The Future of Nematode Management in Cotton

The importance of plant-parasitic nematodes as yield-limiting pathogens of cotton has received increased recognition and attention in the United States in the recent past. This paper summarizes the remarks made during a symposium of the same title that was held in July 2007 at the joint meeting of the Society of Nematologists and the American Phytopathological Society in San Diego, California. Although several cultural practices, including crop rotation, can be effective in suppressing the populations of the important nematode pathogens of cotton, the economic realities of cotton production limit their use. The use of nematicides is also limited by issues of efficacy and economics. There is a need for development of chemistries that will address these limitations. Also needed are systems that would enable precise nematicide application in terms of rate and placement only in areas where nematode population densities warrant application. Substantial progress is being made in the identification, characterization and mapping of loci for resistance to Meloidogyne incognita and Rotylenchulus reniformis. These data will lead to efficient marker-assisted selection systems that will likely result in development and release of nematode-resistant cotton cultivars with superior yield potential and high fiber quality.Cotton (Gossypium hirsutum) is the most important fiber crop in the world, and current lint production in the US accounts for nearly one quarter of world supply. The land devoted to cotton production in the US peaked in 1926 at approximately 18 million hectares. The advent of mechanized farming and the availability of effective, relatively low-cost fertilizers, pesticides and improved cotton cultivars after World War II allowed the production of significantly greater yields per unit area and total hectares planted declined. United States production of cotton lint in the past 5 years has varied from 3.0 x 10⁹ kg to 4.4 x 10⁹ kg from approximately 5 million hectares. Additionally, cotton seed is a valuable source of vegetable oil, ruminant animal feed and other feed products.Since World War II, cotton cultivation has been increasingly dependent on inputs of pesticides for weed and insect control. Historically, the cotton boll weevil, Anthonomus grandis, was the most costly pest of cotton in the US. Until recently, the combination of crop loss due to this insect directly and the expense of insecticides for control amounted to several billion dollars annually. The success of the Boll Weevil Eradication Program coordinated by the US Department of Agriculture has resulted in a major reduction in insecticide usage and improved profitability for growers and has led to a resurgence of cotton production in the southeastern US. In addition, the widespread use of transgenic cotton cultivars (currently 92.7% of the crop) with resistance to herbicides and/or lepidopteran insects has further reduced total pesticide usage on the crop (USDA-Agricultural Marketing Service, 2007). Reductions in losses from weeds and insects as a result of the deployment of transgenic traits and the boll weevil eradication program have allowed the cotton industry to focus on other pest problems, especially nematodes.Modern cotton production in the US is intensive, highly mechanized and dependent on a local infrastructure to support this industry. Equipment for cotton harvesting and lint processing, including cotton pickers, modules for storing seed cotton, and gins, are highly specialized and generally not used for other crops. The necessity of an exclusive infrastructure to support cotton production has two important implications: (i) cotton is frequently grown in monoculture, and (ii) cotton typically has a greater impact on local economies than grain crops because of the jobs created to serve the industry.The damage potential of plant-parasitic nematodes to cotton has been recognized since the late 19th century. Classic work by Atkinson demonstrated the pathogenicity of Meloidogyne incognita to cotton and the role of this nematode in Fusarium wilt of cotton (Atkinson, 1892, 1899). Plant-parasitic nematodes, however, received only limited study as cotton pathogens until the 1950s. Currently, the four most damaging species of plant-parasitic nematodes affecting cotton in the US are the southern root-knot (Meloidogyne incognita), reniform (Rotylenchulus reniformis), Columbia lance (Hoplolaimus columbus) and sting (Belonolaimus longicaudatus) (Blasingame, 1993; Koenning et al., 1999; Starr et al., 2005; Blasingame, 2006). Estimated losses of cotton lint yield by these pathogens in the US have increased from 1% to 2% in the 1950s to more than 4% in 2000 (Blasingame, 2006). This increase in estimated losses due to plant-parasitic nematodes can be attributed to several factors: (i) the lack of resistant cultivars, (ii) limited use of crop rotation in many areas, (iii) increased awareness of pathogenic nematodes as production constraints, especially the reniform nematode, (iv) the loss of highly effective, low-cost, fumigant nematicides, and (v) a recent increase in cotton production in the southeastern US.