Managing Global Genetic Resources Agricultural Crop Issues and Policies (1993) / Chapter Skim
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1. Genetic Vulnerability and Crop Diversity
1. Genetic Vulnerability and Crop Diversity
Pages 47-84
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From page 47... ...
Significant efforts have been made by national and international institutions to collect and preserve crop genetic resources as an insurance policy against future disasters. The recognition, however, that the products of a few major breeding programs are now planted over entire continents under increasingly intensive conditions raises new concerns that are global in scope.
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From page 48... ...
in the United States, became concerned about the potential for similar outbreaks in other major crops. They sought to warn the agricultural community about trends in modern plant breeding programs that may have contributed to the crisis and recommended that the genetic foundations of major crops be diversified to reduce the risk of future outbreaks.
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From page 49... ...
These concerns have prompted surveys of plant breeders on their perceptions of the gravity of the problem and a reevaluation of trends in international varietal development and distribution. Although some breeders and scientists are encouraged by the wider availability of crop gene pools into which exotic plant genes have been introduced, others are worried that genetic uniformity may be increasing on a global scale because of the widespread adoption of modern varieties with similar genetic backgrounds across continents where large numbers and mixtures of landraces were formerly grown.
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From page 50... ...
Breeders can strengthen plant resistance against epidemics by broadening the diversity of resistance genes and "pyramiding" multiple genes from different sources and genes controlling other mechanisms of resistance. One key to the uniformity issue is understanding the genetic basis of the plant-pathogen, plant-pest, or plantstress interaction and identifying preventive measures that breeders can take to build in greater resistance.
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From page 51... ...
In these situations, networks of collaborating scientists monitoring the effect of environmental conditions on the development and spread of pest problems provide the best prediction of possible vulnerability. Vulnerability to abiotic stresses may also arise from genetic uniformity for susceptibility.
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From page 52... ...
In comparison with determination of acreage devoted to major cultivars and pest and pathogen evolution, data on the extent of genetic uniformity among major cultivars are scarce and difficult to assemble. The number of traits involved is enormous, detection methods are laborious, and cultivar pedigrees (when available)
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From page 53... ...
(1985) analyzed the diffusion of rice genetic materials among 27 plant breeding programs over a 20-year period and found a disturbing similarity in the genealogies of improved rice varieties across Asia.
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From page 54... ...
Simmonds (1991) recognizes four broad types of genetically controlled resistance (Table 1-1~: major gene or vertical resistance, which is pathotype specific; polygenic or horizontal resistance, which is TABLE 1-1 The Four Main Kinds of Resistance Kind of Resistance Specificity Geneticsa Durability Pathotype- Very high Oligogenes Mobile pathogens, specific or durability vertical, usually bad VR/ SR Pathotype- Nil Oligogenes Immobile pathogens, nonspecific durability may be major gene good resistance, NR General or Nil/low Polygenes High horizontal resistance, HR/ GR Interaction or Some mixture resistance, IR/ MR Heterogeneous Probably good oligob aOligogenes are single genes that produce a pronounced phenotypic (expressed character)
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From page 55... ...
This means they can be damaged by a different virulent race. There are major concerns about varieties that contain vertical resistance genes effective for dealing only with the prevailing races in the breeders' plots.
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From page 56... ...
For example, a soilborne fungus that is not also seedborne may develop a new race in a particular field and remain highly localized for decades. In general, vertical resistance genes are ephemeral and, when introduced in crops, are soon rendered ineffective by new races of pests and pathogens.
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From page 57... ...
In general, selection in an arid environment not favorable to airborne fungal or bacterial diseases often results in plant genotypes inadequately protected genetically against many fungal and bacterial pathogens when the crop is grown in wetter climates. For viral diseases with insect vectors and for many insect pests, climates that are only periodically moist may favor heavier crop damage.
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From page 58... ...
Rubber in both Africa and Southeast Asia still escapes South American leaf blight (Dothidella ulei) of its Amazonian homeland (Buddenhagen, 1977~.
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From page 59... ...
The semidwarfing genes of both wheat and rice have affected the epidemiology of fungal pathogens by reducing plant height and interleaf distance, consequently reducing air circulation and increasing the humidity around the plant that can contribute to enhanced pathogen densities and rate of development. This has been studied intensively in Septoria leaf blight of wheat (Scott et al., 1982, 1985~.
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From page 60... ...
For many horticultural, orchard, or ornamental crops, product quality is so important that resistance or susceptibility to pests and pathogens is often largely ignored in breeding, on the assumption that these high-priced crops will be protected by chemicals. BREEDING STRATEGIES AND THEIR IMPACT ON GENETIC DIVERSITY Modern plant breeding is essentially an evolutionary process, characterized by a more rapid rate of change than that which occurs in the slow natural selection of wild species or the landraces of primitive agriculture.
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From page 61... ...
Except in extreme environments, new hybrids yield more whether grown under good or bad conditions (Anderson et al., 1988) Germplasm Varietal Development and Vulnerability The breeding system of a crop species whether self- or crosspollinated can have a great effect on its genetic diversity.
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From page 62... ...
This is especially true for crops for which there have been long-standing breeding programs, such as the cereals. This strategy increases the odds for steady performance improvement, but it also tends to enhance genetic uniformity.
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From page 63... ...
. The most important input, however, has been that of improved varieties and hybrids from breeding programs (Duvick, 1987~.
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From page 64... ...
In 1981, Duvick (1984a) examined data from the seed TABLE 1-2 Area and Farm Value of Major Crops in the United States and Extent to Which Small Numbers of Varieties Dominate Crop Area, 1969 Area Area in Value in Number of Planted Hectares Dollars Total Major Varieties Crop (106)
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From page 65... ...
1984. Genetic diversity in major farm crops on the farm and in reserves.
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From page 66... ...
The stability of early farming systems was due not only to the genetic diversity and heterogeneity of their landraces but also to the spatial separation of farms and the temporal separation of crops that reduced vulnerability to pests and promoted more efficient use of water, nitrogen, and light (Gould, 1983~. Alternative agricultural strategies use a mixture of management and technological options that try to take advantage of natural cycles and biological interactions (National Research Council, 1989a)
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From page 67... ...
VULNERABILITY AND CROP DIVERSITY SINCE 1970 The 1970 southern corn leaf blight epidemic focused public awareness on the risks of genetic vulnerability in crops. Trends toward genetic uniformity in crop breeding programs and extensive monoculture were identified by the National Research Council (1972)
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From page 68... ...
Within 5 years of the report, one of the authors had refined a predictive model for dry bean genetic vulnerability, which suggested that pinto beans faced the highest risk of an epidemic of any dry bean commodity class (Adams, 1977~. Adams calculated a genetic distance index based on 36 chemical and agronomic traits of each cultivar to assess the homogeneity of beans grown in each production region.
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From page 69... ...
illustrates two conflicting trends in genetic diversity. In the United States, there has been a significant increase in the number of cultivars and the range of exotic germplasm from which they are derived (Cox et al., 1988a; Dalrymple, 1988~.
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From page 70... ...
At the same time, the use of exotic germplasm by wheat breeders is increasing. Genes from exotic landraces or other species occurred in a quarter of the soft red winter wheats and 7 percent of the hard red winter wheats grown on at least 16,000 ha in 1984.
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From page 71... ...
. 1966 19671968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 Crop Year FIGURE 1-2 The estimated area planted to high-yielding varieties of wheat and rice in southern and southeastern Asian nations increased steadily between 1965-1966 and 1982-1983.
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From page 72... ...
maize varieties and recommended the restoration of genetic diversity through the development of new and unrelated types of maize resistant to southern corn leaf blight (Bipolaris maydis) and other hazards.
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From page 73... ...
Exotic germplasm is a source of resistance to maize bushy stunt mycoplasma, rust (Puccinia sp.) , leaf blight, maize streak virus, maize chlorotic dwarf virus, maize chlorotic mottle virus, rootworms (Crambus caliginosellus)
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From page 74... ...
Continuation of introgression of exotic germplasm into adapted materials can be valuable for future breeding maize programs and most likely will fall to public sector and international programs like Centro Internacional de Mejoramiento de Ma~z y Trigo (CIMMYT, International Maize and Wheat Improvement Center)
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From page 75... ...
A major concern is that the Kenyan and other nationalized seed industries in developing countries may be modeling their breeding programs after the IARCs, but are not yet able to respond rapidly to resistance breakdown in released cultivars (Lipton and Longhurst, 1989~. Maize hybrids developed in both the private and public sectors have spread to developing countries where they were previously absent and, for the present, have brought novel germplasm to those countries.
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From page 76... ...
The use of semidwarf parents in breeding began to increase in 1965. Although the use of locally developed semidwarf lines as females has dominated various national breeding programs since then, nearly all of them can be traced back to sd-1 semidwarf ancestors.
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From page 77... ...
. During the 1970s brown planthopper and the associated grassy stunt virus inflicted heavy losses to the rice crop over a broad region (Dyck and Thomas, 1979~.
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From page 78... ...
assess the status of genetic resources utilization, evaluation, conservation, and vulnerability for their particular crops. The USDA has CACs for all major crops and most minor crops of importance to U.S.
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From page 79... ...
Also, new abiotic stresses from herbicides, ozone, and sulfur dioxide contamination are affecting grapes. Sugar beets also suffer from a narrow genetic base derived from very few parents.
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From page 80... ...
The genetic basis of elite germplasm, however, was found to be shallow because of extensively shared ancestry and limited use of exotic germplasm. Continued efforts to broaden the genetic diversity of primary breeding pools of major crops would provide enhanced and more stable resistance to biotic and abiotic stresses.
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From page 81... ...
The national breeding programs in developing countries are not nearly as well funded nor as effective as are IARC breeding programs. There is some question, therefore, whether sufficient breeding efforts worldwide will be devoted to developing and using new wheat and rice varieties at frequent intervals for the developing world.
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From page 82... ...
Varietal mixtures and varietal deployment can be used to supplement genetic manipulation of resistance. Additionally, the genotypes of successive releases must vary significantly to provide genetic diversity over time.
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From page 83... ...
Genetic Vulnerability and Crop Diversity / 83 Greater emphasis is needed on agronomic management strategies (for example, multiple cropping, crop rotation, tillage practices, crop mixtures, varietal mixtures, multiple lines, integrated pest management, pest trap crops) through extension services and private-sector marketing campaigns as preventative measures to pest and pathogen attack.
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