supported universities in the United States have projects targeted at the introgression of reniform nematode resistance into agronomic upland cotton, from primitive tetraploid accessions of Gossypium hirsutum and G. barbadense, and diploid G. arboreum, and G. longicalyx. This task will take many years to complete. However, significant progress has been made toward developing cultivars carrying resistance from each source, and the first resistant cultivars could appear within three years, with committed follow through by commercial planting seed companies.
Keywords: cotton, Gossypium hirsutum, introgression, molecular marker, reniform nematode, resistance, Rotylenchulus reniformis
The reniform nematode (Rotylenchulus reniformis Linford & Oliveira) (Gaur and Perry, 1991; Lawrence and McLean, 2001; Starr et al., 2002) is an increasing problem in cotton (Gossypium hirsutum L.) production in the eastern half of the United States Cotton Belt. It is estimated to incur annual losses of approximately $130M, with major impact in the states of Mississippi, Louisiana, and Alabama (Blasingame, 2006; Koenning et al., 2004; Robinson, 2007). Because there are no cultivars with high levels of resistance to this nematode (Robinson et al., 1999), management options are limited primarily to nematicides (Kinloch and Rich, 2001; Lawrence et al., 1990; Overstreet and Erwin, 2003; Zimet et al., 1999) and crop rotation (Davis et al., 2003; Gazaway et al., 1998, 2000), which often return only a fraction of the profits lost to reniform nematode damage. Most rotational crops are less profitable than cotton and provide cotton yield boosts only during the first year that cotton is grown following the rotation (Davis et al., 2003). Another, partial solution has been to identify tolerant genotypes that suffer less damage than do typical cultivars. A drawback of tolerance is that tolerant genotypes typically support high levels of nematode reproduction, and thus cannot be used to reduce the nematode population density in the soil. A second drawback is that tolerance appears to be highly environment-dependent, making the development of widely adapted tolerant cultivars unlikely (Koenning et al., 2000). More than a dozen breeding lines and cultivars exhibiting some degree of reniform nematode tolerance have been identified (Cook et al., 1997a, 1997b, 2003; Cook and Robinson, 2005; Jones et al., 1988; Koenning et al., 2000; Sciumbato et al., 2005; Stetina et al., 2006; Usery et al., 2005). Because discovery of tolerance provides an immediate solution to the problem, the search for tolerance remains an important research priority. Recent studies have identified several breeding lines and cultivars with potential for use in the important Mississippi Delta production region (Sciumbato et al., 2005; Stetina et al., 2006).
Reniform nematode reproduction has been evaluated on more than 3,000 genotypes of plants in the genus Gossypium in order to discover sources of resistance for developing highly resistant cultivars (Beasley and Jones, 1985; Carter, 1981; Muhammad and Jones, 1990; Robinson, 2002; Robinson et al., 1999, 2001, 2004, 2006; Robinson and Percival, 1997; Stewart and Robbins, 1995, 1996; Weaver et al., 2007; Yik and Birchfield, 1984). Only weak to moderate resistance has been reported in G. hirsutum, but high to very high levels of resistance have been found in several Gossypium species, including G. anomalum Wawr. & Peyr., G. arboreum L., G. barbadense L., G. herbaceum L., G. longicalyx Hutch. & Lee, G. raimondii Ulbr., G. somalense (Gurke) Hutch., G. stocksii Mast. In Hook., and G. thurberi Tod. (Yik and Birchfield, 1984; Robinson et al., 2004; Stewart and Robbins, 1995).
The most information regarding resistance to the reniform nematode is available for G. hirsutum, G. barbadense, G. arboreum, and G. longicalyx. Most accessions of G. barbadense, a species which freely hybridizes with G. hirsutum, are susceptible to the nematode and resistant G. barbadense accessions usually suppress nematode populations by only 70-90% (Robinson et al., 2004). In contrast, many accessions of G. arboreum, from