Cotton producers in five Southern states may have lost $150 million to damaging infestations of reniform nematodes in 2002, according to estimates. Throw in more than $100 million in losses from other types of nematodes, and you're talking about real money — a quarter of a billion dollars taken out of farmers' pockets by pests that cannot be seen without the aid of a microscope.
“There are many species of plant parasitic nematodes, but only a few have a significant impact on cotton,” says Don Blasingame, a retired Extension plant pathologist with Mississippi State University. “However, this impact can have severe economic consequences to the grower if managed improperly.
“Nematodes feed on plant roots, interfering with the uptake of water and nutrients, thus stunting growth,” he says. “Severe infestations can kill cotton plants.”
In the past, losses due to the microscopic worm-like organisms often were erroneously attributed to poor soil fertility, pH or “weak fields,” according to Blasingame, who works as a private consultant and continues to be active in groups trying to deal with the nematode problem.
Growers have become more aware of the threat posed by nematodes and how to identify nematode damage, but the drought conditions and economic problems of recent years — and the reduction in research dollars for nematodes — have made it difficult for them to make a dent in the problem.
“Breeding for resistance is the best method for control of any disease or nematode,” says Blasingame. “However, this is a long-term process, and we have some short-term needs at the present time. I wish we had varieties with resistance to nematodes, but we don't. So we have to use other control methods.”
If nematodes were like tobacco budworms or bollworms and could eat half the fruit off the plant as they did in 1995, they would have attracted considerable attention from the cotton industry.
But nematodes are much more insidious, causing a gradual, frequently unrecognized drop in crop productivity. “This is the most costly scenario because the grower does not realize how much is being lost,” says Blasingame.
This slow loss in productivity is compounded by the fact that some nematodes are not evenly distributed over the field, while others are.
“The rootknot nematode is found in sandy parts of the field, resulting in weak areas,” says Blasingame. “The reniform nematode is more evenly distributed, and the only thing the grower can detect is a slight stunting and reduced yields.”
The lack of obvious symptoms above ground makes sampling for nematodes below ground especially critical, according to Blasingame and other plant pathologists and nematologists.
“Correct sampling procedures can greatly improve the picture of what is really present in the field,” says Blasingame. “Nematode distribution can be very sporadic, ranging from high levels in one area to perhaps none in others. For this reason, several composite soil samples that cover areas of both high and low infestations should be taken.”
In general, soil samples should be taken in the fall, he says, although recommendations may vary in some states. (See accompanying article.)
In his earlier career, Blasingame directed a program that conducted extensive surveys of the nematode populations in Leflore, Coahoma and Sharkey counties in the Mississippi Delta in 1986. Those counties and others were again surveyed in the fall of 2001 and again in the fall of 2002.
In the time period covered by the surveys, the rootknot nematode population declined from 32 percent to 21 percent while the reniform population increased from 26 percent to 37.3 percent in Leflore County, according to Blasingame.
“Past research has shown that the reniform nematode is very aggressive and is found in high populations,” he noted. “In fields where both nematodes are present, over time the reniform nematode will increase in population, while the rootknot nematode will decrease.”
In Coahoma County, the rootknot population had declined from 26 percent to 18 percent, while the reniform had increased from 19 to 34 percent. In Sharkey County, rootknot had decreased from 10 percent to 2 percent while the reniform had risen from 69 percent to 89 percent.
“For reniform, which is obviously increasing in these and other counties, especially in the central and south Delta, growers must know their population levels and integrate chemical treatments and rotational crops for best control,” he said.
Blasingame said the 2001 survey indicated 34 percent of the total cotton acreage in Leflore County, 38 percent of the acres in Coahoma County and 64 percent of the acres in Sharkey County are above threshold levels and need treatment with a nematicide.
“It is apparent in looking at the percent of acres invested with plant parasitic nematodes found in the 2001-2002 survey is much higher than found in the 1986 survey,” he said. “It is also noted that where both major nematodes are found in a field or county that the reniform populations are, in most cases, replacing the rootknot populations.”
As each county is surveyed, specialists are providing growers showing the location and population of nematodes in the county. The publication also provides the latest control measures for growers to follow in managing nematodes.
More needs to be done, however, according to Blasingame and other specialists. Besides making growers aware of the reniform nematode problem and how to use current tools to try to manage it, researchers need to have more funds to broaden and fine-tune their recommendations.
“We cannot recommend control measures for reniform problems in cotton for the entire South based on research conducted in one location. Nematicides and crop rotation systems need to be evaluated in several regions of a state and over several states,” says Bill Gazaway, a retired Extension plant pathologist from Auburn University.
“We need to refine that knowledge to determine the most efficient application rates for nematicides and crop systems for the different soil types and growing conditions throughout the South.”
Case in point: Gazaway and other researchers have shown that a one-year corn and peanut rotation is highly effective in controlling reniform nematodes in Alabama — a reason peanut acreage has expanded recently throughout southwestern Alabama, where nematode pressure is exceptionally strong.
Even so, what works in Alabama may not work in other parts of the South, such as the Mississippi Delta, Louisiana or Arkansas, he notes.
“Some cotton nematologists in Mississippi say that corn may require two or more seasons to bring reniform down to manageable levels in parts of the Mississippi Delta,” he says. “And I don't know why. It could stem from a difference in reniform populations. Mississippi populations may be more aggressive and capable of producing larger populations than in Alabama.”
Gazaway, who also continues to work on nematode problems even though he is retired, says the fight to rein in nematode infestations with the lack of research dollars and educational efforts has been frustrating for plant pathologists.
“We've got growers going out of business and they don't even know why,” he says. “There is some on-going USDA basic research looking for genetic resistance. And this, if successful, will pay huge dividends 10 to 15 years down the road, but what good is it if we can't keep our growers in business long enough to profit from this research?”
As Blasingame and Gazaway are the first to concede, fine-tuning and improving the research already in place will cost money. What scientists need, they say, are adequately funded, broad-based rotation studies throughout the region provided through USDA and other public and private sources.
The same goes for nematicide and fumigant application studies, he says.
So far, virtually all of the research into reniform nematodes has been carried out through the Beltwide Research and Education Committee composed of university and Extension cotton nematologists throughout the South working on tight budgets. Both men continue to work on the committee.