Nematodes

Plant parasitic nematodes are microscopic roundworms that feed on dry bean roots and survive in soil and plant tissues. Several species may occur in a field at the same time. They have a wide host range and vary in their environmental requirements and symptoms they cause.

Only root-knot nematodes cause significant damage to beans. Yield reductions due to high numbers of root-knot nematodes may range from 45 to 90% and are typically most severe in sandy soils.

In addition to yield losses, these nematodes predispose plants to other soilborne pathogens that cause root rot and wilt diseases. For example, a bean variety resistant to infection by the Fusarium wilt pathogen will become susceptible to it if infected with root-knot nematodes. If root-knot nematodes are present in a field with a history of Fusarium wilt, choose bean varieties that are resistant to root-knot nematodes as well as to the particular Fusarium wilt race present when possible.

Lesion nematodes can be damaging, but are not as common as root-knot nematodes. Other nematodes associated with dry beans in California have not been fully studied (e.g., stunt and stubby root nematodes).

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.

Nematodes develop and reproduce on the roots of bean plants. Infection and feeding by the nematode can stimulate the root tissue to swell into galls around the infection site. Root-galling and nematode reproduction harm the plant because they both take away nutrients and photosynthate from the developing top growth and harvestable bean crop.

Although galls are diagnostic for root-knot nematode, some bean types do not gall much. An example is the blackeye CB-3, a susceptible variety that can support high numbers of root-knot nematodes but shows little galling. Severely galled roots can be short and thick. 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 nematode galls cannot be separated from the root easily.

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 level of infestation in the soil. If a previous crop had problems caused by nematodes that are also pests of dry beans, nematode numbers may be high enough to cause damage to the bean crop. It is therefore important to record monitoring data of root-knot nematodes from all crops in a field and consult these field histories when choosing a field to plant beans.

If nematode species have not been identified previously, 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.

1. Divide the field into sampling blocks that are representative of cropping history, crop injury, or soil texture. An ideal sampling block is 5 acres, but a larger sampling size (no greater than 20 acres) may be more economical.
2. 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).
3. Keep the composite samples from each block in separate plastic bags, seal them, and place a label on the outside with your name, address, block location, and the current or previous crop, and the crop you intend to grow. If plants with symptoms are available, place them in the same bag as the soil.
4. Keep samples cool (do not freeze) and transport as soon as possible to a diagnostic laboratory.

Contact your local 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 variety, crop rotation, sanitation, and fallowing the ground.

Use resistant varieties if there is a field history of nematodes in the soil.

Blackeye varieties are sorted by the year of release.

Growing small grains during the winter followed by a fallow period during the summer helps to reduce root-knot nematode numbers. Clean fallow, cruciferous crops, and green manure will help to reduce numbers of root-knot nematodes. Growing cover crops such as oats (Avena strigosa cv. Saia), marigolds, rattlebox (Crotalaria spectabilis), or hairy indigo (Indigo hirsuta) will reduce numbers of plant parasitic nematodes, including root-knot nematodes.

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 rotation worked because root-knot nematodes were unable to reproduce on the resistant tomatoes, thereby reducing their numbers to below damaging levels. A similar reduction in root-knot nematodes can be achieved by planting beans after a crop of root-knot nematode-resistant cotton.

Clean equipment using a high-pressure wash to remove all traces of soil before moving from infested to non-infested fields.

Weed-free fallow reduces the numbers of most types of nematodes. Fallowing is more effective if the soil is plowed and exposed to the sun. Irrigation during the dry period stimulates nematode egg hatch, further reducing nematode numbers if all food sources are removed via proper weed control.

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 nematicides (e.g., metam sodium, 1, 3-dichloropropene or ethoprop) has not been found to be cost effective on dry bean crops. Contact your farm advisor or agricultural commissioner for further advice on the use of nematicides.