GIS/GPS Techniques help evaluate soil insect and nematode pest control strategies in sweet potatoes

Linda Benedict, Smith, Tara, Burns, Dennis, Burris, Eugene, Wolcott, Maurice C., Overstreet, Charles  |  6/15/2009 8:59:15 PM

Figure 1.

Figure 2.

Eugene Burris, Dennis Burns, Tara P. Smith, Charles Overstreet and Maurice C. Wolcott

Most sweet potato production in Louisiana lies within the Macon Ridge terrace that starts near Eudora, Ark., and ends near Sicily Island, La. Ter­race uplands of the ridge make up about 75 percent of West Carroll and Franklin parishes. The prominent ridge consists of almost entirely early-Wisconsin glacial outwash. The eastern edge of the region is higher by 20 to 30 feet compared with the western edge. A veneer of loess soil covers 75 percent of the region. Manag­ing crops on these loess soils of the Mid­south is difficult because they contain little organic matter, have a permanent hardpan, are droughty and erode easily. Generally, these fields are small com­pared to delta alluvial fields; however, they often have considerable variability in soil texture. This variability is often created by land-leveling for irrigation and drainage improvements. The reni­form nematode is another problem not uniformly distributed within these fields. This variability presents producers with an opportunity to manage these problems in a site-specific manner.

Recently, LSU AgCenter research and extension personnel have attempt­ed to characterize sources of variability within loess soils using the Veris cart, which measures apparent electrical con­ductivity (EC) in the soil. These data combined with real-time kinetic geo­graphical position systems (GPS) can be used to characterize in-field soil texture variability. The Veris cart provides a method for categorizing regions of a field into low and high silt and clay content, and Veris EC values are highly correlated to clay content. Because of the narrow range of electrical conductivity measured in the loess soil regions, three ranges determined by natural breaks were typi­cally used to characterize the field into zones (Figure 1).

Geographic information system (GIS) tools – computerized systems used to compile, retrieve, analyze and display spatially referenced data – were used in combination with GPS to develop pre­scriptions for site-specific applications of lime and fertilizer for correcting soil pH and nutrient deficiencies and fumigation for managing nematodes.

Research using these techniques in Louisiana sweet potato production is limited. The results from these studies, however, show GIS/GPS techniques have the potential to help sweet potato produc­ers fine tune lime and fertilize treatments as well as provide relief from costly blanket applications of nematicides – both major expenses incurred in sweet potato production.

In initial studies conducted on the Macon Ridge, one-acre grids were used to develop soil sample plans (Figures 1 and 2). Grids overlaid on GIS maps served to organize placement of the GPS sample points. The goal for sampling was to collect soil from the dominate EC range featured in the grid (Figure 1). Therefore, the final soil sampling proce­dure was derived from a combination of scientific and subjective techniques. The grid-management technique also was used to track early movement of cucumber beetles on one farmer’s field (Figure 2).

In 2007, researchers used these tech­niques to evaluate insects across irrigat­ed and non-irrigated portions of a field (Figure 1). Insect data were collected routinely throughout the season from one-acre grid plots. Sweep-net collec­tions from individual grids were used to provide data on cucumber beetles, click beetles, armyworms, loopers and crick­ets. The grid pattern allowed observa­tions of early insect movement across an 85-acre field. Sampling using this method indicated that early emergence of cucumber beetles was variable enough to justify use of site-specific treatments for the first one to two insecticide applica­tions. Sampling in this manner, however, is time-consuming and labor-intensive.

In 2008, fields were sampled on a farm in northeast Louisiana near Oak Grove. In cooperation with the producer, Veris data and soil samples from one-acre grids were collected prior to plant­ing in the spring. Several opportunities for site-specific lime applications to cor­rect soil pH were found, and grid-sam­pling data could be used to make site-specific treatments for reniform nema­todes in some fields.

Using these methods, LSU AgCenter researchers consistently have observed and documented GIS-referenced dif­ferences in soil pH, potassium, phos­phorous, zinc, boron and sulfur. Since reniform nematodes thrive and reproduce throughout all EC zones, grid sampling is the most effective means of charac­terizing variability in reniform nema­tode population densities. Site-specific prescription treatments for soil pH, soil fertility and reniform nematodes can be developed using both soil EC data and data from grid sampling.

Equipment for site-specific delivery of nematicides was generally not avail­able at the farms; however, several farm­ers recently purchased fumigation equip­ment with GIS/GPS capability.

Eugene Burris, Professor, Northeast Research Station, St. Joseph, La.; Dennis Burns, Agri­cultural and Natural Resources Agent, Tensas Parish, St. Joseph, La.; Tara P. Smith, Assistant Professor, Sweet Potato Research Station, Chase, La.; Charles Overstreet Professor, and Maurice C. Wolcott, Instructor and Extension Specialist, Department of Plant Pathology & Crop Physiol­ogy, LSU AgCenter, Baton Rouge, La.

(This article was published in the spring 2009 issue of Louisiana Agriculture.)

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