How to Manage Pests
UC Pest Management Guidelines
Plant parasitic nematodes are microscopic roundworms that feed on plant roots. They survive in soil and plant tissues, and several species may occur in a field. They have a wide host range and vary in their environmental requirements and in the symptoms they cause.
Only root knot nematodes are known to cause significant damage. Yield reductions due to high populations of root knot nematodes may range from 45 to 90%. Yield losses due to root knot infestation are typically most severe in sandy soils. These nematodes are also known to predispose plants to other soilborne pathogens that cause root rot and wilt diseases. Lesion nematodes can be damaging but are infrequently encountered compared to root knot nematode. Other nematodes (e.g., stunt and stubby root) have been associated with dry beans in California but have not been studied.
Symptoms described below are indicative of a nematode problem, but are not diagnostic as they could result from other causes as well.
Aboveground symptoms of severe root knot infestation include patches of chlorotic, stunted, necrotic, or wilted plants. Infested plants that are also under moisture or temperature stress may wilt earlier than other plants. Feeding by root knot nematode incites cell enlargement and proliferation resulting in swellings, called galls, on roots. These galls are diagnostic for root knot nematode, however, some bean types do not gall much. An example is the blackeye CB3, which is susceptible to root knot nematode and can support high populations of this nematode but shows little galling. Severely galled roots may be shortened and thickened. Galls caused by root knot nematodes may be confused with nodules of nitrogen-fixing Rhizobium bacteria. Rhizobium nodules, however, are pink inside and come off the root easily when rubbed. Root knot galls cannot be separated from the root.
Roots of bean plants infested with lesion nematodes are likely to be poorly developed and may exhibit brown-black lesions. Damage to roots by lesion nematodes may be more severe in the presence of other soilborne pathogens.
To make effective management decisions, it is important to know the nematode species present and to estimate their population density in soil. If a previous crop had problems caused by nematodes that are also listed as pests of dry beans, population levels may be high enough to cause damage. Develop and track field histories to monitor the presence of root knot nematode on previous crops.
If nematode species have not previously been identified, take soil samples and send them to a diagnostic laboratory for identification. The best time to collect samples is soon after harvest or preferably just before harvest of the previous crop. Divide the field into sampling blocks that are representative of cropping history, crop injury, or soil texture. An ideal sampling size is 5 acres, but a larger sampling size (no greater than 20 acres) may be more economical. In each block randomly take several subsamples from the root zone of the previous crop, mix them thoroughly and make a composite sample of about one quart (1 liter). Place 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. If plants with symptoms are available, place them in the same bag as the soil. Keep samples cool (do not freeze), and transport as soon as possible to a diagnostic laboratory. Contact your farm advisor to help you find a laboratory for extracting and identifying nematodes, and for help in interpreting sample results.
Management of nematodes in dry beans requires a careful integration of several cultural practices, including choice of cultivar, crop rotation, sanitation, and fallow.
Resistant varieties. Large lima bean cultivars that are resistant to Meloidogyne incognita are White Ventura N. Maria, UC-90, and UC-92. The baby lima bean cultivar, Cariblanco N, has resistance to root galling and reproduction by M. incognita and root galling by M. javanica. UC 302 has resistance to some races and is due to be released in the next year or two. Blackeye #5 and CB-46 are highly resistant to most, but not all populations of M. incognita but susceptible to M. javanica. CB-50 is a new cultivar with strong resistance to M. incognita and moderate resistance to M. javanica.
Crop rotation and cover crops. Growing small grains during the winter followed by a fallow period during the summer helps to reduce root knot nematode populations. Clean fallow and green manure will help to reduce populations of root knot nematodes. Growing cover crops of oats (cv. Saia), marigolds, rattlebox (Crotalaria spectabilis), hairy indigo (Indigo hirsuta), etc., is known to reduce populations of plant parasitic nematodes. Also, research in Stanislaus County demonstrated that a rotation of root knot nematode resistant processing tomatoes with common and Lima beans was successful in preventing root knot damage to beans. This is because root knot nematode were unable to reproduce on the resistant tomatoes, thereby reducing soil populations to below damaging levels. A similar benefit from reduced soil populations can be achieved by a preceding crop of root knot nematode-resistant cotton, NemX.
Sanitation. Clean soil from equipment with water before moving from infested to noninfested fields.
Fallow. Weed free fallow reduces most nematode populations. Fallowing is more effective if the soil is plowed and exposed to the sun. Irrigation during the dry period stimulates nematode egg hatch, and so further reduces nematode populations if proper weed control is maintained.
Monitoring and treatment decisions. During the flower bud to bloom period inspect plants for nematode damage along with other pests and their damage. If a plant looks stunted, check its roots for galling. Damage thresholds have not been established for nematodes on beans. In California, the use of chemical treatments has not been found to be cost effective. Contact your farm advisor or agricultural commissioner for further advice on the use of chemical treatments.
UC IPM Pest Management Guidelines: Dry