Every year, cotton growers face two invisible pests that strike without warning, leaving rows of stunted, withered plants. The root knot and reniform nematodes are among a cotton grower’s most devastating enemies. Because they do most of their damage at the roots, they can destroy entire rows of young cotton plants before growers realize they have a problem. In fact, both species of nematodes together cause average losses of over 2% of the total U.S. cotton crop every year, valued over $100 million.
But controlling these nematodes poses a number of dilemmas for growers. Do they use a soil fumigant or spread nematicide on the soil surface? Do some areas need more treatment than others? Because each species prefers a different soil type, and some populations are more intense than others, treatment decisions can be both complicated and costly. Scouting the field by taking soil samples to find those populations is time intensive, and lab analysis of those samples is expensive. Conversely, applying a soil fumigant or a surface nematicide can reduce profits if the grower applies the wrong nematicide or an inaccurate amount.
Several researchers say that the key to nematode management may lie in the soil preference of each nematode species. Since root knot nematodes prefer sand, while reniform nematodes cluster in silty loam, finding the soil types present in a field may help predict locations where nematode problems will occur. To do that, researchers are testing electrical conductivity (EC) systems that analyze soil texture. (A popular example of an EC system is the Veris cart from Veris Industries.) From that analysis, soil-type maps are created.
Louisiana State University nematologist Charles Overstreet uses the maps to identify which places in a field are most likely to house root-knot and reniform nematode populations. “The lower the reading, the more sand in the field,” Overstreet says. “The higher the number, the more clay you have. Once you have clay, you don’t have as many problems with nematodes.”
The maps provide Overstreet and his team – GIS expert Maurice Wolcott, entomologist Gene Burris and plant pathologist Boyd Padgett – with clues about why some areas of the field were unproductive even with nematicide treatments.
“Growers assume the areas of low yield are those where the nematodes are,” Overstreet says. “But low yields may be just from other soil problems, or they may be from nematodes.”
The maps help Overstreet’s team create “management zones” based on soil type. These zones indicate where nematicide may be needed, and where it won’t have any benefit.
USDA soil scientist Dana Sullivan has had similar results in Georgia, where she and University of Georgia researchers Calvin Perry and George Vellidis, Extension Agent Keith Rucker and doctoral student Brenda Ortiz have been testing electrical conductivity measurements since 2005 to locate “hot spots” of root knot nematodes. They use the technology to create homogeneous zones by soil type and other geological variables such as topography and slope.
“Electrical conductivity measurements provide an important key to delineating areas most likely to have high populations of nematodes,” Sullivan says. “For the fields that were not completely infested, we may be able to tell growers, ‘If your field is going to have problems, it is most likely to be here.’”
While using soil EC as a flag to point to potential nematode management zones is a good first step, simply having soils that are favorable for nematodes is not the whole picture. The next questions are: Are nematodes present in these zones? And are the population densities high enough to justify the expense of a nematicide?
Those questions are currently driving the research into this technology. Sullivan’s team is beginning this year to evaluate nematicide rates in each zone. Overstreet, who began testing the EC system for nematode management in 2001, divides the management zones into “verification strips,” half treated with a soil fumigant, and half left untreated as a control area. The strips help validate not only the presence of nematodes in a zone, but also that the pests are the primary cause of any crop loss.
“We still don’t know where the transition zones are; that’s where the verification strips come into play,” Overstreet says. “Some farms we worked with had four or five textures in a field; that makes it very difficult to break it down without knowing where the textures change.”
Stand damage from the reniform nematode is often
As the name implies, root knot nematodes cause knotty galls
that interfere with moisture and nutrient uptake.
Example of nematode data from a field, broken down into 4 classes.