How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PEST

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 disease complex on cotton. A given nematode population is relatively more damaging to cotton in the presence of Fusarium than when it occurs alone. Fusarium wilt race 1 damage is consistently associated with root knot nematode infection.

DAMAGE

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.

MANAGEMENT

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 hosts that support root knot nematodes.

Cultural Control
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. Crop rotation is also useful for reducing root knot nematode populations. For successful control of nematodes in a rotation program, susceptible weeds or grasses must be controlled completely during the non-cotton parts of the rotation. 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 5, CB 27, and CB 46), sorghum and sudangrass.

Small Winter Grain Rotation. Delay fall planting until soil temperatures are below 65°Fat 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 (a list of alfalfa varieties is available at http://alfalfa.ucdavis.edu). Maintain the alfalfa crop for 2-3 years to reduce root knot nematode populations 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 has been shown to increase yields when root knot nematode is present. This practice involves the use of a ripping shank that is pulled through the soil where the center of the future cotton beds will be located. Generally the soil is deep tilled at least 18 inches deep during winter before the rains. While this practice does little to 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 tap root 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.

Resistance
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.

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.

Monitoring and Treatment Decisions
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 System or WNR, which assesses root gall symptoms in fields where cotton will follow cotton.

Weighted Nematode Rating System (WNR). An easy and inexpensive way to rate root knot nematode damage where cotton follows cotton is the WNR method. This method does not require soil samples or a laboratory to extract nematodes because roots are rated in the field. Rating is based on relative intensities 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 subarea. 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 UC/ANR Publication 3305, Integrated Pest Management for Cotton, 2nd edition.

Also in Integrated Pest Management for Cotton, 2nd edition, is a detailed explanation of sample interpretation. UC researchers have determined the relative yield loss expected for a given preplant nematode population level. Thus, your sample results can indicate potential loss to cotton from your nematode infestation. Using this yield loss estimate, you can determine whether a nematode management option is economically justified.

Chemical Control
When nematodes are at levels requiring treatment, aldicarb (Temik) at planting has increased 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.

Common Name		Amount/Acre	R.E.I.+	P.H.I.+
(trade name)			(hours)	(days)
AT PLANTING
A.	ALDICARB*
	(Temik) 15G	4–7 lb	48	90
	MODE OF ACTION GROUP NUMBER1: 1A
	COMMENTS: Provides only temporary early season protection of developing roots. No difference in yield response was noted in field trials by increasing rate.
+  Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest. * Permit required from county agricultural commissioner for purchase or use. 1 Rotate chemicals with a different mode-of-action Group number, and do not use products with the same mode-of-action Group number more than twice per season to help prevent the development of resistance. For example, the organophosphates have a Group number of 1B; chemicals with a 1B Group number should be alternated with chemicals that have a Group number other than 1B. Mode of action Group numbers are assigned by IRAC (Insecticide Resistance Action Committee). For additional information, see their Web site at http://www.irac-online.org/.

PUBLICATION

UC IPM Pest Management Guidelines: Cotton
UC ANR Publication 3444
Nematodes
P. A. Roberts, Nematology, UC Riverside
D. J. Munier, UC Cooperative Extension, Glenn County
P. B. Goodell, UC IPM Program, Kearney Agricultural Center, Parlier

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