The aim of precision agriculture is to help growers maximize their economic returns, and new methods of detecting and controlling nematode populations in a field is certainly in keeping with that aim.
With precision agriculture, we try to refine things in order to achieve the goal of maximizing returns,” says Richard Davis, USDA-ARS plant pathologist in Tifton, Ga. “And when we talk about precision agriculture in nematode management, what we really end up talking about is more effective targeting of nematode applications.”
Many of the things growers do for managing nematodes involve making applications over entire fields rather than specific parts of a field, says Davis.
“One of the things we can do to more effectively target nematicide applications is to increase our precision in nematode sampling. The idea is to find out where nematode pressure is in the field — where it’s high, where it’s low, and where to target your nematode management zones,” he says.
The areas in the field with similar characteristics, including soil texture, slope and elevation, among others, are more likely to have similar nematode levels and to suffer similar levels of damaged compared to areas with different characteristics, explains Davis.
“Management zones are designed to minimize variability within a zone and maximize the differences among the zones,” he explains. “Once you’ve done that, you’ll still have to sample the zones to find out what is actually there. You’re just trying to draw boundaries so you can better target where the pressure is likely to be high or low.”
When GPS first became widely available for use in agriculture, grid sampling was one of the first approaches, says Davis. “This is where you draw a grid in the field and then go out and collect your samples from each of the cells in that grid. But it’s not practical because it’s very costly and takes a lot of time and labor. It’s not that it doesn’t work — it’s just not very practical. One method that has allowed us to get around this problem is the development of the Veris rig that allows us to collect soil electric conductivity (EC) data. It’s fast, easy, cheap and very well correlated with soil texture,” he says.
The result of this method, he adds, is that growers have a very good map of where soils are sandier or heavier in a specific field. Nematologists have known for a long time that soil textures can have an effect on nematode population levels, he says.
“Certainly, soil textures can also have an effect on other factors like plant growth,” explains Davis.
As part of a large project funded in part by Cotton Inc. and the Georgia Cotton Commission, the Precision Farming Team at the University of Georgia has been evaluating a number of techniques for delineating areas within fields at high risk for nematodes.
The fact that root-knot nematodes prefer sandy areas has encouraged researchers to find ways to rapidly measure soil texture — either directly or indirectly — and one of the most promising techniques is to directly measure soil EC. Soil EC is a function of soil texture and soil moisture. Sandy soils produce low soil EC while heavier soils result in higher values of soil EC.
While different instruments have been developed to measure soil EC, one of the most popular is the Veris 3100. This instrument has six coulter-electrodes (disks) mounted on a toolbar. As the Veris is pulled through the field, one pair of disks transmits an electrical current into the soil while another pair of disks measures the drop in voltage. The separation between the disks determines the depth to which soil EC can be measured. In the most commonly used configuration, soil EC is measured simultaneously from 0 to 1 foot (shallow) and 0 to 3 feet (deep).
In addition to directly measuring soil EC, there are other promising methods for indirectly measuring soil texture. These include using real time kinematic (RTK) GPS to rapidly create detailed topographic maps of fields. Elevation and slope of the terrain frequently dictate where coarse textured soil particles are deposited by erosion.
Soil color is generally associated with differences in soil texture, so bare soil aerial photographs or images taken from satellites can be used to identify these features.
Nematodes are found throughout the U.S. Cotton Belt, and in Georgia, root-knot nematodes have been found to infest as many as 68 percent of sampled fields in 67 cotton-producing counties. Research by nematologists have shown that root-knot nematodes prefer sandy soils and consequently aggregate in sandy patches of fields.
Root-knot nematodes damage the roots of cotton plants by forming knots on the roots. These knots interfere with the plants’ ability to take up water and nutrients and result in stunted plant growth and reduced yields.
Because root-knot nematodes are often more widespread than reniform nematodes, most of the research in precision agriculture up until now has been with managing root-knot nematodes, says Davis.
“In Georgia, for example, we have drawn nematode management zones based on soil physical characteristics like soil EC and elevation. This has been reasonably effective with root-knot nematodes, with soil EC being the primary driving force in driving these maps. And there’s software available that draws these maps for us. It takes the variables we want to use and groups them to minimize the variability within the zone and maximizes the variability among the zones.
“But we wanted to see if what we were doing with root-knot nematodes would transfer and work reasonably well for reniform nematodes.”
Davis says objectives of this latest research included evaluating the effect of soil physical characteristics on reniform nematode populations and the yield reductions they cause in cotton. In addition, an aim of the research is to evaluate the influence of field physical characteristics on the effect of nematicides in reniform nematode-infested cotton field.
Results of the study have shown that defining nematode management zones based on a field’s physical characteristics may be useful for managing reniform nematodes, though the utility is likely to increase as the range of soil textures and elevations in the field increase.