Reniform nematodes are marching through the Mid-South like Sherman rumbling through Georgia, according to one plant pathologist. But the reniform may have a slight edge on the Civil War general. Not many farmers are even aware that it's here.
The reniform nematode “is a tropical nematode that started in Louisiana and has been around 15 to 20 years,” says Gabe Sciumbato, plant pathologist at the Delta Research and Extension Center in Stoneville, Miss. “It's been moving around on farm implements and is now quite a serious problem in cotton.”
It's also a stealthy pest. Outward symptoms of a reniform nematode infestation include yield reduction and stunting. When the latter occurs across the entire field, it's difficult to tell that a problem exists. The reniform nematode can be seen only through a microscope.
On the other hand, infestations of the more-well-known root knot nematode can be picked up fairly easily because they leave visible symptoms on the roots of infested plants, noted Sciumbato.
Plant pathologists and others suspect that the relatively recent “plateau” in cotton yields in some parts of the United States may be related to reniform nematodes, although it's not the single reason.
The spread of the reniform nematode has become so alarming that a group of cotton producers and Sen. Thad Cochran, R-Miss., recently convinced the federal government to allocate funds for reniform nematode research at the USDA Jamie Whitten Southern States Research Station in Stoneville. Nematologist Lawrence Young was hired to head up the research, which will focus on both short- and long-term approaches to reniform nematode control.
For the long term, Young and USDA/ARS plant breeder Bill Meredith have begun developing the screening protocols for reniform-resistant cotton varieties. Growers should expect to see results from the project in 10 years.
In the meantime, Young is cooperating with Sciumbato on testing short-term alternatives like crop rotation, nematicides and the use of anhydrous ammonia to slow down the reniform's march through the Mid-South.
The most effective options may be to rotate into a crop that is not a host of the species of nematode in the field. Growers should routinely take soil samples to know what their treatment and rotation options are, plant pathologists say. The best time to sample is in the fall.
For the reniform nematode, rotation options include corn, grain sorghum, peanuts and nematode-resistant soybeans. Rotation partners for root-knot nematode include nematode-resistant soybean varieties.
Making the wrong choice of a rotation partner can be costly, according to Terry Kirkpatrick, plant pathologist at Arkansas' Southwest Research and Extension Center.
“If you put corn in a field that has a root knot nematode problem, you may not notice much damage to the corn, but the year you go back into cotton, your problem will be much worse than if you had just mono-cultured cotton. Before rotating, know your nematode.”
Unfortunately, many Mid-South producers have been reluctant to go to corn because of low prices and anxiety over the aflatoxin outbreak of 1998. “It's hard to justify going to a corn rotation when corn is $2 a bushel,” one cotton producer said.
But if cotton producers really want to reduce reniform nematode populations significantly in the short-term, rotation will have to be a serious option. “With the reniform, you almost have to go one year in corn and one year in cotton,” Sciumbato said.
There are other practices to control reniform nematodes, but don't expect consistent results. The applications of in-furrow nematicides like Temik have been erratic for reniform nematode control, according to Sciumbato. “Some producers have gotten some good results with it. Some people haven't. It has a lot to do with rainfall and how many nematodes you have.”
Nematicides applied in a side-dress application or the use of the Vydate as an over-the-top at pinhead square in conjunction with an in-furrow Temik treatment may produce better results than an in-furrow alone, but those results have been erratic, too.
Neither of those treatments may be as effective as Telone, a fumigant. But Telone II is expensive and the application can be time-consuming.
However, Kirkpatrick said that cotton producers in southeast Arkansas are finding that fumigation with Telone II is paying off. “They're going in preplant with a $35-an-acre Telone treatment because it's showing that they can make money with it.”
Sciumbato observed that where he put out Telone or Temik for nematodes, those plots tended to have higher numbers of nematodes at the end of the year. “You're controlling the nematodes until the cotton can make a crop. But those cotton plants have more roots at the end of the year and the nematodes are producing so fast.”
Kirkpatrick noted that root knot nematode is still the number one nematode in Arkansas in terms of costs to growers, but reniform nematode “is rumbling through like Sherman through Georgia.” “In 1990, we had verified reniform nematodes in only two counties. In one of those counties, it was only known to occur in one field. Now, it's in 12 counties spread as far north as Mississippi County.”
Kirkpatrick said some growers are rotating to rice to control reniform nematode “because in Arkansas, reniform tends to be in soils with a little higher clay content and a little better hardpan.”
One Mississippi cotton producer traces his problem with reniform nematodes to a change in the way he applied his nitrogen. “We were managing them pretty well when we were using anhydrous ammonia and Temik. Four years ago, because of safety problems with anhydrous ammonia and the cost of Temik and the fact that liquid nitrogen had become competitive, we left off Temik and anhydrous.”
The potential for anhydrous ammonia acting as a fumigant to control nematodes is being looked at closely by Sciumbato and Young. In 2000, Young reported a 26 percent increase in lint yield where he used anhydrous ammonia as a nitrogen source.
“It's been shown that higher rates of ammonia will disable the reniform nematode,” Young said. “The question is how big an area are we actually treating and is that enough. We'll be looking at different rates and applying on one side of the row versus both sides of the row.”
Sciumbato added that while the root knot nematode and reniform nematode are related, they're far from kissing cousins. “If you have both of them in the field, the reniform will take over and drive the root knot out.”