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TITLE: Managing Plant-parasitic Nematodes in Sustainable Agriculture with Emphasis on Crop Resistance.
DURATION: October 1, 1997 through September 30, 2002.
STATEMENT OF PROBLEM:
Plant-parasitic nematodes cause yield suppression in many crops and plant species grown under a variety of conditions. Cotton and soybean losses are 3-6 % (Tables 1 and 2). These losses require that management options must be explored. Nematicides, plant resistance, and cultural strategies have been routinely used to control nematodes in field crops and high value vegetable crops. However, many nematicides have been removed from the market because of environmental contamination problems and prohibitive re-registration costs. In addition, the U. S. Clean Air Act (Section 602) mandates that methyl bromide, the leading soil fumigant used in vegetable culture, be withdrawn from production, importation, and use in the U. S. by the year 2001. Therefore, safe alternative methods for managing plant-parasitic nematodes in field and vegetable crops are critically needed for the development of sustainable cropping systems. Through the use of classical breeding and genetic engineering techniques, new sources of resistance to nematodes must be incorporated into cotton and certain vegetable cultivars, and improvements made in the breadth and levels of resistance in peanut and soybean. These resistant cultivars also must be integrated with biological and cultural control, and improved methods to monitor nematode field populations, in sustainable cropping systems.
JUSTIFICATION:
Importance of the problem - Cropping systems in the southern U. S. are changing as growers diversify the crops they grow in response to ongoing changes in government set-aside programs and cropping incentives, including the termination of certain support programs. Successful growers will have to manage changes in land use patterns, varietal selection, crop rotations, and prescription application of fertilizers and pesticides, as well as monitor rapidly changing market conditions. With the increasing acreage of intensive, multi-product farming practices come increasing demands for cost containment and environmentally sound pest management practices. Management of plant-parasitic nematodes is in the midst of a dramatic change from reliance on nematicides to more wide-spread use of resistance and cultural and biological controls. Moreover, no new nematicides have emerged on the market since 1974, and the remaining ones are under scrutiny by federal and state regulatory agencies, environmental advocates, and the general public, due to concern about pesticide contamination of ground water and food safety. In fact, pesticides registered before 1984 must be re-registered by the end of 1997. Nematicides are judiciously used in field crops because of the potential for inadequate return on the investment and because of the availability of resistant cultivars in certain crops such as soybean. However, nematicides and soil fumigants are commonly used to control nematodes in high input vegetable crops. The fumigant, methyl bromide, is commonly used to control nematodes, pathogens, and weeds in vegetable crops such as tomato, pepper and cucumber. However, methyl bromide is scheduled to be removed from the U. S. market in the year 2001 due to concerns that this chemical is reducing the earth’s ozone layer. Resistance to nematodes has been identified in some vegetable crops, and a limited number of resistant cultivars have been developed. Still, many such cultivars are not horticulturally acceptable and hence, not widely grown. Acceptable resistance is not available in adapted cultivars of bell peppers and cucumbers, and although high levels of resistance to root-knot nematodes are available in certain sweetpotato cultivars, they have not been readily accepted in the marketplace. Since 1980, cotton acreage has increased, as have double-cropped small grains, especially wheat, after soybean and cotton. For example, cotton has increased in acreage 320% from 1980 to 1995 in Virginia, 782% in Georgia, and 1215% in North Carolina. Much of that increase has occurred during the period 1990 to 1995; for example, cotton acreage in Georgia and North Carolina has increased over 300% in that period while soybean acreage has declined slightly. Vegetable production, excluding home gardens, has been variable from year to year in terms of both the types of products and the amounts grown.
Urgency of the problem - Research is needed to identify and introduce nematode resistance into vegetable crops, such as southernpea, pepper, and cucurbits, and into cotton. Vegetables can serve as partial replacement crops for tobacco, but resistance to the major nematode pathogens must be identified and incorporated into horticulturally acceptable vegetable cultivars. The identification and development of nematode-resistant soybean cultivars and peanut germplasm by members and collaborators in the
S-253 Regional Project is a remarkable success story (Boerma and Hussey, 1992; Starr et al., 1996). Similar research efforts and results are greatly needed for most vegetable crops and for cotton. Moreover, cotton, soybean, vegetables, and peanut constitute a large proportion of the crops grown in the southern U. S. These crops are often planted interchangeably with each other and suffer damage from such commonly occurring plant-parasitic nematodes as Meloidogyne spp., Heterodera glycines, and Rotylenchulus reniformis. In addition, planting of resistant varieties can result in the buildup of more virulent races of the target nematode species within a field, affecting the durability of the plant resistance genes (Kaloshian, et al. 1996; Young, 1992a).
Molecular techniques have particular potential for the development and use of plant resistance to nematodes as outlined in this project. The following techniques are critical for the rapid identification and introduction of resistance genes into useful crop cultivars: 1) the isolation, cloning, and bioengineering of plant resistance genes into desirable cultivars, and the use of molecular markers for plant resistance genes in plant breeding programs; 2) identification of nematode and plant gene products that are essential for successful parasitism, and the potential development of ways to disrupt these processes to develop novel, transgenic plant resistance to nematodes; 3) identification of nematode virulence genes and the isolation of molecular markers to monitor shifts in virulence gene frequency within field populations; 4) understanding the molecular, biochemical, and cellular mechanisms of plant-nematode interactions in both susceptible and resistant responses; and 5) utilization of DNA, protein, or antibody markers to identify nematodes at the species and subspecies level.
During the next several years, many transgenic and genetically enhanced new crop cultivars will be available to growers. Already in 1996, the following such cultivars are in the market (Sulecki, 1996): Roundup Ready TM soybean with tolerance to Roundup TM, STS TM soybean with tolerance to sulfonylurea herbicides; Bollgard TM cotton with the Bacillus thuringiensis (Bt) toxin that provides a level of protection against cotton bollworm, tobacco budworm, and pink bollworm; BXN TM cotton with tolerance to bromoxynil for control of most annual broadleaf weeds in cotton; IMI TM-corn tolerant (t) or resistant ( r ) to imidazolinone herbicides; Poast Compatible TM corn tolerant to sethoxydim herbicide for control of certain grasses; Bt corn for control of the European corn borer; and NewLeaf TM potato with Bt protection against the Colorado potato beetle. Liberty Link TM corn, resistant to glufosinate-ammonium; and BXN/Bt TM cotton for dual protection against worms and weeds are due to be marketed in 1997. These cultivars are not considered to be a panacea for pest problems and will likely be used in conjunction with other means of pest management. The technology now exists to make rapid advances in disease and pest management through molecular biology. The members of the Southern Regional Project see an urgent need to identify and introgress new nematode resistance genes into crop cultivars and to complement host resistance with biocontrol and cultural management options that will enhance the level of nematode control, while extending the durability of resistance.
Extent of the problem --- Plant-parasitic nematodes can be controlled by application of nematicides, although chemical control of large infestation levels of root-knot and cyst nematodes is often insufficient, and environmental concerns mitigate against continued wide-spread use of these pesticides. The economics of crop production for many of the crops grown in the South do not allow sufficient return on investment to justify use of nematicides. Therefore, research on new, more effective, broader crop resistance to nematodes supplemented and complemented by biological and cultural management is most critical. These are the areas of research considered most important in sustainable cropping systems. Among the major accomplishments of the scientists affiliated with the Southern Regional Nematology Project S-253 are the identification of new sources of crop resistance to nematodes in soybean and peanut and the development of new cultivars with improved resistance to Meloidogyne spp. and Heterodera glycines. Developmental research remains to be done on these two crops to improve the breadth of resistance and assess the durability of the new cultivars to evolving races of target nematodes. Environmentally compatible nematode management options are critically important to Southern agriculture, specifically, genetic resistance is needed in cotton, vegetables, peanut, and soybean for managing several nematode species. The extensive progress made in the area of nematode management so far in the current project will provide the impetus and structure for further significant advances (See Critical Review).
Need for cooperative work --- A structure for cooperative work exists within the current S-253 Project, as demonstrated by the successful development of resistance in soybean and peanut (see Critical Review). The proposed research on nematode resistance and knowledge of how to integrate it with other control strategies and tactics can best be done by cooperation among the diminishing number of nematologists in the participating states and USDA ARS. Cooperating scientists in the current S-253 Project are presently developing resistant soybean and peanut cultivars and are constructing protocols for development of resistance to M. incognita and R. reniformis in cotton and vegetable cultivars. We propose a revised project through which to build on the accomplishments of the current project and expand into new areas in response to and anticipation of changes in cropping systems and public concern for food safety and environmental protection. Specifically, we propose to develop new resistant cultivars in cotton and certain vegetable crops, enhance the level and breadth of nematode resistance in soybean and peanut, and gain knowledge on how best to deploy those resistance genes to maintain their durability, despite the potential selection of new nematode races and species which may occur in response to new resistant cultivars. Project members will cooperate in identifying new sources of nematode resistance and characterizing them using both classical and molecular genetics techniques. Scientists in cooperating states will evaluate resistant germplasm along with promising strategies for the integration of resistant cultivars with biological and/or cultural control methods. Research on components of a multi-faceted management system for nematodes necessarily requires cooperative effort and local evaluation. The proposed research will provide resistant cultivars that can be used for nematode management in sustainable systems and extend the knowledge and techniques in the discipline.
The proposed research should reduce use of nematicides, resulting in decreased costs to the producer, while protecting the environment. Cotton and soybean are among the most widely planted and therefore most important field crops in the southern region, while certain vegetables play an ever increasing role of importance in the diversification of agriculture in the South (Tables 3-7). Nematologists and other cooperating scientists in the southern region have differing expertise to develop resistant cultivars and to develop methods for utilization and integration of biological and cultural controls, molecular markers for resistance, pathogenicity determinants, nematode species and race identification, variability, host-parasite interactions, and nematode population assessment. We are committed to working together to provide new and practical means of nematode control. The results of this research will provide alternative, integrated methods for nematode management, some of which can soon be implemented by producers. Some of these alternative nematode management systems may be put in practice during the longer developmental time required for release of resistant cultivars.
Nematode management success over the long term requires several alternatives and combinations of approaches to be economically feasible. This is especially true for low-value crops. For high-value crops, such as certain vegetables, with the loss of methyl bromide, non-chemical measures are urgently needed. Methyl bromide is highly effective for control of weeds, fungi, and nematodes. New means of nematode management must be found for these crops. The proposed research to develop management options for different crops will affect crop producers throughout the South. Cooperative projects, by their very nature, achieve success through true interaction among the participants. In this regard, more than one institution will study an individual component, and two or more may work on a single aspect that will tie into a larger component, such as for the rotation and cultural control studies. Each institution will provide component parts (resistant cultivars) through breeding or testing that will all become parts of the management system. Several institutions working together will assess locale differences that might affect results, such as the effectiveness of biological control agents or cultivar resistance, variation in nematode populations, alternative crops or rotations. Still, the project will greatly enhance the productivity of the scientists by limiting unnecessary duplication of efforts and providing new ideas for regional adoption.
Benefits to be derived - New vegetable, cotton, soybean, and peanut genotypes and cultivars that have multiple mechanisms of durable resistance to multiple species and races of nematodes will result from the cooperative research. Identification of molecular markers linked to resistance genes in plants and to virulence genes in nematodes will make varietal development more efficient, increase durability of resistance, and extend fundamental knowledge in the associated scientific disciplines. Resistant plant genotypes must be integrated with biological and cultural controls into environmentally sound cropping systems that prolong their utility and impede the selection and increase of resistance-breaking nematode species and races. Nematode resistant cultivars will substantially reduce the production costs of cotton and vegetables which rely heavily on pesticides and will also reduce environmental contamination. Resistant cultivars of soybean already contribute to the economic sustainability of this crop, and new sources of durable resistance will further enhance the benefits of resistance as the primary means of nematode control in that crop. The ratio of economic benefit to research costs was 400:1 for development of one soybean cyst nematode-resistant cultivar (Bradley and Duffy, 1982). Identification of resistance genes, molecular resistance markers, and knowledge of the interaction of nematode virulence and plant resistance genes can ultimately supply resistance to those crops lacking it and provide biological explanations for nematode virulence. Identification of resistance genes is necessary before resistant cultivars can be developed in some crops. Identification of molecular markers greatly facilitates the use of these genes. Identification of virulence genes in the nematodes will assist in the development of cropping systems for deployment of the resistant cultivars. Replacement crops for tobacco and alternatives for methyl bromide require that crops such as vegetables have nematode resistance if we are to continue to have a bountiful and economically viable agriculture. In addition, developmental research on the utilization of biological control with nematode suppressive cropping systems integrated with nematode resistant cultivars will result in wide-spread field applications by producers. The economic benefits to be attained through control of plant-parasitic nematodes are estimated to be in the billions of dollars. Up to 90% of the value of some crops would be lost without the information gained from previous nematology research (Barker et al., 1994). Generally, estimated annual losses due to nematodes are in the range of 6-15% (Sasser and Freckman, 1987), although nematode-induced losses in cotton are estimated at about 3% (Table 1). If the use of insecticide-nematicides can be reduced through our proposed efforts, the economic and social benefits would be substantial. Aldicarb (Temik TM), in particular, is a highly toxic insecticide-nematicide with potential for groundwater contamination and has a high frequency of use on both cotton and peanut.
Relationship to current priorities ---- The National Integrated Pest Management Initiative is a cooperative and focused effort involving USDA and Land Grant University programs, farmers, consultants, and industry to achieve the national goal of IPM implementation on 75% of crop acres by the year 2000. Certainly, an important component of this initiative should be management of plant-parasitic nematodes through providing farmers with alternatives to pesticides, including improved resistant cultivars, biological control products and cultural tactics. This revised regional project reflects and augments the national evolution of IPM toward crop production practices that are compatible with ecologically-based pest management (Barnett et al., 1996). These practices require knowledge of the nematode’s biology so that resistance gene deployment, rotations, green manures, biological control, and management of suppressive soils can be effectively implemented and integrated with other on-farm systems of agronomic management. The development of this new strategy is reflective of the pressure from consumers for sustainable approaches to pest management and of the need of producers to have economically feasible alternatives for pest control. Sustainable agriculture, biologically-based pest management technologies, and exploiting value-added genes through conventional breeding programs and molecular biology are ranked 1, 2, and 4, respectively, among 15 prioritized research opportunities as perceived by the Southern Association of Agricultural Experiment Station Directors (Merrifield and Wynne, 1994).
Management of plant-parasitic nematodes has evolved during the past two decades from great reliance on nematicides to the use of alternative practices, such as cultural management and resistance gene deployment. Moreover, the utility of durable plant resistance can be enhanced by cultural and biological management. The Report by the Committee on National Needs and Priorities in Nematology, 1994, sponsored by the Cooperative States Research Service, U. S. Department of Agriculture, and the Society of Nematologists (Barker et al., 1994) indicates that a strong scientific foundation in nematode biology and management is needed in order to implement the national goals of "preserving the biosphere and developing an environmentally compatible and sustainable agriculture". As nematologists seek to effectively manage plant-parasitic nematodes, they also must continue to understand the beneficial features of some of the microbivorous forms that are important in nutrient cycling and soil remediation.
The research and education priorities identified by the Committee on National Needs and Priorities fall into three main sections, 1) lessening the societal impact of plant-parasitic nematodes, 2) advancing our knowledge of fundamental nematode biology, and 3) promoting the beneficial use of nematodes. Section 1 lists development of nematode-resistant crops through new and traditional approaches and enhancement of the activity of antagonistic organisms as being high priorities. Section 2 priorities include improvement of methods to identify nematode species and races quickly and accurately and design of ecologically balanced, sustainable systems for nematode control. Notably, this revised Regional Project Proposal includes these priorities among its objectives.
The Council for Agricultural Science and Technology (CAST) in its publication entitled, "Sustainable Agriculture and the 1995 Farm Bill" (Clark et al., 1995) noted that protecting plants from insects, diseases, nematodes and weeds is critical to maintaining food and fiber quality. Domestic and international consumers are concerned not only with food safety and quality but with the environmental and social consequences of food production. The methods of integrated pest management (IPM) and integrated crop management (ICM) have been used to "strike a compromise minimizing the use of chemical pesticides" and to encourage the use of biological and cultural controls (including plant resistance). Scientists must build on these technologies and progress to the adoption of newer technologies to provide the "next generation of plant protection for sustainable systems". The CAST report identified plant protection strategies as a critical component of total plant systems management and indicated strongly that research should be accelerated in the use of "predators, parasites, competitors, pathogens, attractants, repellents, insect sterility, resistance, culture, improved pesticides, application technology and area-wide management strategies", looking at both established and emerging pests. Also, the identification and development of new sources of resistance will enable growers to circumvent the potential damage of ever evolving genotypes of these nematodes. In addition to the earlier removals of nematicides (DBCP, EDB) from the market, the effect of the projected loss of other nematicides over the next decade should be diminished in part by the development of a greater array of nematode resistant cultivars and other management options.
Impact on Science ---- The identification, development, and characterization of natural and novel crop resistance to plant-parasitic nematodes will have significant impact on our understanding of basic biology. The study of these nematodes represents the opportunity to investigate an animal system that is parasitic on plants—yielding fundamental information on the biology of both entities individually, and the biology of their unique and varied interactions. Natural plant resistance to nematodes is expressed in several ways and the new sources of resistance discovered in this project may present new molecular and cellular mechanisms of resistance, as well as enhance our understanding of known mechanisms of resistance. Molecular markers developed to assist in breeding selection may also be used to isolate and clone nematode-resistance genes, potentially making them available to engineer into crop species with no known natural source of resistance genes. Cloned resistance genes may be identified and compared with other known plant resistance genes, to other types of pathogens, to infer functional roles of nematode resistance genes in plants. This information may even be used to infer the evolution of plant resistance to pathogens on a more global scale. In much the same way, investigations of the mechanisms of parasitism by plant-parasitic nematodes will also have a major impact on our knowledge of fundamental molecular and cellular events in biological systems. The root-knot, cyst and reniform nematodes studied in this project all make dramatic modifications of host plant cells for feeding. One aim of this project is to identify and disrupt these unique, fundamental mechanisms to develop novel resistance to nematodes in desirable crop species. In-depth analyses of mechanisms of both resistance and parasitism will yield information on molecular signaling events and gene expression in both nematodes and plants that may be applicable to many biological systems.
Significant impact in the sciences of soil ecology, microbiology, and population genetics may also be derived from the integration of nematode-resistant crop cultivars into sustainable agricultural systems. Nematode populations are key components of soil ecosystems, and their population densities and species composition are strongly influenced by many selection pressures, including plant hosts, environment, and biological antagonists. Resistant plant hosts or rotations to a nonhost crop can dramatically reduce soil populations of target plant nematodes. This, in turn, has a strong influence on existing soil microflora and microfauna, which can exert selection pressure for increases in nematode virulence frequency within the nematode population. Evaluation of the ecological ramifications of incorporating pest resistant transgenic plants in cropping systems will be needed to prevent development of highly virulent nematode pest populations. Identification and monitoring of changes in soil microbes and nematode populations is an important component of this project that may have application to other ecosystems. Antagonists of nematodes that exist or are introduced into the soil are not only critical for use as potential nematode management tactics, but they strongly influence (and are influenced by) biotic and abiotic factors of the soil environment. Investigations of the ability of biocontrol agents to be enhanced, effective, and persistent in sustainable cropping systems will require an integration of fundamental aspects of ecology and microbiology, and contribute to our understanding in these areas of science.
Societal impact - Many of the outcomes from this regional research project will have considerable impact on society, both directly and indirectly. Perhaps the greatest benefit from this regional research will be to maintain and increase the quality and quantity of food and fiber crops produced in the southern United States while reducing the environmental impacts of nematode management. The potential environmental risks of nematicides have incited severe restrictions in their use and delivered a severe blow to cost-effective nematode management in many cropping systems. The proposed regional research is designed to provide alternative nematode management options to nematicides while maintaining agricultural productivity and economic benefits to both growers and consumers. Host plant resistance is a highly effective means of reducing nematode-related crop damage, and these beneficial effects can be amplified when properly integrated with cultural and biological tactics to manage plant-parasitic nematodes. Indirect beneficial effects of developing these nematode management strategies include reduced soil erosion, increased soil tilth, enhanced nutrient cycling, and greater compatibility with integrated pest management systems. It is critical to extensively study these systems to avoid potential adverse effects such as increased costs and reduced productivity for both growers and consumers due to inefficient nematode management practices, undesirable effects of management tactics on non-target organisms, and reduced longevity of efficacy of these management options in sustainable cropping systems. The combined efforts of the regional researchers in this project will be able to address some of these aspects in an efficient manner as they relate to the southern agricultural regions of the United States.
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