Nematode management in the south
This is the third article of a four-part series on soybean cyst nematode management (SCN). This article focuses on the southern U.S. While SCN is a problem in the South, root-knot and reniform nematodes are actually a greater problem because they are more active in warmer climates compared with SCN. Therefore, this article will address all three nematodes.
Microscopic nematodes are the most numerous animals on earth. “There are 3 billion nematodes in just an acre of soil and more than 1 million species,” says Travis Faske, University of Arkansas Extension plant pathologist. Given its warm climate, it is not surprising that the southern U.S. has several soybean nematode management concerns beyond soybean cyst nematode (SCN). Because they populate crop roots and lower yields, that’s a lot of invisible pests to manage!
It’s hard to address SCN in the South without the context of many other crop nematodes in the same environment. “The main southern nematode concerns are root-knot nematode and reniform because they are more active in warmer climates compared with SCN,” Faske says.
“In soybeans, there is usually either SCN, or we see root-knot and reniform nematodes,” says Greg Tylka, Iowa State University nematologist and SCN expert. “Root-knot and reniform nematodes can feed and reproduce on other southern row crops, such as cotton, corn, tobacco, and wheat, whereas SCN feeds only on soybeans.”
Having cotton in rotation cuts SCN populations, but cotton hosts reniform and southern root-knot nematode populations, says Kathy Lawrence, Auburn University plant pathologist. Reniform nematodes cut cotton yields by 49–62% in her seven-year Auburn replicated study (2013–2019). This field study was on adjacent fields having the same soil types and production practice with and without reniform nematodes (see Figure 1).
Both reniform and root-knot nematodes feed on different parts of the same crop root, Lawrence says. “Reniform tends to have the advantage, having a broader feeding area. But the two nematodes compete with each other. If we can get cotton to emerge successfully the first six weeks while protecting that root system (with nematicides), it can generally handle moderate nematode population and yield well.”
The best tools to prevent and manage nematode spread are nematode-resistant crop varieties, nematicides, clean equipment, and rotations with non-hosts.
Reniform Nematodes: What We Know
- Prefer hot climates and survive long periods in very dry soil.
- Not seriously affected by soil type.
- Hosts include cotton, soybeans, tomatoes, and cucurbits (gourd family).
- Non-hosts are rice, sugarcane, some milo/sorghum varieties, and corn.
- Populations are usually highest in fields in continuous cotton or those rotated with soybeans.
- Reproduce rapidly and are usually found at high densities within only a year or two after entering a field. It is common to find population densities of 10,000 or greater per 100 cm3 of soil in Arkansas cotton fields. Once introduced, the reniform may become the dominant nematode.
- Can feed on many root parts, conferring an advantage over other nematode species.
- Where cotton is grown continuously, SCN populations will drop, and reniform nematode populations will continue to increase.
- Damage on cotton and soybean is more subtle; somewhat analogous to SCN, although SCN causes more obvious stunting in patches, according to John Mueller, plant pathology professor, Clemson University.
- Weed nematode hosts include pigweed, morning glory, sickle pod, prickly sida, and purple nutsedge.
- Reniform-resistant cotton varieties are in the pipeline that might appear next year, according to Auburn University’s Lawrence.
- Targeted nematicides manage reniform at most for the first 30–40 days, Lawrence says.
- A map of county reniform nematode populations is available at https://bit.ly/ReniformPopulations.
Southern Root-Knot Nematodes: What We Know
- The most important nematode attacking cotton. See Table 1 for the economic thresholds for root-knot and reniform.
- In South Carolina, root-knot might be the number 1 nematode problem across all crops, says Clemson’s Mueller. “It affects soybean, cotton, and corn to a lesser extent. It is in more than 35% of our row crop fields. It tends to severely damage, even kill, plants in patches across a field.”
- Causes much higher losses in soybeans than reniform nematodes, Arkansas’ Faske says.
- Prefers coarse, sandy, and sandy loam soils with a history of cotton production, Auburn’s Lawrence says.
- Just one root-knot nematode can reproduce into thousands, Faske says.
- Resistant root-knot cotton and soybean varieties are commercially available.
- “Resistance does not mean immunity; the nematode still attacks, so you need a systematic approach: nematicides, increased soil organic matter, plus all the tools you have to reduce stress and give that plant every opportunity to emerge successfully,” Lawrence says.
- “We have to manage our resistance and not lose a mode of action that takes so long to develop.”
- Management has traditionally relied on nematicides and crop rotation.
- Not typically seen in the same fields with reniform nematodes unless corn (a host) has been a rotation crop.
- The presence of this pest increases the presence and severity of Fusarium (soybean sudden death syndrome). The nematodes may also interact with seedling diseases such as Rhizoctonia and Pythium, resulting in very severe stand loss early in the season.
- Thrives on winter legume clover cover crops (see Table 2).
- Targeted nematicides manage nematodes for the first 35 days.
- Radishes, some mustards, and sunn hemp cover crops are a very poor host to root-knot nematode, but do not reduce their populations.
- A map of root-knot nematode populations by county is available at https://bit.ly/Reniform_Populations.
Table 1. Soil population density economic thresholds for cotton nematodes. Source: University of Arkansas
| Nematode | Time of year | |
|---|---|---|
| December–May | June–November | |
| no/100 cm3 soil | ||
| Root-knot | NAa | 50 |
| Reniform | 250 | 1,000 |
Table 2. Hosts and non-hosts to major Southern nematodes
| Crop/weed | S. root-knot | SCN | Reniform |
|---|---|---|---|
| Soybeans | Y | Y | Y |
| Cotton | Y | N | Y |
| Grain sorghum | Y | N | N |
| Peanuts | N | N | N |
| Corn | Y | N | N |
| Small grains | Y (variation here) | N | N |
| Alfalfa | Y | ||
| Tobacco | Y | N | Y |
| Annual morning glory | Y | ||
| Annual sow thistle | Y | ||
| Bahiagrass | Y | N | |
| Barnyardgrass | Y | N | |
| Beggarweed | N | Y | |
| Black nightshade | Y | Y | |
| Chickweed | Y | Y | |
| Coastal bermudagrass | N | N | |
| Common bermudagrass | Y | ||
| Coastcross | N | ||
| Cocklebur | Y | ||
| Crabgrass | Y | ||
| Cranesbill | Y | ||
| Crotalaria spectabilis | Y | ||
| Dallisgrass | N | N | |
| Dandelion | Y | ||
| Hemp sesbania | Y | ||
| Henbit deadnettle | Y | ||
| Horsenettle | N | ||
| Horseweed | Y | ||
| Jimsonweed | N | ||
| Johnsonsgrass | N | ||
| Lambsquarter | Y | ||
| Low hop clover | Y | ||
| Marigold | N | ||
| Mexican teaweed | Y | ||
| Mullein | Y | ||
| Mustard | Y (lots of variation) | N | |
| Nutgrass | Y | ||
| Nutsedge | Y | ||
| Penn. smartweed | Y | ||
| Pokeweed | Y | Y | |
| Purslane | Y | ||
| Ragweed | N | ||
| Redroot pigweed | Y | ||
| St. Augustinegrass | Y | N | |
| Sericea lespedeza | N | Y | |
| Sicklepod | Y | Y | |
| Spiny pigweed | Y | ||
| Spurge | Y | ||
| Tall ironweed | Y | ||
| Turnip | N | ||
| Tall fescue | N | ||
| Vetch | N | Y | Y |
Soybean Cyst Nematode: What We Know
- Can rob 30 to 50% of yield, says Iowa State’s Tylka.
- Prefers dry soils but adapts almost anywhere.
- Prefers more moderate temperatures than reniform and root-knot.
- In South Carolina, “SCN is definitely a problem in our soybean fields,” Clemson’s Mueller says. “We still have growers that monocrop soybeans. In these fields, it is a real problem. SCN and root-knot build to high levels.”
- In southern Illinois, “SCN isn’t the only big dog on soybeans in the South because root-knot and reniform feed on soybeans too,” says Jason Bond, plant pathologist at Southern Illinois University. However, in this area, winters still prevent reniform and root-knot nematodes from making inroads.
- Neither cotton nor corn is a SCN host, so they are good rotation crops to control SCN,” Arkansas’ Faske says. “However cotton is a host for reniform and root-knot nematodes, which cut cotton lint yield.”
- Choosing an effective resistant variety can make a 30-bu difference in your yields, according to annual Iowa State University SCN-resistant variety trials. Among SCN-resistant varieties, there’s a 51 to 81 bu/ac yield difference.
- Resistant soybean varieties are readily available and will perform much better than susceptible varieties when SCN is present; however, some resistant varieties may not work as well as they once did, says Carl Bradley, University of Kentucky plant pathologist. This is due to SCN populations evolving to be less affected by the primary source of resistance in most resistant soybean varieties. This source of resistance comes from the genetic soybean line PI 88788.
- What we once referred to as “races” are now known as “HG types.” Specifically, the population of SCN that has evolved to become less affected by the PI 88788 source of resistance is known as HG Type 2. HG Type 2 SCN populations have become the most frequent HG type observed across several soybean-producing states. In some areas, soybean varieties with different sources of SCN resistance may be available. Planting different SCN-resistant soybean varieties every time a field is planted to soybean will help improve SCN management overall.
- Planting Group V or earlier soybean varieties allows for an extra SCN generation to develop, which can double or triple a field’s population density.
- Rotate away from soybeans for at least two years to control SCN populations, says Lindsey Thiessen, North Carolina State University Extension plant pathologist.
- Corn is not an SCN host.
- In soybeans, there is usually either SCN or root-knot and reniform nematodes, Tylka says. “I have never seen or heard of all three nematodes in a field at the same time. We do not know why that is the case. So as far as I know, root-knot and reniform nematodes do not affect SCN and vice-versa.”
Nematode Control
A systems approach, using all your tools, is the most effective approach, Lawrence advises.
Soil Testing
Test your soils at least every three years, whether you think you have nematodes or not. Do this when plant biomass is the highest, right before harvest. You want to know what your highest nematode populations are, and that’s when they have the most food. It is vital to do this when the soil is moist, so as to capture nematode populations accurately. Dig 6-8 inches deep, capturing soil and roots.
Crop Rotation
A three- or four-year rotation with resistant crops is an effective prevention tool. Most of the cereal crops are fairly resistant to nematodes. Where peanuts are an option, cotton–cotton–peanut controls nematodes well, Mueller says. “Only peanut is a non-host for Columbia lance nematodes, SCN, southern root-knot, and reniform. We do not generally grow peanuts and soybeans in the same field because of the buildup of fungal disease (such as red crown rot and white mold/southern blight).
“Otherwise, cotton–corn–soybean, in any order, helps to control SCN because two-thirds of the crops are not SCN hosts,” Mueller says.
Seed Treatments and Nematicides
Seed treatments and nematicides are effective only against low nematode levels on cotton and soybean primarily, Mueller says. “We often have combinations of nematode species in a field. So if planting a resistant variety to root-knot or SCN, the seed treatment or low in-furrow rates may help with the ‘other’ nematodes or SCN races your variety is not resistant to.”
“Planting a resistant variety with a seed treatment usually enhances yields in the South,” Lawrence says. “The nematicide applied at planting will protect the growing seedling for approximately 7–40 days after planting. Both chemical and biologic seed treatments are effective against low nematode levels on cotton and soybean primarily.”
Here are some nematicide options from Mueller and Lawrence:
- AgLogic 15G, a granular, and Velum formulations, in-furrow sprays, offer some control on cotton, soybean, and peanut. They cost roughly $30/ac. See sidebar on the Velum Total reniform nematicide.
- Telone II, a fumigant, works on all species and all crops but is about $60/ac. It is effective in sandy soils but does not penetrate clay and silty soils well.
- Counter 20G, a granular, is available on corn for about $20/ac.
- Propulse, in-furrow sprays and seed treatment formulations, are being tested now on corn and cotton.
- Vydate C-LV is a foliar-applied nematicide available for nematode control in cotton.
Biological Controls
Nematode fungi or bacteria that specifically attack a given nematode are available and will become more common. “The ones available now drop the populations gradually,” Lawrence says.
Dig deeper
Zenglu Li, Nicole Bachleda, Brice Wilson, E. Dale Wood, James W. Buck, Thomas E. Carter, M. A. Rouf Mian, Benjamin Fallen, James Noe, Blair Buckley, Registration of G11‐7013 soybean germplasm with high meal protein and resistance to soybean cyst nematode, southern root‐knot nematode, and stem canker, Journal of Plant Registrations, 10.1002/plr2.20204, 16, 2, (430-437), (2022).
Text © . The authors. CC BY-NC-ND 4.0. Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.
