Root-knot nematode reproduction and root galling severity on related conventional and transgenic cotton cultivars.

The root-knot nematode (Meloidogyne incognita Kofoid & White), a widespread and serious pest of cotton (Gossypium hirsutum L.) throughout the Cotton Belt, is managed in many areas in part through cultivar resistance. Recently, commercial cotton cultivars modified with genes for resistance to the tobacco budworm (Heliothis virescens F.), to glyphosate herbicide (e.g., Roundup, Monsanto, St. Louis, MO), or in some cases to both the budworm and the herbicide have been released. The objective of this study was to compare the root-knot nematode resistance or susceptibility of several transgenic cotton cultivars with that of their unmodified parent cultivars. The cultivars were evaluated in a field naturally infested with the root-knot nematode and in a growth room in pots infested with the nematode. A dramatic increase in root-knot nematode susceptibility was seen in the transgenic cultivar, Paymaster 1560 BG, compared with its nontransgenic parent, Paymaster 1560. Although only a limited number of cultivars were studied, the data demonstrate that differences in susceptibility to the root-knot nematode exist between some transgenic cultivars and their nontransgenic parents. These data indicate the importance of screening transgenic cultivars for resistance to pests other than the particular pest species targeted by the genetic modification before the transgenic cultivars are recommended for planting. The root-knot nematode Meloidogyne incognita is a widespread and serious pest of cotton throughout the Cotton Belt (Kinlock and Sprenkel, P.D. Colyer, W.D. Caldwell, and P.R. Vernon, Louisiana Agric. Exp. Stn., Red River Res. Stn., P.O. Box 8550, Bossier City, LA 71113-8550; and T.L. Kirkpatrick, Univ. of Arkansas, Southwest Res. and Exp. Stn., Hope, AR 71801. Received 6 Jul. 2000. *Corresponding author (pcolyer@agctr.lsu.edu). 233 COLYER ET AL.: ROOT-KNOT NEMATODE DAMAGE SEVERITY ON COTTON CULTIVARS 1994; Martin et al., 1994; Baird et al., 1996; Bateman et al., 2000). Yield suppression in cotton is due primarily to root damage and extensive galling that are produced by nematode infection and result in the inhibition of translocation of water and nutrients to the plant (Bridge, 1992). In addition to direct damage, such as a reduction in the number and size of bolls, nematode infection also may delay maturity (Walker et al., 1998). Moderate to high levels of resistance to M. incognita have been reported in upland cotton (Shepherd, 1974; Jones et al., 1988, 1991). Although breeding lines with this resistance have been developed (Kirkpatrick and Shepherd, 1989; Cook et al., 1997; Robinson and Percival, 1997), only three commercial cultivars exhibit moderate resistance: Stoneville LA 887, Paymaster 1560, and CPCSD Acala Nem-X (Robinson and Percival, 1997). While these cultivars have been effective in lowering yield losses due to the nematode in some regions, limitations in both cultivar adaptation and seed availability have prevented their widespread use across the Cotton Belt. Several genetically modified cotton cultivars are commercially available, including Paymaster 1560, which contain genes that provide resistance to the tobacco budworm, to glyphosate herbicide, or in some cases to both the budworm and the herbicide. In many cases, these cultivars are genetically modified selections from existing conventional cultivars. Many cotton producers have assumed that phenotypic and genotypic characteristics of the transgenic cultivar are similar or identical to the parent, except for the expression of the introduced trait. The objective of this study was to compare the root-knot nematode resistance or susceptibility of several transgenic cotton cultivars with that of their unmodified parent cultivars. MATERIALS AND METHODS