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Development of Genetic Resistance to Disease and Pests and its Implementation in Industry BENJAMIN F. GEORGE Heinz U.S.A. Since 1905âwhen R.H. Biffen first reported on the heritability of striped rust disease resistance in wheatâplant breeders have been attempting to improve yield and reduce dependency on chemical methods to control diseases by breeding disease-resistant genes into crops (4). While success has been achieved in some crops for certain diseases, the potential envisioned by researchers in the first half of this century still has not been realized. In general, where disease resistance has been found within the species and has behaved as a single Mendelian trait, plant breeders have bred the resistance into commercial crop cultivars. Where the resistance has not been controlled by a single gene, this process has usually been difficult for diploid crops that are bred intensively to produce many new cultivars. Conversely, where single gene resis- tance has been in commercial use for 20 to 30 years, resistant strains of fungi have developed as predicted by plant pathologists, such as in resistance to Verticillium and Fusartum species. With a few excep- tions, insect- and pest-resistant cultivars have not made a significant impact on commercial crops of the food processing industry. The complexity of breeding resistance to insects, mites, nematodes, and other pests into food crops has been very great. Although levels of resistance have been found in various crops, breeding problems have limited development. For example, resistance to nine different species of insects has been reported within the tomato genus, yet none have been commercialized (8). While farmers, food processors, and the fresh market produce 170
171 industry have generally embraced disease-resistant cultivars, there are a number of exceptions which reveal the historical priorities of the agricultural industry. In one sense, the industry is driven by the consumer, with the quality and value of the finished food product determining whether disease-resistant cultivars will be implemented by the industry. In reality, however, the food processor's percep- tion of the consumer's desire for quality and value determines the implementation decision. The form in which disease resistance is presented to the industry is very important. The genetic resistance comes in at least three forms of readiness to use: new cultivars, inbreds, and germplasm. The tomato industry offers us an opportunity to review the results of substantial efforts to breed disease resistance into a popular food crop; in 1952, nearly every U.S. state had an active tomato breeding project to search for resistance to at least one disease (9). NEW DISEASE-RESISTANT CULTIVARS Historically, plant breeding in the United States was a public service and a responsibility of land grant universities and the U.S. Department of Agriculture (USDA). Today some new crop cultivars are developed by universities and USDA, but more are developed by private industry. In all cases, a need perceived by the industry is a prerequisite for the commercial introduction of a new cultivar. Working through crop science and food science departments, university extension services perform a valuable function by focusing university researchers on industrial problems and on areas for poten- tial improvement. University/industry committees review university research results and develop financial support for research projects. The tomato breeding program at the University of California is an example of industry-supported research that has been very benefi- cial for farmers and the food processing industry. Such cultivars as VF145-7879, VF134, UC82, and UC204 have been widely used in California and all over the world. The Food Science Department of the University of California at Davis has helped industry considerably by evaluating new cultivars for product yields. Whenever a cultivar is improved for disease resistance, it must be tested for standard product yields. These results are adopted by industry on a needs basis, again drawing on the communication efforts of the extension service.
172 The extension service has also played a role in evaluation of prod- ucts from private industry. In addition to new cultivar evaluation, the extension service is continually testing new fertilizers, irrigation methods and products, herbicides, pesticides, and seed treatments that, along with new disease-resistant cultivars, lead to an improved agricultural system. In the past, support of private industry for farmers and food processors was limited mainly to seed companies. For the past seven years, however, new research and development companies have emerged as developers or modifiers of cultivars either directly for food processing companies or for traditional seed companies. These new companies propose to bring biotechnology or molecular biology to the traditional process of breeding improved crop cultivars. This type of research is generally considered proprietary, done on a con- tractual basis with the objective of giving the companies involved a competitive advantage in the marketplace. Exclusive rights agree- ments or profit-sharing agreements are usually established between the concerned parties. Varieties developed through this process are tested privately, not through the university agricultural extension systems. Examples reported to date are celery and carrot cultivars developed by DNA Plant Technology for the Kraft Company and a high-solid tomato cultivar also developed by DNA Plant Technology for the Campbell Soup Company. Other projects in progress are herbicide, disease, and insect resistance activities of Monsanto, Calgene, and other genetic engineering companies. However, traditional seed companies still are the primary source of improved cultivars for disease resistance and improved quality features. Their products are tested in the university agricultural extension system for the benefit of farmers and food processors. Private internal research is conducted by some food companies for their own needs. In the larger food companies, this type of research is dispersed through the entire vertical chain from seed cultivars of crops to retail product development, including recipes and manufacturing processes. In order to assemble a critical research mass, many companies perform research within central corporate units and then must convince their field operation units on the merits of their research. The probability of success for internal company disease-resistance development is much the same as that for research by the universities or private research and development companies,
173 except for knowledge of the food product and process for which the crop is destined. Most of the major food processing companies in the United States, such as Del Monte, Pillsbury, Campbell, and Heinz, have successfully developed their own disease-resistant cultivars to meet some of their product needs. For example, resistance of tomatoes to Bacterial Canker disease (Corynebacterium michiganense) has been developed proprietarily by the Heinz Company in response to a seri- ous need in a limited but growing area of the company. This research is expected to provide a strong competitive advantage to Heinz in that procurement area. In addition, Basic Foods Company uniquely combined very high solids with pink root resistance (Pyrenchaeta terrestris) in their proprietary hybrids for the onion dehydration industry. INBREDS AND BREEDING LINES Universities and USDA are unique suppliers to the agricultural industry of such semi-finished products as inbred lines for hybrid use and breeding lines possessing disease resistance or other improved qualities, but require further selection or backcrossing to make a finished cultivar. This approach has a multiplier effect on the original research by allowing the line to be used by different seed companies to produce hybrids designed for different products and for different growing areas. The current situation in regard to nematode resistance in toma- toes illustrates the value of this approach. In contrast to the very limited availability of nematode-resistant cultivars for the process- ing tomato industry, the fresh market tomato industry now has 50 percent of its cultivars resistant to nematodes (2). The first of the current major fresh market cultivars was in- troduced in 1973 by Ferry Morse Seed Company. In this instance, the need was certainly no greater than for processing tomatoes (6). The same nematicideâDBCP (l,2-Dibromo-3-chloropropane)âwas used by fresh market tomato growers and processing tomato grow- ers. The difference was a program at the University of California, Davis, that developed suitable nematode-resistant inbreds for the fresh market industry which had accepted hybrids and was able to use the university inbreds. Although the need was not great, the university lines could be used by a seed company to create a unique proprietary hybrid and
174 thus offer a slight cost savings to the farmer. The nematode-resistant hybrids reduced the growers' production costs and enhanced safety for applicators by eliminating the nematicide chemical. With the shift to utilization of hybrids even in the processing industry and the opportunity for universities and USDA to become more involved with biotechnology, the job of developing finished cul- tivars and hybrids is increasingly moving to private industry. Conse- quently, all university programs are releasing more breeding lines. In the California tomato processing industry, the seven major cultivars currently in use are all derived from breeding lines at the University of California, Davis. Many of them carry the resistance to Fusarium oxysporum Race 2 from the university lines. GERMPLASM The providers of germplasm to industry are land grant uni- versities and USDA, particularly USDA plant introduction stations that screen new germplasm received from explorations. Although re- search results in this area are clear, industry has not been successful in transferring the research into practice. In addition to establishing need, industry must be able to utilize new methods. Indeed, there are numerous cases where industry and private agencies have not taken advantage of improved disease resistance. Nematode resistance in tomatoes again provides an interesting case history for germplasm research. This pest is a major problem for tomatoes and many other crops in areas where the soil does not freeze during winter, and was first identified in 1855 in England. As reported by Bailey at the Tennessee Agricultural Experiment Station, genetic resistance was found in 1941 in a wild relative of the tomato Lycopersicon peruviannum var. dentatum (3). Sexual crossing with the commercial tomato L. esculentum was difficult as it required embryo culture techniques, but was nonetheless achieved in 1943 by Smith at the University of California (7). This work eventually resulted in the release of commercial cultivars in the early 1960s. However, wide-scale commercial use of processing tomato cultivars with nematode resistance has begun only recently. There have been occasional cultivars developed and released in the interim but they have not been not adapted to industry. Why did it take more than 20 years for industry to use this research? The need was not there, as perceived by the industry. DBCP was available, having been introduced as a soil nematicide in
175 California in the late 1950s. It was an inexpensive ($10-$15/acre) farm chemical that gave excellent control of nematodes and could be applied in irrigation water or by tractor-mounted shank injection equipment. From 1960 until 1977 when use of DBCP was banned, about 3,000,000 pounds were used each year on California farms at a rate of 20-80 pounds per acre. It was a fine chemical, persistent, highly nematicidal, was able to spread through a soil profile, and had low phytotoxicity. It could be used on 21 crops in addition to tomatoes, including cotton, grapes, and orchard fruit and nuts. When DBCP usage was banned as a potential carcinogen, the annual cost impact to farmers was estimated to be $7,700,000 per year. Genetic resistance to Bacterial Speck (Pseudomonas syringae pv. tomato) has also been underutilized by industry. The resistance occurs in a single dominant gene and the screening methodology is simple and economical, yet it is not being bred into cultivars with any sense of urgency by the seed industry. In this case, providing resis- tance to this single disease would not eliminate spraying of chemicals to control Bacterial Spot (Xanthomonas campestris pv. vesicatoria). While the disease is widespread and common in the Midwest, it is not a serious economic threat to most growers. Similarly, lack of need has deterred introduction of the newly discovered resistance to Lepidoptora species using the "Bt" gene in tomatoes. The farmer will still have to use the same insecticide for other insects. C.F. Andrus understood the absence of perceived need in his review of tomato disease resistance in 1953 (l). He described it as follows: (T)he breeder striving to produce disease-resistant varieties for benefit of fanners must have a proper perspective of the entire list of diseases that may seriously damage the crop in a given area, and he would be well advised to strive for combinations of resistances that permit elimination of expenditures by classes of control measures. Young, enthusiastic plant pathologists and plant breeders have often found it strange that farmers are seldom interested in a new cultivar because of its disease resistance. Again, Andrus concluded after 20 years of breeding experience that if a new disease-resistant cultivar is to be acceptable to farmers, it must be as beneficial as the old susceptible cultivar in all features that influence the net value of the crop, even when the disease is absent. A prime example of this conclusion is the sweet corn hybrid, Jubilee. Developed in the early 1950s and susceptible to all diseases, it continues to be the single major cultivar for the freezing process.
176 Many cultivars are available now with resistance to as many as five diseases, but none has the freezing quality of Jubilee. Along with the evaluation of need, the method of breeding a given disease resistance will often make the difference in its utilization. In the case of the tomato nematode resistance described earlier, the selection methods prior to 1975 required maintenance of the pest in infested soil. Also, field screening, with the inherent problems of potential contamination of nematode-free soils in greenhouses and research plots and problems in growing non-resistant lines in infested fields, constituted a very difficult screening method. In 1974, Rick and Fobes published a new method employing electrophoresis in a laboratory to detect a gene product (acid phosphatase) which is tightly linked to the Mi gene for nematode resistance (5). The acid phosphatase test is conducted on leaf extract and clearly detects homozygous and heterozygous resistant genotypes. A survey of U.S. seed companies in 1975 found none with a program to breed nematode resistance in tomatoes. The original screening technique devised in 1974 has been greatly improved for cost and efficiency, and breeding for nematode resistance is now a top activity in all seed companies. The processing industry expects to have five percent of its tomato acreage in 1987 planted with nematode-resistant varieties, eliminating both the grower's cost of nematicide use and the environmental impact of such use. CONCLUSION In summary, the task of introducing disease-resistant crop culti- vars to the food industry has proven to be complicated and difficult. The benefits should be reduction of farmers costs and reduction of environmental contamination by reducing the need for chemical con- trol. In practice, the disease-resistant cultivar must be equally as good as the susceptible one it replaces in terms of field yield, pro- cessed product yield, and quality under all growing conditions, or it generally will not be adopted (barring severe epidemic losses with standard cultivars). Closer contacts between cultivar-developing or- ganizations and the food industry have helped improve the develop- ment/implementation problem. The release of disease-resistant breeding lines by universities and USDA to seed companies and other sectors of private industry provides a multiplier effect to the value of their fundamental research.
177 The seed companies must customize the disease resistance into a finished cultivar that is equal to or better than the present cultivar. For the safety of the environment, public agencies must look past inexpensive chemical control measures and push ahead with breeding line development where good genetic disease resistance can be found. The chemical control class approach, advocated by Andrus, must be used to overcome the economic barrier to introduction of disease- resistant cultivars. REFERENCES 1. Andrus, C.F. 1953. Evaluation and use of disease resistance by vegetable breeders. Proc. Amer. Soc. Hort. Sci. 61:434-446. 2. Angel, F. 1987. Personal communication. Asgrow Seed Company. 3. Bailey, D.M. 1941. The seedling test method for root-knot nematode resistance. Proc. Amer. Soc. Hort. Sci. 38:573-575. 4. Biffen, R.H. 1905. Mendel's laws of inheritance and wheat breeding. Jour. Agric. Sci. 1:4. 5. Rick, C.M. and J.F. Fobes. 1977. Linkage relations of some isozyme loci. Rep. Tomato Gen. Coop. 27:22. 6. Sims, W. 1987. Personal communication. University of California, Davis. 7. Smith, P.G. 1944. Embryo culture for tomato species hybrid. Phytopathol- ogy 34:413-416. 8. Tigchelaar. 1986. Tomato breeding. In Breeding vegetable crops. Westport, CT: AVI Publishing Co., Inc. 9. Walters, J.M. 1967. Hereditary resistance to disease in tomato. Annual Review of Phytopathology 5:131-162.
