppress growth in greenhouse tests. A variety of other nematodes, such as another species of stunt nematode (Merlinius brevidens), dagger nematode (Xiphinema americanum), ring nematode (Criconemella spp.), and spiral nematode (Helicotylenchus sp.), are found in alfalfa fields. Their influence on the plant growth has not been thoroughly studied, but they are not believed to cause significant alfalfa injury.
SYMPTOMS:
The symptoms described below are characteristic of a nematode problem, but are not diagnostic as they could result from other causes as well. Plants infected with the alfalfa stem nematode have distorted and bloated tissues with spongy appearance, shortened internodal length, and reduced number of shoots. Heavy infestations cause leaf curling and distortion. Root knot infestation produces galls on lateral roots and excessive branching of roots. Galls induced by M. hapla and M. thamesiare very small and difficult to see, whereas, galls produced by other root knot species are larger. Root systems infested with stubby root nematode can be severely stunted and lack feeder roots or they can exhibit brooming, a symptom that occurs when the growing tip of a root is killed and several new roots are produced where the old one died. The other nematodes listed do not produce any distinctive symptoms, but may suppress growth and weaken the plants by feeding on roots.
Nematode injury typically occurs in areas or pockets of the field and is not evenly distributed throughout the field.
FIELD EVALUATION:
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, soil samples should be taken and sent to a diagnostic laboratory for identification.
If symptoms of stem nematode are evident, 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- 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 one 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. (See IPM for Alfalfa, UCANR publication 4104 for more details). 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.
MANAGEMENT:
Cultural practices: For stem nematode control, use certified seed. Growing nonhost crops such as beans, cotton, corn, sorghum, lettuce, carrots, tomatoes, and grains for 2 to 3 years has also been shown to reduce stem nematode populations. Volunteer alfalfa growing around the edges of fields planted to rotation crops will continue to perpetuate nematodes; all volunteer alfalfa plants must be destroyed in order for crop rotation to be effective. Burning weeds in infested fields during fall has been shown to decrease problems from stem nematode the following spring, but burning weeds in spring seems to make the problem worse. Clean soil from machinery and equipment with water before moving between fields to prevent the movement of infested soil. Proper irrigation and nutrition reduces stress and may reduce nematode damage.
When root-parasitizing nematodes are present, planting in fall is preferable over spring planting. With fall plantings the alfalfa plant can become established while soil temperatures are low and nematode activity is minimal. When soil temperatures rise in spring and summer, the alfalfa plants have become established and more vigorous, enabling them to better withstand the deleterious effects of the nematodes. Crop rotation is not a feasible management practice in fields with root knot nematodes because of their wide host range.
Resistant/Tolerant cultivars: Resistant/tolerant cultivars available are specific to nematode species. Resistant/tolerant cultivars are available for all the areas of California where stem nematode problems have been reported. In wet years, however, stem nematode symptoms may appear unless the cultivar has a high level of resistance. Less information is available regarding the resistance of alfalfa cultivars to root-feeding nematodes. The majority of nondormant cultivars and a few dormant cultivars are resistant to the root knot nematodes M. incognita and M. javanica. Some cultivars resistant to M. hapla have been developed. Contact your farm advisor for more information regarding resistant cultivars for your area.
Chemical treatment. Methyl bromide, metam sodium, and chloropicrin are registered for preplant use on alfalfa. In California, the use of these chemicals has not been found to be cost effective except possibly in seed production fields. Contact your farm advisor or agricultural commissioner for further advise on the use of these chemicals.
REFERENCE:
Alfalfa Pest Management Guidelines. U.C. Kodira, B.B. Westerdahl. July 1992. PUBLICATION: UCPMG2