U.S. cotton production losses to nematodes have climbed from 1 percent to about 5 percent since 1988, with the reniform nematode increasing in severity during that time, said Bill Gazaway, Auburn University Extension plant pathologist/nematologist and emeritus professor.
“Part of the increase is due to more awareness by our scientists. But most of the losses are due to the reniform nematode,” said Gazaway at the Beltwide Cotton Production Conference in New Orleans.
The U.S. cotton industry lost an estimated $402 million to nematodes in 2004. About $214 million (548,000 bales) was lost to the root-knot nematode, $166 million (425,000 bales) to the reniform nematode, and $22 million (57,000 bales) to lance and other nematode species.
“This is a significant increase, and it's one we need to be concerned about,” said Gazaway. “The root-knot is responsible for 53 percent of our total nematode losses; the reniform is responsible for another 41 percent. The remaining 6 percent is due to lance and other nematode species.”
The root-knot nematode, he said, has been around since cotton has been grown. “This nematode appears throughout the Cotton Belt, from California into Virginia and North Carolina. This is the most commonly known nematode. However, it is pretty much static. It's not spreading, and its damage generally is restricted to sandier soils. It causes damage to localized areas of a field and doesn't necessarily cause damage throughout the field,” he said.
The lance nematode is mostly restricted to the Southeast, said Gazaway, with the majority of the damage being found in Georgia, South Carolina and North Carolina.
Prior to the 1980s, the reniform nematode was considered a minor pest, he said. “Since then, it has spread at an alarming rate to all areas throughout the Southeast. In fact, Louisiana, Mississippi, Alabama and parts of Arkansas sustain about 80 percent of the damage from reniform nematodes.”
What's more alarming, he said, is that the reniform nematode continues spreading to other states. “Unlike the root-knot, the reniform can damage an entire field. We've documented losses in fields up to 75 percent under stress conditions.”
The National Cotton Council's Nematode Research and Education Committee, supported by various industries, has been responsible for the majority of the nematode research and education efforts of the past 15 to 16 years, said Gazaway.
“Looking at our current recommendations, the first line of defense is nematicides — Telone II and Temik 15G. They have done a tremendous job for us. Often, we've relied too heavily on these materials. They're very effective provided they are used in conditions which are favorable for their activation. The problem is that they're not always effective when they're used in less-than-favorable conditions, or when nematode populations have reached the point where they can't be controlled.”
Crop rotation also has proven effective in controlling nematodes, especially the reniform species, said Gazaway. “We've found that crop rotation is greatly effective in reducing reniform populations. In Alabama, we've been able to do one year out of cotton in a non-host crop such as corn or peanuts, and we've found that to be just as effective as a nematicide.”
However, there are disadvantages to rotation, he said. One disadvantage is the lack of a suitable, economical, non-host crop.
“In our part of the country, we have corn and peanuts. In other parts of the country, most growers have only corn or grain sorghum. Another problem with rotation, of course, is that it isn't always economical in most situations because many producers rest their land. They don't have an agreement with their landlord to give them a break on their rent if they grow a non-host crop.”
As for resistance or tolerance, Gazaway said, there is some tolerance to the root-knot nematode. “We've had a breeding program for years with the fusarium wilt/root-knot complex, and thanks to that, we've had pretty good varieties to resist that complex. We have no state-controlled screening programs for the root-knot.”
There is no resistance in commercial varieties to the reniform nematode, he said.
Exclusion is another means of controlling nematodes, said Gazaway. “Simply keeping them out of your fields is very effective. The reniform nematode is extremely proficient in spreading by dirt particles and dirt clods on old, used equipment. We've made an effort to tell growers and other people who buy used equipment to at least knock off the dirt clods when going from an infested to a non-infested field. But we haven't had a lot of success with that.”
There are challenges for the future in improving nematode management systems, he said. “First, we need a quick way of identifying and locating the culprits. Research is being conducted with remote sensing and infrared remote control technology. In identifying the nematode itself, we're looking at methods that use carrier fungi and viruses. We're also looking at DNA analyses.”
Improvement in soil sampling technologies is also needed, said Gazaway. “We're sampling in 6-inch zones. But we've seen evidence that these nematodes often are far below that level. Research in Texas has shown that most of the nematode populations are not in that 6- to 8-inch zone but closer to a foot or a foot and a half down. We're obviously missing some populations whenever we take samples.”
Researchers also are looking at more efficient means of applying existing nematicides, he said. “We're looking at precision application at variable rates. We're also looking at applying according to soil type or nematode population. We already can spot applications, and that is especially helpful for root-knot nematodes, which occur in spots in a field. Growers can save a lot of money by concentrating only on affected areas.”
Another research priority, he said, is looking at nematicides — especially fumigants — in relation to no-till or minimum tillage systems.
“We also need to look at the biology of the reniform nematode. We know something about it, but there are areas that we don't completely understand. We have indications that population differences and ability to reproduce vary from region to region and from state to state.
“In addition, we need to know more about the reniform nematode host ranges, particularly for an effective rotation program.”
Researchers also need to understand the role of the environment on the reniform nematode, said Gazaway. More rotation studies also are needed, he adds.
“We need more rotation studies over a wider area. This is due to the fact that we suspect we're dealing with different populations of reniform in different growing conditions and in various regions. We also need more economical rotation crops. In addition, we need more information on the role of weeds in controlling nematodes.”
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