Louisiana cotton farmers are facing increasing threats from high populations of nematodes – microscopic, parasitic worms that feed on plant roots. Of the two types most common, reniform nematodes are relatively new to the Louisiana delta cotton fields.
“The current population densities of nematodes Louisiana farmers must deal with are basically isolated to and seriously limiting production on the silt loam and fine-sand soils in the Louisiana delta as well as in other parts of the state,” said Gene Burris
, an entomologist at the LSU AgCenter’s Northeast Research Station
at St. Joseph.
Burris attributes the growing problems to continuous cotton planting. Cotton is such a good host for these worms that they continue to increase in population as cotton is planted in the same fields over several years. In time, he said, reniform populations can grow to exceed 100,000 per pint of soil and root knot populations can be found in excess of 5,000-6,000 per pint.
The consequences of nematode damage include not only reduced harvest but also lost efficiency of applied fertilizer, particularly nitrogen, which has become particularly expensive the past few years. Within fields, variations in soil texture and elevation can result in varying populations of nematodes and varying response rates to applied nitrogen. To address field variability, producers can use new technology that incorporates geographic positioning systems – GPS – to vary the application rate of fertilizer, applying different amounts based on the yield potential of the soil.
Burris and a team of researchers have been conducting studies to determine the causes of variation in cotton yield, define the correct nitrogen rates based on these variations and optimize yields. Other team members include Dennis Burns and Ernie Clawson at the Northeast Research Station; Keith Morris, an engineer in the Department of Biological & Agricultural Engineering; Kevin McCarter in the Department of Experimental Statistics; and Charles Overstreet and Maurice Wolcott in the Department of Plant Pathology & Crop Physiology. The researchers have used soil maps and measuring devices in conjunction with GPS to map fields based on soil type by measuring electrical conductivity in the soil, which can be used as a surrogate for soil type. Different soil types respond differently to nitrogen applications, and soils with the lowest electrical conductivity have the potential for the highest nematode populations.
Burris and his team conducted a nitrogen fertility experiment in a 108-acre field on the Helena Plantation near Waterproof, La. in 2005. The field was divided into strips that included a number of different soil types, and the researchers applied different rates of nitrogen to the strips. Some strips were given constant rates of nitrogen throughout. For other strips, however, the researchers used GPS technology to vary the nitrogen rated based on soil electrical conductivity. A similar test was done in 2006 in a 90-acre field with various soil types on a farm near Newellton, La.
In both tests, the researchers observed that dividing the fields into zones based on electrical conductivity was useful in identifying regions of the fields that responded to nitrogen treatments, Burris said. In addition, the same data showed which areas of the fields were affected by nematodes.
“Severe nematode damage and lack of nitrogen response were found to be associated with the low electrical conductivity portions of the Helena test field,” Burris said. And data from the nitrogen tests at the Newellton farm in 2006 suggested that high population densities of multiple nematode species resulted in excessive nutrient consumption and water use.
“In the test field, reniform and root-knot nematodes were found to be above the economic threshold,” Burris said of the Newellton farm.
Farmers have historically controlled nematodes through crop rotations and by using seed treatments and in-furrow and foliar applications of nematicides. These approaches haven’t been as successful as producers would like as nematode populations increased, Burris said. In addition, the nematicides are usually applied uniformly across a field, resulting in over-application and increased cost.
An alternative to these practices is to use a soil fumigant. These products, however, require specialized equipment and are expensive to use. One way to cut down on the expense, however, is to use GPS technology to identify the areas where nematodes are most likely to be present and selectively apply the fumigant in those areas.
“We want farmers to learn how to use fumigants to optimize yield,” Burris said. “They need to understand the seriousness of the yield losses nematodes can cause.”
He explained that fumigants, because they’re applied as liquids that form gasses, must be handled carefully. In addition, they must be applied at least three weeks before planting. Waiting time after fumigation is more critical when the weather and soils are cool and wet in early spring. This is more critical when the products are used on corn, which is planted earlier in the year than cotton.
One other advantage cotton has over corn is a later planting date. Because of this, the window for applying fumigants can include both late fall and early spring.
Burris and his research team are currently evaluating cotton yields with no nematode treatment, a seed treatment for nematodes and a soil fumigant. They’re also comparing these treatments with a variety of nitrogen fertilizer application rates.
“There’s a relationship between nematode activity and nitrogen,” Burris said. “Nematodes destroy roots, limiting the uptake of nitrogen. Farmers apply more nitrogen to compensate for lost roots and end up applying too much fertilizer and costing themselves money.” Rick Bogren