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Mesa Redonda 4: Manejo Integrado dos Nematoides NEMATODE OCCURRENCE, CROP DAMAGE, AND MANAGEMENT IN COTTON IN THE USA: PAST,

PRESENT, AND FUTURE.

Richard F. Davis U.S. Department of Agriculture, Agricultural Research Service, Crop Protection and Management Research Unit, P.O. Box 748, Tifton, GA 31793, USA.

In the United States, plant-parasitic nematodes typically cause greater yield and economic loss in cotton (Gossypium hirsutum) than other pathogen groups. In fields where one or more major nematode parasites of cotton are present, they are often a primary limiting factor to crop production. Estimated cotton crop loss due to nematodes in the US is approximately 4.3% with a value of more than US$275,000,000 per year. Reductions in pest pressure resulting from the use of transgenic plants with resistance to insects or herbicides and from the boll weevil eradication program in the Southeastern states have allowed farmers and the cotton industry to focus more effort on other problems, including nematodes. The primary nematodes damaging cotton in the US are Meloidogyne incognita (the cotton root-knot nematode), Rotylenchulus reniformis (the reniform nematode), and Hoplolaimus columbus (the Columbia lance nematode). Across the cotton growing regions of the US, M. incognita is believed to cause approximately twice as much yield loss as all other nematodes combined. Although M. incognita was recognized as a significant pathogen of cotton in the 1890s, relatively little research was conducted on managing this pathogen in cotton until the 1950s. Rotylenchulus reniformis has become a more common problem in recent decades. The geographical range of H. columbus is restricted, although it is very damaging where it occurs. Curiously, Pratylenchus brachyurus, which is very widespread and believed to cause damage in Brazil, is not believed to cause significant damage to cotton in the US. Tactics that have been studied for their ability to reduce losses due to nematodes include cultural controls, chemical controls, biological controls, and host plant resistance. The most widely studied cultural control method has been the use of rotation crops that are poor or non-hosts for the nematode being managed. The most common examples in the US are using peanut (Arachis hypogaea) or resistant soybean (Glycine max) to suppress M. incognita, or using peanut, corn (Zea mays), or resistant soybean to suppress R. reniformis. Other crops are also used to a lesser extent. Highly-resistant or poor host crops appear to be almost as effective as non-host crops in suppressing nematode levels and damage in a subsequent cotton crop. The beneficial effect of crop rotation typically only protects the first cotton crop following the rotation crop before nematode levels rebound to the levels found in continuous cotton production. The most important factor limiting the use of rotation crops is economics: the alternative crop must provide an adequate income to the grower and result in sufficient yield increases in the subsequent cotton crop to justify using crop rotation instead of continuously growing cotton. Another cultural control method similar in concept to crop rotation is the use of cover crops, but the benefits are not consistent or well documented. Rye (Secale cereale) is often planted to reduce soil erosion and suppress winter weeds, some of which are good hosts for nematodes. Rye that produces high levels of certain benzoxazinoids that are nematicidal may be useful in suppressing nematode levels, but the rye residue may need to be incorporated to be beneficial. Most

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leguminous winter cover crops are good hosts for M. incognita and often allow two generations of nematodes prior to planting cotton the following spring. Reproduction on winter cover crops has been shown to increase levels of M. incognita resulting in increased damage to the subsequent cotton crop. There are a few cultivars of vetch (Vicia sativa) and clover (Trifolium pratense) that are resistant to M. incognita, and such cultivars should be used to reduce nematode reproduction resulting in levels similar to those in fallow soil: resistant cover crops do not actively reduce nematode levels, but they prevent any increase. The primary method to reduce losses to nematodes in cotton in the US since the 1950s has been the application of nematicidal chemicals. The cost-effective use of chemicals to kill plant-parasitic nematodes began soon after the First World War as a way to dispose of surplus chloropicrin. In the 1940s and 1950s, the nematicidal effects of other fumigants were discovered. Fumigant nematicides are considered to be the most effective nematicides and are still used in fields with the greatest expected losses to nematodes. Organophosphate and carbamate insecticides were created in the 1960s, and their nematicidal properties were soon discovered. These non-fumigant compounds were less phytotoxic than the fumigants and could be applied at planting, which farmers found to be advantageous. Aldicarb, a non-fumigant carbamate nematicide, was the most widely used chemical control of nematodes in cotton in the US for more than 30 years until recently when the manufacturer chose not to reregister the product for use in the US. With the loss of aldicarb, fumigant use has increased and several seed treatments have gained wide acceptance in the US for nematode control in cotton. A significant benefit of seed treatments is that they require no additional equipment and do not require calibration to apply the correct rate. At least one seed treatment product is a biological control rather than a chemical control. One nematicide is currently labeled for post-plant application; it has been used relatively little in the past because more cost-effective options were available. In the last decade, significant advances have been made in applying the techniques of precision agriculture to nematode management in cotton. By dividing fields into distinct management zones based on differences in soil electrical conductivity (an indirect but fast and inexpensive method of estimating soil texture) and elevation, farmers can better determine which areas of fields should benefit from nematode management. Targeting nematode management only at those areas where it is needed can be a significant cost savings. Using management zones increases the likelihood that areas needing treatment will be identified. Nematode management zones have proven to be beneficial when used to control variable rate or type of nematicide application (site-specific nematicide application). As nematode management in cotton moves increasingly toward seed treatments and resistant cultivars, both the ability and the need to use site-specific nematicide applications will be decreased. Host plant resistance is usually the best choice for nematode management, but it has only recently become a viable tactic in cotton production. Germplasm with high levels of resistance to either M. incognita or R. reniformis are available. The first source of resistance to M. incognita in cotton was developed through transgressive segregation more than 40 years ago, but markers for the resistance genes have only recently been identified thereby allowing breeders to begin using high-throughput, marker-assisted selection. Two major genes for resistance to M. incognita are known, one on chromosome 11 and one on chromosome 14. In recent years, potentially novel sources of resistance to M. incognita have been suggested. Resistance to R. renifomis was first developed through a complex process that introgressed resistance from G. longicalyx into G. hirsutum. Markers were identified for that resistance gene, which is on chromosome 11 in the

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same region as one of the resistance genes for M. incognita. Other sources of resistance and markers for those genes have since been identified. Cotton cultivars with resistance to M. incognita are beginning to be available to farmers, but the level of resistance is not as high as the level in resistant germplasm. In many areas, farmers are finding that the yields of nematode-resistant cultivars are not as high as some non-resistant cultivars in the absence of nematode parasitism. Cultivars with resistance to R. reniformis are not yet available. For the next few years, nematode management in US cotton production will continue to be based primarily on the use of nematicides. Because of its potential cost savings, increasing adoption of the use of nematode management zones will likely continue. A fumigant nematicide will likely be used in fields with the greatest damage potential, possibly in conjunction with a resistant cultivar (for M. incognita) and a nematicidal seed treatment. Growers who use a fumigant are the most likely to also use site specific nematicide applications. In fields where moderate damage is expected, a seed treatment will probably be the most widely used management option, but some growers will choose to use a resistant cultivar. Some growers will supplement a seed treatment with a post-plant nematicide application. The major cotton seed producers are putting significant effort into breeding M. incognita-resistant cultivars, and the next generation of resistant cultivars is expected to have a higher level of resistance and yield as well as the best non-resistant cultivars. When such cultivars are released, they will likely become the preferred method of nematode management. Several new products, both chemical and biological, are in development and are expected to be released to farmers in the next few years. In the more distant future, the use of RNA interference (RNAi, also called post transcriptional gene silencing) holds great promise as a unique method of imparting nematode resistance to plants.