Nematodes

Root-knot nematodes are microscopic roundworms that are widely distributed throughout California on many crops, and cause varying degrees of damage to cotton. Root-knot nematodes are damaging to cotton as a single pest problem and as part of the Fusarium wilt race 1 and race 4 disease complex. Nematode populations are more damaging to cotton in the presence of Fusarium than when nematodes are present without Fusarium. Fusarium wilt race 1 damage is consistently associated with root-knot nematode infection. Fusarium wilt race 4 is less often associated with root-knot nematode infections. Fusarium race 4 is by itself a serious problem in cotton, but may be exacerbated by the presence of root-knot nematodes.

Nematodes damage cotton by attacking the young tap and secondary roots. Their feeding stimulates the production of galls. These galls interfere with the roots' ability to absorb water and nutrients, and provide locations for other disease-producing organisms, such as fungi (Fusarium) or bacteria, to readily enter the plant.

To manage root-knot nematodes efficiently, you should know the cropping history, soil texture, and history of injury in local soils, and you need an estimate of the nematode population level. In sandy soil with a history of root-knot injury to cotton, injury is likely to recur each season as long as cotton or other susceptible crops are planted without soil fumigation. If injury is severe enough and if the affected area is extensive, soil fumigation may be needed, or it may be necessary to plant a resistant cotton variety or a resistant variety of another crop such as tomato or alfalfa. Good field sanitation helps prevent infestations from spreading, and weed control is important in eliminating weed hosts supporting root-knot nematodes.

Clean fallowing (i.e., weed-free) during years when land is to be left unplanted is effective in controlling root-knot nematodes for cotton in California soils, but clean fallow can reduce beneficial mycorrhizal fungi to crop-damaging levels if the land is left plant-free for more than one year. Crop rotation is also useful for reducing root-knot nematode populations. For successful control of nematodes in a rotation program, susceptible weeds must be controlled completely in the rotation crop. Rotation crops that help to reduce cotton root-knot nematodes include alfalfa, winter small grains, corn, root-knot resistant cultivars of processing tomatoes, resistant cowpea (cvs. California Blackeye CB 46, CB 27, CB 50, and CB 5), sorghum and sudangrass. Corn is known to sustain several root-knot nematode species also found in cotton. So far there have been few problems reported in cotton following a rotation with corn.

Small Winter Grain Rotation

Delay fall planting until soil temperatures are below 65°F at planting depth; when soil temperatures are below 65°F, root invasion by root-knot nematodes ceases.

Alfalfa Rotation

Select an alfalfa variety with known resistance to the southern root-knot nematode. (See the publication Winter Survival, Fall Dormancy & Pest Resistance Ratings for Alfalfa Varieties) The alfalfa crop must be grown for 2 to 3 years and weeds must be controlled to reduce root-knot nematode numbers enough so cotton can be grown for at least one season without soil treatment. Be sure to sample for nematodes before planting back to cotton.

Precision Tillage or Precision Ripping

Precision tillage or ripping increases yields when root-knot nematodes are present. This practice involves pulling a ripping shank through the soil where the center of the future cotton beds will be located. Typically the soil is ripped at least 18 inches deep during winter before the rains. While this practice does not reduce the nematode population, replicated tests have shown significant yield increases with this technique. It is thought that the channel created under the planting bed allows the cotton seedling to extend its taproot deeper into the soil earlier in the season, thus making use of more soil volume than in unripped soils. The resulting enhanced root system of the cotton plant has greater access to soil water and can escape some nematode infection by growing into deeper soils at an earlier stage of development.

A resistant variety of Acala cotton, NemX HY, developed in the San Joaquin Valley of California, suppresses initial infection by southern root-knot nematode and thus reduces yield loss of cotton plants grown in infested soil. Because reproduction of the southern root-knot nematode on roots of NemX HY is limited, the population of this nematode in the soil in fall is much lower than the level in soil following susceptible cotton varieties. Check with seed companies for availability of this variety or its comparable replacements, as they are used on minimal acreages and may not be readily available.

The use of NemX HY is especially effective for managing root-knot nematode in rotations with other nematode susceptible crops. In addition to resisting the southern root-knot species, Meloidogyne incognita, it also acts as a nonhost for the other common species of root-knot nematodes, including M. arenaria, M. hapla, and M. javanica. Therefore, the resistant cotton can effectively prevent the buildup of all the root-knot nematode species likely to be encountered; this will relieve the nematode infection and damage potential on other crops grown in rotation.

Although resistant varieties provide excellent protection, do not plant them year after year in the same field. Experience with resistance in other crops has shown the root-knot nematode populations have the potential to overcome host plant resistance when exposed repeatedly to varieties with the same genetic resistance.

Root galls are the only distinctive symptoms of root-knot nematode injury on cotton plants. Above ground, infested plants have a nonspecific poor growth appearance. Early in the season, check for galls on the roots of cotton to determine if root-knot nematodes are present. Later in the season, from harvest until plowdown, use the Weighted Nematode Rating.

Weighted Nematode Rating System

An easy and inexpensive way to rate root-knot nematode damage where cotton follows cotton is the weighted nematode rating system. This method does not require soil samples or a laboratory to extract nematodes, because roots are rated in the field. Rating is based on the relative intensity of root galling. Start sampling at harvest time. Before you begin, draw a map dividing any areas in the field that differ in soil texture, cropping history, or crop injury. Further divide the areas where conditions are most uniform into smaller blocks, using a grid pattern.

Sample each block separately; blocks should be no larger than 10 acres. Use a root lifter such as a carrot or beet lifter attached to a light tractor to lift cotton roots easily out of the soil. See the Monitoring procedures and form for Weighted Nematode ratings.

Soil Samples

Sample soils of fields known or suspected to be infested with root-knot nematodes to establish the population density of each sub area. Find a laboratory that will process the samples and give population estimates of the number of juveniles per weight of soil. Details of the appropriate sampling procedure and handling samples are given in Integrated Pest Management for Cotton, 2nd edition.

Integrated Pest Management for Cotton, 2nd edition, gives a detailed explanation of sample interpretation. UC researchers have determined the relative yield loss expected for a given preplant nematode population level. Using this yield loss estimate, you can determine if nematode levels in your soil may lead to yield losses and whether a nematode management option is economically justified.

When nematodes are at levels requiring treatment, Mymik (a replacement for Temik) at planting, which is pending registration, can increase cotton yields. Other products such as metam sodium (e.g., Vapam) are registered for preplant use on cotton, but extensive tests in California have not shown economic yield increases with treatable populations of nematodes. Also, the fumigant 1,3-dichloropropene (Telone II) is effective on nematodes in cotton but is currently not economic for southern root-knot nematode because of its high cost.