Smaller than the head of a pin. More damaging than boll weevils (in some locations). Able to increase populations rapidly. Widespread across the Cotton Belt. Difficult to control economically.
Nematodes, primarily root-knot and reniform, take huge chunks out of cotton farmers' incomes each year, and the best management technique may still be several years away.
“Nematodes have become significant pests across the Cotton Belt and take a heavy toll on production,” said Bob McLendon, a Leary, Ga., cotton farmer and former president of the National Cotton Council.
McLendon moderated a panel discussion on nematode management strategies recently at the Beltwide Cotton Conferences in Atlanta. Panelists from across the belt, representing growers, research and Extension, shared infestation data, loss potential, research needs and management techniques.
“This is an age-old problem,” said James Starr, Texas A&M University. Agricultural scientists “first discovered nematodes in the 1890s. We know they cause significant yield and economic losses and that management will improve profit potential.”
Starr said nematodes come in a wide variety of species and races but root-knot is the most widespread. “In Texas, we see a lot of root-knot nematodes in the Panhandle. Reniform nematodes are increasing in Louisiana and are severe in Alabama. The Columbia lance primarily infests South Carolina fields but also shows up in North Carolina and Georgia. Damage from the lance can be severe.”
Infestations often go undetected, Starr said, because nematodes do their work below ground. “Often, they leave no discrete symptoms, but a farmer can lose from 5 percent to 10 percent of his yield and see no warning signs. Fortunately, we're aware the problem has increased in the last 10 years, but we still need more support for research.”
Critical needs include resistant or tolerant varieties, he said.
John Shackleford, a Bonito, La., farmer, said reniform nematodes have emerged as a major pest problem. “I know way more about reniform nematodes than I wish I knew, but I still don't know enough.”
He agrees that resistant varieties hold the most promise for adequate management. “But that's still a ways down the road. With transgenic varieties, however, we may be closer. Unfortunately, little research is being done.”
He hopes a reniform nematode action committee will help bring the problem to the forefront. “We need to begin identifying some common varieties that have tolerance.”
For the time being, Shackleford depends mostly on rotation to manage the problem. “With high numbers, we have to rotate or treat with pesticides,” he said. “We've used Telone, but with cotton so cheap, we can't afford it.”
He said cotton and peanuts provide an excellent combination. “I also plant corn two years in a row to reduce the numbers. Even with that, I can still find counts as high as 20,000 per sample.”
David Wildy, Manila, Ark., farmer, battles more root-knot than reniform nematodes. “I have a lot of sandy soil,” he said. “Boll weevils and boll worms are not as big a threat as they are further south, but nematodes are gaining on us. We're beginning to pick up numbers above treatment levels. The trend on my farm is alarming.”
In 1998 he found 50 nematodes per sample. By 2000, that number had jumped to more than 800 per sample.
Populations, he said, are distributed across the field and range from four per sample to more than 700 (per 100 cc of soil).
“Telone gives us a yield increase of 53 pounds per acre, but (at current prices) the expense is not justified. Temik, applied side-dress, provides a 50-pound increase and a slight profit improvement,” Wildy said.
He agrees with Shackleford that variety tolerance holds the key for management. LA 887 may be a possibility, he said.
More-accurate treatment thresholds, better sampling techniques, improved procedures for handling living organism samples, and economical control options top his list of research needs.
Gary W. Lawrence, professor with Mississippi State University, said the reniform nematode “is becoming the most serious pest in the 11 Southeastern states. The pest infests some 1.2 million acres, 19 percent of the region's cotton. Infestation levels range from 1.4 percent to 55 percent. Mississippi, Louisiana, Alabama and Georgia have high numbers of reniform nematodes.
“The pest is difficult to see,” Lawrence said. “We know yields decline year after year. If we pull plants, we can see the pests, including small egg masses and kidney-shaped nematodes in the root system. Root necrosis may also be apparent.”
Lawrence said 1,000 nematodes per 100-cc sample could reduce yield by 7 percent. “As populations increase, yield loss goes up. In Mississippi, we've seen a 153,000-bale reduction from nematode damage.”
Management, Lawrence said, depends on a combination of tactics, including rotation (resistant grain sorghum, corn, peanuts and wheat), tolerance (DP 420 RR), and chemicals (Temik, Telone, Vydate).
Precision application techniques, according to Texas A&M researcher Terry Wheeler, currently provide little help in controlling nematode populations. “Sampling costs,” she said, “limit our opportunity to profit from precision ag technology. Sampling is expensive, difficult and time-consuming. And we've seen no significant improvement in yield with variable rate applications versus total field treatments.”
Wheeler said sampling techniques are not accurate enough. Field populations range from 30 percent with no infestation, 30 percent with moderate and 30 percent with high levels.
“Also, yield losses vary according to weather and other factors. Chemicals must be applied at an effective rate to reduce nematode numbers and pesticides must be applied accurately.”
She said nematodes have “to be costing farmers yields before treatment is justified.”
Gus Lorenz, University of Arkansas, said nematodes “make life difficult for growers. After they put seed in the ground, they begin to lose choices. Farmers have to know what they need to do before the season starts. They need to know if they have nematodes and then they can consider control options.”
Lorenz said pesticide options for nematode and thrips control include Orthene, Temik, Di-Syston, Thimet, Gaucho/Adage, Telone and Vydate. “Temik is the best choice for in-furrow treatments for nematodes and thrips,” he said. “We get about a 50-pound yield advantage. Still, nematicides will not turn a sorry field into a good one, and they work better some years than they do others.
“Nematodes are like weeds,” he said. “We can never control them, but we have to manage them. We usually don't see a crop failure, but nematodes will take the high off a good year and make a bad year even worse.
“We have no new management techniques, no new chemicals, no transgenic cultivars and no fertilizers or plant growth regulators to help control nematodes. And we can't manage them if we don't know what's out there.”
Phil Roberts, University of California, Riverside, said an assay of root galls may prove a better sampling technique than counting pests in soil. He said an Acala type cotton, NemX, shows some resistance to root-knot nematodes. “Even without treatments, it shows tremendous yield differential compared to non-resistant. This is not the greatest variety so it behooves us to put resistance traits in good varieties.”
Until that happens, he said, farmers have to rotate.
Root-knot nematodes are the major problem in southwest Georgia, said Eddie McGriff, Decatur County agent. “We find some reniform in heavier soils.”
Two root-knot species infest the area, the Southern and the peanut. “Cotton is a non-host to the peanut root-knot nematode. And peanuts are non-host to the Southern, so we have a good rotation option.”
McGriff said farmers should watch for stunting, reddening of leaves and interveinal necrosis. “And pull roots up carefully for samples or the galls will fall off.”
He said routine sampling techniques will not differentiate between the two root-knot species. “We have to pull samples and grow them on a tomato so we can tell by the galls.”
Sampling is necessary to determine if chemical treatment is justified, McGriff said. He said Telone and Temik are excellent options. Temik side-dressed is “very good.”
He recommends that farmers “consider the field history. On acreage with multiple years of cotton, walk the fields carefully and look for symptoms.”
McGriff said rotation helps, but some corn varieties are not as good as others. “Farmers in this area have changed corn hybrids and some are more susceptible to nematodes. Southern nematode populations will carry over from corn, so we prefer not to plant cotton behind corn.”
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