W1147: Managing Plant Microbe Interactions in Soil to Promote Sustainable Agriculture
Statement of Issues and Justification
The future of sustainable agriculture in the U.S. will increasingly rely on the integration of biotechnology with traditional agricultural practices. Although genetic engineering promises enhanced yields and disease resistance, it is also important to recognize that plants exist in intimate associations with microorganisms, some of which cause plant disease while others protect against disease. Identifying, understanding and utilizing microorganisms or microbial products to control plant disease and enhance crop production are integral parts of sustainable agriculture. Biological control has the potential to control crop diseases while causing no or minimal detrimental environmental impact. For this proposal, we define biological control as the manipulation of microbial populations through cultural, physical or biological means including plant mechanisms. Some of the benefits of utilizing microorganisms include:7 reduced dependence on chemical pesticides, important because of the increasing restrictions on chemical usage due to environmental and public concerns;
7 lack of development of pathogen resistance to biological control organisms, important due to the observed increase in resistance to many chemical controls;
7 selective action against specific groups of pathogens and not against beneficial organisms;
7 biodegradability of microbial pesticides and the by-products of their manufacture;
7 lack of danger to humans or animals;
7 improvement of soil and enhancement of agricultural sustainability.
During the past project period, W-147 made major contributions towards our understanding of microbial biological control of plant disease. To date several commercial biocontrol products and processes are available and registered by the EPA. These include fifteen bacterial and seven fungal products, and three activators of plant defenses (http://www.apsnet.org/online/feature/biocontrol/). However, the complexity of this area requires further research in order to overcome continuing problems in production, storage, delivery, reliability, efficacy, establishment and the understanding of the mechanisms of action.
Why a Multi-State, Multi-Disciplinary Approach? Due to the broad nature of the complex problems, this research must be multi-disciplinary and collaborative. No single research institution has sufficient resources and diversity of expertise to solve the problems. Many of these pathogens occur in multiple states and need a coordinated effort for research and to prevent duplication of effort. Because the results of our efforts are only now beginning to affect U.S. agriculture, continuation of the W-147 project for another five years will lead to further improvements in the use of biological control in agriculture.
JUSTIFICATION:
Economic Costs Due to Soil Borne Plant Pathogens
Soil borne plant pathogens result in severe yield and economic losses for growers. Economic losses due to pathogens are estimated at 10-20 % of the attainable yield for many crops (Pimentel, 1991). Yield failures resulting from acute diseases such as vascular wilts, take-all of cereals, Phymatotrichum root rot, Verticillium and Phytophthora may be even more severe and have destroyed entire agriculture industries. The soybean cyst nematode and Phytophthora stem and root rot are the most severe diseases of soybeans, and reduced yields by 7 million tons in the northern states during 1997 alone (Wrather et al. 2001).
7 For root diseases of mature crops, there are few effective and economical post-plant strategies for control.
7 About 90% of the 2000 major diseases of the principal crops in the US are caused by soil borne plant pathogens (Lewis and Papavizas, 1991).
7 Monetary losses due to soil borne diseases in the U.S. are estimated to exceed $4 billion per year (Lumsden et al., 1995), and losses due to parasitic nematodes exceed $8 billion per year (Barker et al. 1994).
7 Several of the top 15 restricted, invasive quarantine pathogens listed by APHIS are soil borne, and could represent a biosecurity risk.
Environmental Costs of Soil Borne Plant Pathogens
The cost of soil borne plant pathogens to society and the environment far exceeds the direct costs to growers and consumers. The use of chemical pesticides to control soil borne pathogens has caused significant changes in air and water quality, altered natural ecosystems resulting in direct and indirect affects on wildlife, and caused human health problems. For example, methyl bromide, a fumigant used to control soil borne diseases, has become notorious in recent years for depleting the ozone layer and changing the climate of our planet. The production and importation of this product will be banned by 2005, and is the subject of an intensive search for alternative methods. Many other chemicals are being removed from the market due to regulatory and public concerns. Additionally, plants evolved in the presence of microorganisms and are dependent on them in order to carry out many activities necessary for growth and reproduction. Thus, long-term chemical applications may permanently alter the microbial community structure sufficiently such that sustainable agriculture may be impossible.
Societys Expectations
As is readily apparent from reading the popular press, consumers are demanding plentiful low cost but safe food while simultaneously requiring the use of fewer chemical controls. New specialty and organically-grown crops will also require non-chemical methods for management of diseases. This has resulted in numerous new pesticide regulations and the loss of more and more pesticides to control soil borne diseases. Several soil borne diseases, for example, those caused by Phytophthora, Verticillium, Gaeumannomyces and Fusarium, remain major problems after more than 100 years of study. Soil borne pathogens are well adapted to soil conditions, and once established are very difficult to eliminate by any known method of control. Chemical controls are often too expensive to be economically practical and chemicals effective against many pathogens have yet to be identified. Other approaches with great potential include the development of transgenic crops engineered with resistance genes to several pathogens. However, there is widespread public reluctance to accept these crops as evidenced by protests both here and in Europe. This public concern, combined with the natural ability of pathogens to overcome introduced resistance genes, has frustrated efforts to maximize this approach.
The ultimate goals of this collaborative work of W-147 are:
7 To provide society with a safe, low cost food supply.
7 To reduce the environmental impact of food production
Biological Control As an Attractive Alternative
Biological control is an attractive approach for the control of soil borne diseases (Cook, 1993; 1990; Cook and Baker, 1983; Jacobsen and Backman, 1993; Lewis and Papavizas, 1991; Weller, 1988, 2002, Paulitz and Belanger, 2001, Boland and Kuykendall, 1998, Whipps, 1992, 1997, McSpadden Gardener and Fravel, 2002, Mathre et al. 1999. Advantages of a biological approach to disease control include a lack of environmental damage, reduced human health risks, lack of resistance development in the pathogen, selectivity in mode of action, lack of activity against most beneficial microorganisms, and improved soil conditions and agricultural sustainability.
Biological control of soil borne plant pathogens has made large strides over the past several years. Much of this success is due to activities of the members of W-147. Today the EPA lists more than 24 commercial biocontrol agents that are registered and commercially available in North America. Nearly all of them have been registered during the past five to ten years. However, most of these products are for seed and seedling diseases. W-147 project is unique in emphasizing biological control of root diseases of mature crops, including avocado, citrus, wheat, and turfgrass, which are generally not treatable with chemicals or other methods (Tables 2 and 3).
Interest and enthusiasm about biocontrol have never been greater. A recent analysis of articles published in 1996 in Phytopathology, the premier plant disease journal in the U.S., shows that nearly 20% of the articles dealt with biocontrol. In fact, two new journals were launched in the 1990s-the journal Biological Control, which covers both arthropod and microorganism-mediated control methods, and Biocontrol Science and Technology. Combined with the increasing resistance in parts of the world to transgenic plants, it appears that the W-147 regional project is both very timely and successful.
In spite of the strides made in biological control research, there are many areas that require work before biocontrol will be used extensively. Current areas of research include:
7 Identification of more effective agents. Workers are isolating potential antagonists from soils where many pathogens originated.
7 Understanding the genetic diversity of biocontrol agents.
7 Identification of natural disease suppressive soils.
7 Development of lower cost production, storage, and distribution systems.
7 Improved quality control assays.
7 Improved stability of the agent during production, storage and application.
7 Integration of biocontrol into current agronomic practices.
7 Identification of parameters affecting efficacy and survival after application.
7 Understanding the mechanisms of action of control, especially at the molecular and biochemical level.
7 Investigation of manipulation of cultural parameters that advance biological control (compost, green manures, rotation crops).
7 Understanding the role of the plant in biological control (induced resistance, genetic resistance)
7 Microbial community and plant-microbe interactions with biocontrol agents.
The promise, public acceptance and environmental benefits of biocontrol continue to make research on this area both timely and of critical importance to the future of U.S. and world agriculture.
Despite over 30 years of research, biological control of plant pathogens is not widely used in commercial agriculture and sales of biofungicides represented less than $1 million compared to total fungicide sales of $5.5 billion (Powell and Jutsum, 1993). Powell (1991) summarizes the current status of plant biocontrol agents when he says, the real problem for biological control is to deliver an active agent to the site where it is required and keep it there while activity is required. We are yet unable to do that efficiently with most of our current biocontrol agents. Clearly there is much to be done in order to improve biocontrol agents so that they will become major factors in the control of soilborne diseases. Biocontrol agents isolated by participants of W-147 at ARS-WA, ARS-CA, CA-R, OR, MT, AK and NY have the ability to suppress a wide variety of plant pathogens that cause serious diseases of food, fiber and ornamental crops. The need for high quality biocontrol agents has never been more critical because of the pending loss of fungicides and fumigants upon which agriculture has been dependent for the last 50 years. Consider the billion-dollar-a-year commercial strawberry industry in California, which relies exclusively on soil fumigation with a combination of methyl bromide and chloropicrin at about 250 lb/acre for disease, nematode and weed control. The mandated 75% reduction in the use of methyl bromide by 2003 will leave this industry vulnerable to soilborne nematodes and pathogens. Biocontrol may provide a safe, environmentally sound alternative to methyl bromide and other valuable agricultural chemicals that may be lost in the future. Understanding the complex biological and environmental interactions that must occur for biocontrol to be effective requires the combined efforts of multiple investigators at multiple institutions focusing on different aspects of the problem, from applied to basic research. This logical approach is an area in which the W-147 regional project has excelled and will continue to depend on during the next five years.
This project also fits the goals of other CSREES initiatives, including the National Integrated Food Safety Initiative of 1998, and other programs, such as Integrated Pest Management (IPM), Methyl Bromide Transition Program (MTB), Pest Management Alternatives Program (PMAP) and Sustainable Agriculture (SARE).
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