Late summer or early fall before or just after cotton harvest, as nematode populations usually reach their peak, is an ideal time to take nematode samples. Samples should be taken with a soil probe or nematode collection device in moist soil six to eight inches deep in the root zone of cotton.
One quart of soil and root fragments from 15-20 sampling cores should be collected and sealed in a plastic bag. It would be wise to also send off a soil sample from the soil collected to determine if there are fertility problems. If trouble shooting, a sample should be taken from the area that nematode damage is suspected and an adjacent healthy area for comparison.
The sample’s location (sample id), crop grown, and farm should be marked on the bag. Next year’s crop should be reported on the form sent to the lab to determine if any nematodes present would pose a risk to succeeding production. Since nematodes are living organisms, special care should be exercised in short term storage and shipping.
Samples should be stored out of direct sunlight, preferably in an insulated cooler, and shipped to the nematology lab as soon as possible. Ideally, nematode samples should be stored in a refrigerator or cool location if stored overnight before shipping. It is best to ship the samples at the first of the week, so they are not in transit over the weekend.
Forms and more detailed directions for taking and sending nematode samples to Alabama Extension Plant Diagnostic Lab can be found in Nematode Sample Submission Form ANR-F7.
Samples should be taken when soil moisture is good.
Plant parasitic nematodes are a serious threat to Alabama cotton and are responsible for losses averaging almost 7 percent annually. There are many of nematode species that parasitize cotton, but only two cause significant economic damage in Alabama. The first one is the southern cotton root-knot nematode (Meloidogyne incognita), which is most often found in sandy soils in south Alabama.
The other is the reniform nematode (Rotylenchulus renidormis), which is most commonly found in finely textured, silty soils. These are the two major species of nematodes that attack cotton in Alabama but others such as lance and stubby-root nematodes may occasionally be a problem.
In fields infected with nematodes, symptoms will usually appear in localized areas or “hot spots” and are then spread to other parts of the field through farming equipment. In general, it is difficult to determine if a field is infested with nematodes based on above ground symptoms alone and a nematode analysis of the soil is required to confirm the presence of nematodes. Damage to cotton plants can range from mild to severe depending on the species of nematode and its population level.
Interveinal chlorosis or yellowing is a common foliar symptom of southern root-knot nematodes but other foliar symptoms, including stunting, may be present as the plants often express nutrient deficiencies since root galling reduces nutrient and water uptake.
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Yield losses due southern root-knot nematodes in field corn are not as severe as other crops, but their populations can build to high numbers on the corn hybrids planted in the SE U.S. The peanut root-knot nematode (M. arenaria) is a serious pest in peanuts. Cotton is not a host to the M. arenaria while peanuts are not a host to M. incognita so cotton and peanuts make an excellent rotation to reduce root-knot nematode populations.
Later-occurring, above ground symptoms associated with reniform nematodes range from slightly stunted plants with reduced fruiting to severely stunted plants with few squares or bolls. Effects depend on the number of reniform nematodes present as well as environmental stresses on the plant. Reniform-infected plants under ideal growing conditions and in the absence of stress may not exhibit any detectable symptoms but yield reductions can still occur. Cotton continues to produce lower yields each year if a good rotation is not followed.
The most effective methods to control reniform nematodes is through rotation with a non-host and resistant varieties. Crop rotation is one of the most important things that producer can do to reduce nematode pressure. Corn, sorghum, and peanuts are not hosts for reniform nematodes and will reduce populations.
The winter grain crops such as wheat, rye, oats, and barley also are nonhosts; however, legume winter cover crops such as vetch and clover are hosts and should be avoided when followed with a susceptible crop. Good weed control will also help reduce reniform nematode populations as they can infect and reproduce on cocklebur, cowpea, crotolaria, sow thistle, jimson weed, Florida beggarweed and Florida pusley.
There are some commercial cotton varieties resistant to either southern root-knot nematodes or both root-knot and reniform nematodes available for use in Alabama. Southern root-knot nematode resistant varieties include PHY 350 W3FE, PHY 400 W3FE, PHY 480 W3FE, PHY 500 W3FE, PHY 580 W3FE, ST 5600 B2XF, and DP 1747NR B2XF. More recently, cotton varieties with dual resistance to both root-knot and reniform nematode have become available including DP 2141NR B3XF, PHY 332 W3FE, and PHY 443 W3FE.
Before planting a selecting a nematode resistant variety, producers should determine the species of nematode(s) present in the field and should take a look at a UGA Report on Planting Resistant Cotton Varieties for additional considerations. Additional information on variety performance can also be found on the Auburn University Variety Selection Platform.
Nematicides are a very important tool in nematode management, but they should only be applied when and where they are needed. If you don’t have nematodes, then you won’t see a yield gain by applying nematicides. Depending upon nematode pressure (numbers), application technology and recommendations vary for different nematicides and control measures (Table 1).
Telone II, a pre-plant fumigant, is the only “stand alone” nematicide for use in fields with high nematode pressure. The remaining nematicides can be used in fields with low to moderate nematode populations. Vydate C-LV be applied as a post-emergence in combination with seed treatments such as Aeris, Avicta Duo, and Poncho/Votivo for additional control in fields with low to moderate nematode pressure.
Bottom line when selecting nematicides- don’t guess, do a soil test! See the ACES extension publication IPM-0415, “Cotton IPM Guide” for more information on recommended nematicides for nematode control in Alabama.
Root-knot nematodes can also introduce disease from soil-borne pathogens to cotton plants. Fusarium wilt, for instance, is greatly increase in cotton where root-knot nematodes are present. It usually requires 100 times more of the individual Fusarium wilt pathogen to cause the same of damage to cotton as the Fusarium wilt/root-knot complex.
Any cultural practices, such as in-row subsoiling to prevent j-rooting, that lessen the stress on the plant can help reduce losses from nematodes.