1
Genetic Vulnerability and Crop Diversity

Over the ages the tendency of crop improvement efforts has been to select varieties with traits that give the highest return, largely by concentrating on genetic strains that combine the most desirable traits. The resulting homogeneity and uniformity can offer substantial advantages in both the quantity and quality of crop harvested, but this same genetic homogeneity can also reflect greater susceptibility or pathogens. Thus it appears the more that agricultural selection disturbs the natural balance in favor of variety uniformity over large areas, the more vulnerable such varieties are to losses from epidemics. The increased risks presented by genetic selection and the increased cultivation of only a few selected cultivars are easily perceived. Chapters 1 and 2 of this reports focus on crop vulnerability, because it is a broadly recognized problem. The issue of genetic vulnerability, however, is only one of several important problems affecting the management of global genetic resources.

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. These concerns prompted the committee to reassess where the world agriculture community stands today with respect to the risks of genetic vulnerability and progress in diversifying crop gene pools.



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 47
Managing Global Genetic Resources: Agricultural Crop Issues and Policies 1 Genetic Vulnerability and Crop Diversity Over the ages the tendency of crop improvement efforts has been to select varieties with traits that give the highest return, largely by concentrating on genetic strains that combine the most desirable traits. The resulting homogeneity and uniformity can offer substantial advantages in both the quantity and quality of crop harvested, but this same genetic homogeneity can also reflect greater susceptibility or pathogens. Thus it appears the more that agricultural selection disturbs the natural balance in favor of variety uniformity over large areas, the more vulnerable such varieties are to losses from epidemics. The increased risks presented by genetic selection and the increased cultivation of only a few selected cultivars are easily perceived. Chapters 1 and 2 of this reports focus on crop vulnerability, because it is a broadly recognized problem. The issue of genetic vulnerability, however, is only one of several important problems affecting the management of global genetic resources. 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. These concerns prompted the committee to reassess where the world agriculture community stands today with respect to the risks of genetic vulnerability and progress in diversifying crop gene pools.

OCR for page 47
Managing Global Genetic Resources: Agricultural Crop Issues and Policies The concerns became apparent in 1972 with the report Genetic Vulnerability of Major Crops (National Research Council, 1972). It was prepared by scientists who, alerted by the 1970 epidemic of southern corn leaf blight (Helminthosporium maydis) 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. Nearly 2 decades later, the issue of genetic vulnerability is still fresh, and debate continues on the risk that it poses. This chapter addresses genetic vulnerability, first by defining it. The chapter discusses when vulnerability becomes a cause for concern and how the risk it presents can be measured. It examines the trends that contribute to genetic vulnerability and the strategies for reducing it. It evaluates the impact of modern plant breeding programs on genetic vulnerability, and the changes that have occurred in recent years that have increased or decreased genetic vulnerability. Finally, it assesses what remains to be done to minimize vulnerability. WHAT IS GENETIC VULNERABILITY? The term genetic vulnerability has been widely used to invoke fears of disastrous epidemics and threats to global food security, often without clarifying what is meant by the term. This chapter uses genetic vulnerability to indicate the condition that results when a crop is uniformly susceptible to a pest, pathogen, or environment hazard as a result of its genetic constitution, thereby creating a potential for disaster. Critical Factors and Assumptions Two important factors interact to increase the potential for crop failure: (1) the degree of uniformity for the trait controlling susceptibility to the hazardous agent or environmental stress, and (2) the extent of culture (often monoculture) of the susceptible variety. The greater the uniformity for a susceptible trait and the more extensive the area of cultivation, the greater the risk of disaster. In the case of pest or pathogen attack, two additional factors enhance the risks: (1) a highly dispersible disease or insect agent, and (2) favorable environmental conditions for the multiplication of the agent. Susceptibility to diseases, insects, and environmental stresses is a natural phenomenon characteristic of all plants, no matter how primitive

OCR for page 47
Managing Global Genetic Resources: Agricultural Crop Issues and Policies or highly bred. Natural epidemics, drought, or other adverse conditions may affect any landraces or cultivars grown by farmers. The extent and impact of the disaster on the food supply and regional economy may, however, be lessened on agricultural systems that incorporate a variety of crop and landraces. There is cause for concern when extensively planted cultivars of major crops are derived from limited gene pools and, hence, are uniform for a high percentage of traits with narrow based resistances to common pathogens or other agents. 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 scientist 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. Farmers in developing countries who intensify their cultivation practices and adopt improved varieties may increase their chances of losses from epidemics. One of the rationales for crop germplasm conservation is that plant genes have utilitarian value. When they are effectively used they can decrease crop susceptibility to natural predators, pathogens, and stresses. Thus, genetic vulnerability can be characterized in terms of genetic resources use. Genetic vulnerability results from the improper or inadequate deployment of these genetic resources, in conjunction with biotic and abiotic stresses. It may also be more inherent in some agricultural systems than in others. The Role of Uniformity Genetic uniformity is desirable for many agronomic traits of concern to farmers, processors and consumers and does not, by itself, make a crop vulnerable. In other words, uniformity and susceptibility need not be synonymous. Crops can be vulnerable even when they are genetically quite dissimilar if they have in common a trait (often governed by a single gene) that renders them susceptible to a pathogen, pest, or environmental stress. The chestnut blight (Cryphonectria parasitica) epidemic is a classic case in which genetically heterogeneous species was virtually destroyed by a fungal pathogen that is highly damaging to the American and several other species of chestnut trees (Anagnostakis, 1982). However, such cases are unusual.

OCR for page 47
Managing Global Genetic Resources: Agricultural Crop Issues and Policies More commonly, genetically similar crop varieties that may have in common the same critical gene for resistance to important diseases or insects are grown over large areas in successive seasons. This dependence on a single source of resistance is, arguably, the crux of the problem. 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 plant-stress interaction and identifying preventive measures that breeders can take to build in greater resistance. Another is to reduce contiguous areas of land planted to the same varieties. The Role of Monoculture Intensive and continuous cultivation of uniform crop varieties enhances opportunities for pathogen or pest evolution and the natural selection of new strains able to attack their hosts successfully. In a monoculture of a single variety or genetically uniform group of As Russian wheat aphids feed, the wheat leaves tend to roll around them, making them difficult to kill with conventional pesticides. Credit: U.S. Department of Agriculture, Agricultural Research Service.

OCR for page 47
Managing Global Genetic Resources: Agricultural Crop Issues and Policies varieties, the pest must overcome only one genotype, as opposed to numerous genotypes in mixed cultures. Dense stands can facilitate the spread of the attacking population. The effect is particularly severe in tropical environments, where natural reproductive cycles of the insect or disease often are not curtailed by changing climatic conditions. Failure to rotate crops, or sequential planting of two crops that share a common genotype and the same pathogens or insect pests, produces similar results. Nevertheless, the requirements of mechanization and consumer demands follow-cost, high-quality food make it difficult to alter these patterns. Extensive monocultures clearly pose a risk, yet the spread of closely related, high-yielding varieties (HYVs) continues, despite the risk of vulnerability. Farmers are generally not concerned about vulnerability until production is affected. They depend on plant breeders to supply replacement varieties when existing ones fail. Replacement varieties are often readily available in industrialized countries, but may not be in many developing countries because breeding or seed production programs may be limited or nonexistent. The Role of Environment Environmental conditions play both direct and indirect roles in triggering genetic and physiologic plant responses. Weather conditions may promote crop susceptibility. Warm conditions tend to favor the development of fungal and bacterial pathogens, whereas climates that are only periodically moist may enhance damage from insect-vectored viral diseases and seedborne toxins (for example, aflatoxins or toxins of Fusarium kernel rot). Also, the weather greatly influences off-season survival of the attacking population and the amount of off-season inoculum available to initiate a seasonal epidemic. 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. Plants may vary considerably in their tolerance of or susceptibility to abiotic stresses. Drought or excessive salt levels can cause complete crop failure. Cold or hot weather during flowering may induce pollen sterility, and high temperatures may cause flower drop and poor seed set. Although it is sometimes possible to incorporate resistance into crop varieties, the genetic control of stress resistance traits is usually complex. Experimental testing, and thus varietal development, are often difficult.

OCR for page 47
Managing Global Genetic Resources: Agricultural Crop Issues and Policies Measuring Genetic Vulnerability Three components must be considered when assessing genetic vulnerability. First, the area devoted to major cultivars must be monitored by analyzing volume and distribution of seed sales (which is closely held information in the private sector) through actual farm surveys. Such information was more readily available in the past than it is at present, because many national agricultural agencies (including the U.S. Department of Agriculture [USDA]) have ceased gathering such data in recent years. Second, trends in pest and pathogen evolution must be monitored to identify those that pose the greatest threats to common cultivars under specified conditions. Given the global patterns of varietal distribution, this should be done on a worldwide basis. For rice, maize, wheat, beans, and the several other commodities, information could be developed through the international agricultural research centers (IARCs). Hundreds of collaborating scientists around the world grow and evaluate breeding lines developed by the IARCs. They collect data on agronomic factors and on susceptibility to pests and pathogens. Similar public networks (although much more limited in scope) operate for a few crops in the United States through the USDA and state agricultural experiment station collaborators and in Europe. These networks can be excellent sources of information on developing pest or pathogen problems. However, information is not readily available for minor crops and commodities traditionally developed in the private sector (for example, hybrid corn, coffee, bananas, and other plantation crops), although major multinational seed companies often find it worthwhile to develop their own networks. Networking can be a very cost-effective means both of disseminating germplasm and monitoring evolving pest problems. Third, the extent of genetic uniformity among major cultivars must be determined. 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) usually do not reveal the ecogeographic origins of important resistance genes. Biotechnological methods (see Chapter 7) could be used to develop gene probes and to construct genetic maps, circumventing some of these problems, but the costs are high and the techniques have not yet been broadly applied. One of the few thorough studies of genetic uniformity was done on rice by scientists based at the International Rice Research Institute

OCR for page 47
Managing Global Genetic Resources: Agricultural Crop Issues and Policies (IRRI). Hargrove et al. (1985) analyzed the diffusion of rice genetic materials among 27 breeding programs over a 20-year period and found a disturbing similarity in the genealogies of improved rice varieties across Asia. Some narrowly focused studies have been done on the geographic distribution of specific traits, such as genes for fertility restoration in hybrid rice based on cytoplasmic male sterile germplasm (Li and Zhu, 1989). Nevertheless, there are few such studies, and key pedigree information is not available for varieties created by the private sector. The product of these analyses is not a quantitative measurement but a pattern formed by correlation of the trends in the amount of land planted to certain crops, pest-pathogen development, and extent of uniformity for major resistance genes. Congruent with the definition used here for genetic vulnerability, the greatest risk results when widely planted cultivars are exposed to increasingly severe pathogen or pest problems for which they lack broadly based resistance. Measurement of the trends discussed above, as well as others, indicates the degree to which crops are at risk from genetic vulnerability, a risk that is aggravated by intensive cultivation practices. The following is an examination in greater depth of biological inter-actions that lead to vulnerability and the impact of breeding programs design to manipulate those interactions. PLANT, PATHOGEN, PEST, AND ENVIRONMENTAL RELATIONSHIPS The genetic theories of mutation, selection, population biology, and host-pest genetic interaction apply equally to both pathogens (bacteria, viruses, and fungi) and invertebrate pests (aphids, ants, weevils, other arthropods, and nematodes), both of which are used throughout this section as examples. When plant, pest, or pathogen responses to environmental stresses are genetically controlled, the same theories apply. Evolutionary Nature of the Relationship A crop variety's responses to a pathogen— whether it is resistance or susceptibility— is a result of its evolutionary and genetic relationship with that organism. This accounts for the phenomenon known as the breakdown of resistance, in which a variety previously resistant becomes susceptible to new forms or races of the pathogen population. In fact, the crop's genetic resistance is unchanged and is still effective against the original pest or pathogen population. It is the genetics of the pathogen that has been altered through natural selection so that it

OCR for page 47
Managing Global Genetic Resources: Agricultural Crop Issues and Policies can attack the crop variety. The variety is then considered susceptible, although no change in its genetics has occurred. The task of the plant breeder becomes to incorporate into the now-susceptible variety additional resistance alleles that are effective against the new pathogen population. Seldom, if ever, is total genetic protection from disease possible. Resistant varieties are those with significantly decreased disease or pest susceptibility that does not have a negative impact on the growth and biomass production of the plant, and therefore the yield. This decreased susceptibility, however, lasts only until the pathogen or pest population gains new genetic ability to overcome the resistance. 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-specific or vertical, VR/SR Very high Oligogenes Mobile pathogens, durability usually bad Pathotype-nonspecific major gene resistance, NR Nil Oligogenes Immobile pathogens, durability may be good General or horizontal resistance, HR/GR Nil/low Polygenes High Interaction or mixture resistance, IR/MR Some Heterogeneous oligob Probably good a Oligogenes are single genes that produce a pronounced phenotypic (expressed character) effect, as opposed to polygenes, which have individually small effects. b Some authors (Simmonds, 1979) are inclined to attribute some weight to heterogeneity for polygenic systems, but this matter appears to be undecided. SOURCE: Adapted from Simmonds, N.W. 1988. Synthesis: The strategy of rust resistance breeding. Pp. 119-136 in Breeding Strategies for Resistance to the Rusts of Wheat, N.W. Simmonds and S. Rajaram, eds. Mexico, D.F.: Centro International de Mejoramiento de Maíz y Trigo. Reprinted with permission, ©1988 by Centro Internacional de Mejoramiento de Maíz y Trigo.

OCR for page 47
Managing Global Genetic Resources: Agricultural Crop Issues and Policies pathotype nonspecific; major gene resistance, which is pathotype nonspecific; and mixture or interaction resistance. Types of Relationships Under the genetically controlled resistance patterns described above, the susceptibilities of crop plants to pests or pathogens can develop in many different ways. These host-pathogen relationships fall into three general groups: those characterized by (1) changes in the pathogen or its effect, (2) movement of varieties or pathogens to new biomes, or (3) changes in agronomic practices or pest control strategies. In the first group, major pathogens may evolve, minor pathogens may become more serious, or the environment may enhance the severity of the pathogen's attack. In the second group, a crop may prove to be susceptible to newly encountered pathogens when grown in a new location, or exotic pests introduced into the crop's home region may be successful in attacking it. In the third group, new agronomic traits or practices may render the crop susceptible to new pathogens, or the crop may be vulnerable to previously discounted pests when control strategies or priorities change. Most these responses are intensified by genetic uniformity, ecologic uniformity, of agronomic conditions, or both. Examples illustrating the complexity of interactions that plant breeders must deal with when seeking pest and pathogen resistance in the crops are discussed below. Evolution of Major Pathogens Plants with resistance conferred by a single gene or with resistance to a specific pathogen race are said to have vertical resistance. 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. The critical question is: How likely is the new race to develop or to be introduced? Put differently, how soon will there be severe losses because of breaching of the resistance by the rest or pathogen species? The answer depends on how frequently different races arise in the existing pathogen population, how well adapted they are for survival in the off season, and the size and the dispersion of the pest or pathogen population. The record of single major genes in providing lasting or durable resistance is mixed. The R genes for potato late blight (Phytophthora infestans) resistance and the Sr genes for stem rust resistance in wheat are two classic examples. In both cases, at least some resistant varieties

OCR for page 47
Managing Global Genetic Resources: Agricultural Crop Issues and Policies collapsed because of the selection and spread of new pathogen races able to attack varieties carrying the resistant genes. In wheat, certain combinations of Sr genes in new varieties led to longer-term resistance to stem rust (Puccinia graminis) . In the United States, no widespread epidemic of wheat stem rust has occurred since the late 1950s. However, outbreaks have appeared in other regions. The biology of the pest and its interaction with the host plant greatly influence the epidemic potential of any new race or biotype that may be selected (Simmonds, 1979). 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 pest and pathogens. This is specially true for cases of pathogens that are windborne (for example, the powdery mildews) or of pests that are highly mobile or that have high reproductive rates (such as the brown plant hopper, (Nilaparvata lugens) . It is important to consider the evolutionary relationship of the resistance genes and the pest or pathogen. If the two have coevolved in the primary center of diversity, it is highly likely that the pest population contains genes effective against each plant gene conferring resistance (Ashri, 1971; Browning,1972). In developing new varieties, plant breeders must consider whether resistance is sufficiently durable to be economically useful. Several guidelines emerge from past experience. For pests with a high epidemiologic potential that have coevolved with the host plants, major genes for resistance are ephemeral in their utility, if used singly. In some coevolved systems, pyramiding of many genes may work in augmenting the durability of the resistance. Genes for vertical resistance can be used for cases in which experience has shown that selection of new races is slow, the spread of new races is slow, or pathogen survival from season to season is low. Minor Pathogen Enhancement In addition to the major pests and pathogens that a breeder addresses in a breading program, there are many others that have only a minor impact on crop performance. It is assumed that selection for high yield will ensure sufficient resistance against yield reduction by these minor pests. However, minor pathogens can become important if there is a lack of challenge in the breeders' plots and a difference in environmental conditions between the plots and farmers' fields. The inoculum for

OCR for page 47
Managing Global Genetic Resources: Agricultural Crop Issues and Policies the minor pathogen may be so low in the plots that increased susceptibility will not be detected. Changes in the agronomic environment could result in an increased pathogen population. Thus, without any shift in the genetics of the pathogen, a minor pest may become a major problem. A good example is the recent development of gray leafspot disease (Cercospora zeae-maydis) of maize into a disease of potential importance in the United States. Environmental Enhancement of Pathogen Effects The environment affects disease levels by altering, increasing, or decreasing pest development. Thus, levels of resistance adequate in one environment may be much too low in another. If the environment is not conducive to pathogen or pest increase. It is difficult to select for resistance. 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. In 1985, rains broke the worst drought in a decade in the Sahel, but they also caused an outbreak of the Senegalese grasshopper. An observer (right) from the Food and Agriculture Organization examines the remains of a farmer's crop of millet. Credit: Food and Agriculture Organization of the United Nations.

OCR for page 47
Managing Global Genetic Resources: Agricultural Crop Issues and Policies to certain viruses (Nault and Findley, 1981). Many maize breeders report that they can also find resistance in already adapted elite lines (Duvick, 1984a). They may be picking up resistance factors from the earlier introgression of exotic genes, however. 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 Maíz y Trigo (CIMMYT, International Maize and Wheat Improvement Center). The international maize outlook has changed considerably over the past 20 to 30 years. During the period 1961–1965 to 1983–1985, world maize production increased by 84 percent, representing an annual growth rate of 4 percent (2.8 percent in developing countries) (Centro Internacional de Mejoramiento de Maíz y Trigo, 1987b). By 1985, improved genotypes were grown on 40 million ha or about one-half of the total maize area in developing countries. Thirty million hectares are planted with hybrids, with the People's Republic of China, Brazil, and Argentina accounting for 80 percent of that area (Centro Internacional de Mejoramiento de Maíz y Trigo, 1987b). Ten million ha are planted with improved open-pollinated varieties. In the remaining half of the total maize area in developing countries, however, adoption rates of improved varieties are low. This may be due to seed production systems that restrict the diffusion of improved varieties (Timothy et al., 1988) or because farmers prefer landraces. The rate of variety turnover to hybrid maize in developing countries can be illustrated by using Kenya as an example. Maize is the major staple in Kenya. Until the 1950s, nearly all varieties grown in Kenya were local landraces traceable to Tuxpeno-Hickory King landraces brought from North America in the nineteenth century (Timothy et al., 1988). In the late 1950s, two exotic varieties from Latin America, Ecuador 573 and Costa Rica 76, were introduced as breeding stocks to hybridize with local landraces. By the mid-1960s, the first inbred lines and hybrids were released in Kenya and were grown on half of the large-scale farms there. The impact of the hybrids' high yields was so great (30 percent higher than earlier synthetic hybrids) that Kenya's hybrid maize area increased from 0 to 600,000 ha between 1963 and 1981. Both large- and small-scale farmers have become dependent on hybrids, so that 61 percent of Kenya's total maize area was devoted to hybrids in 1985, and another 5 percent to improved open-pollinated varieties (Timothy et al., 1988). CIMMYT has been one of the primary suppliers of germplasm to Kenyan national programs, even though most of the CIMMYT populations are generally susceptible to common rust (Puccinia sp.) and

OCR for page 47
Managing Global Genetic Resources: Agricultural Crop Issues and Policies turcicum blight (Helminthosporium turcicum), both of which are problems in Kenya (Timothy et al., 1988). Streak virus tolerance has been transferred to inbred lines from a maize composite developed by the International Institute of Tropical Agriculture (IITA). Despite these introductions of exotic germplasm, the germplasm, base in Kenya remains limited because of the policies of the Kenyan Seed Company. From 1963 to 1981, the Kenyan Seed Company and its government-controlled predecessors released only five open-pollinated varieties, four varietal crosses, three double-cross hybrids, two three-way-cross hybrids, and two top-cross hybrids; some of these used the same CIMMYT parents (Timothy et al., 1988). It is therefore not surprising that a severe outbreak of leaf blight (Alternaria triticina) began in 1978 and became widespread by 1979 (Singh et al., 1979). It affected two HYVs and several of the remaining local landraces as well. 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. Dominance by just a few hybrids or a few sources of germplasm may now be of concern in some of those countries. However, 49 percent of the total maize-growing area in developing countries remains planted to locally adapted landraces and the progeny of purchased, open-pollinated seed (Centro Internacional de Mejoramiento de Maíz y Trigo, 1987b). Those countries with a high proportion of area sown to the new HYVs face a dilemma. They probably cannot return to cultivating the indigenous maize varieties without reducing yields, yet they cannot continue to plant the same high-yielding varieties indefinitely because new pest races likely will appear, causing disastrous epidemics. The vulnerability is particularly acute in tropical and subtropical areas that lack a cold season (and often little dry season). Lack of support for public plant breeding efforts in many developing countries makes it unlikely that they will be able to mobilize new varieties in sufficient time to prevent disaster. Rice Genetic uniformity is a greater concern in rice than it is in wheat because of this crop's limited diversity for three major traits: the single locus for semidwarfism (sd - 1); a common cytoplasmic ancestry

OCR for page 47
Managing Global Genetic Resources: Agricultural Crop Issues and Policies for many HYVs; and a widely used, single source of cytoplasmic male sterility in Chinese hybrid rices (Chang, 1984a; Chang et al., 1985). Cytoplasmic uniformity, a concern in maize since 1970, is now a concern in rice and other major crops (Hargrove et al., 1980). Resistance or susceptibility to disease is influenced by cytoplasmic inheritance not only in maize and rice but also in strawberries, potatoes, wheat, grain sorghum, cotton, and grapes(Hargrove et al., 1980). In rice, 38 percent of the female parents used worldwide in rice breeding in 1983 and 1984 traced maternally to one Chinese-Indonesian variety named Cina, which is commonly used at IRRI and elsewhere. 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. Hargr et al. (1985:9) concluded that "the genealogies of improved rice varieties across Asia appear disturbingly similar." Although no weakness in Cina cytoplasm or sd-1 semidwarfs has been reported, in strong recommendations were made to diversify the genetic base of future rice varieties. The area planted to rice HYVs (more than 73 million ha in 1982 and 1983) exceeds those planted to wheat and maize HYVs in developing countries (Dalrymple, 1986b). It is well known that a very small number of varieties accounts for a very large proportion of the area planted to rice HYVs. In Malaysia, for example, three varieties accounted for about 40 percent of all Malaysian rice plantings in 1981 and 1982. In Indonesia, two varieties accounted for 54 percent of the total rice plantings in 1983 and 1984. In the Philippines, it was estimated that two IRRI varieties occupied about 90 percent of the entire rice-growing area during the 1984 dry season (Dalrymple, 1986b). The widespread planting of a few closely related varieties with intensive cultivation practices and multiple cropping has led to a breakdown in varietal resistance and to outbreaks of tungro virus, grassy stunt virus, and the brown planthopper (Nilaparvata lugens) in parts of Bangladesh, southern India, Indonesia, Malaysia, the Philippines, Thailand, and southern Vietnam, although none of these was as destructive as the southern corn leaf blight was in the United States (Chang, 1988). In contrast, no serious epidemics have been reported from parts of Thailand, where traditional varieties are commonly grown in rotation with HYVs (Chang, 1984a). Multiple and continuous cropping of the same HYV and staggered planting dates across vast production areas have contributed to the breakdown of monogenic (or vertical) resistance because of changing biotypes of

OCR for page 47
Managing Global Genetic Resources: Agricultural Crop Issues and Policies the brown planthopper in the Philippines, Indonesia, Sri Lanka, and Vietnam. Sequentially released new varieties (for example, IR26 with the gene bph-1, IR36 and IR42 with the gene bph-2, and IR56 and IR60 with the gene bph-3) with related genetic backgrounds were each developed by adding a gene to deal with an emerging insect biotype that had rendered resistance in earlier releases ineffective. Although these varieties provided some relief, new pest biotypes soon evolved (Heinrichs et al., 1985). The new insect populations also transmitted virus more effectively, as in the case of the tungro virus transmitted by the green leafhopper (Nephotettix spp.) (Dahal et al., 1990). Through the misuse of host resistance, rice farmers have suffered severe losses from the resulting "boom-and-bust" cycles of oligogenic resistance (Chang, 1984a). 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). Following a decade of planting, the virus resistance gene (Gsv) obtained from Oryza nivara was rendered ineffective when a second biotype of the virus emerged. Similarly, when semidwarfs were widely grown, the predominant stem borers shifted from striped borers (Chillo zacconius) to yellow borers (Scirpophaga incertulas ) (Chang, 1988). Indiscriminate use of insecticides has also been linked to a resurgence of the brown planthopper in some areas (Chelliah and Heinrichs, 1980). By contrast, Indonesia has made notable progress controlling pest populations by varietal rotation, pest monitoring, community wide pest control, and synchronization of planting dates (Manna et al., 1985). With more intensive cultivation of fewer varieties, rice diseases and insects continue to grow in number, intensity, and geographic distribution (Chang, 1984a, 1988). Examples include bacterial blight (Xanthomonas campestris), which reduced seed yield by 50 percent, largely on an HYV under irrigation in Niger in 1983 (Reckhaus and Adamou, 1986), and rice yellow mottle virus, which spread from irrigated paddy rice in Niger, Burkina Faso, and Mali and has infected HYVs from IRRI in Côte d'Ivoire and neighboring countries (Awoderu et al.,1987). The Republic of Korea initially reported spectacular increases in grain yield after using the semidwarf gene in Tong-il and other varieties. In 1978, about 76 percent of the rice area was planted to HYVs. However, a cool season and widespread blast epidemic in 1979 affected the HYVs more severely than they did the traditional varieties and led to heavy losses (Rural Development Administration, 1985). The area of HYVs dropped to 20 percent in 1988, and national rice production slumped.

OCR for page 47
Managing Global Genetic Resources: Agricultural Crop Issues and Policies Hybrid rice in the People's Republic of China represents another potentially vulnerable situation. The F1 hybrids grown in the major rice production areas share a common cytoplasmic male sterility source and the sd-1 locus. The area planted with hybrids has rapidly expanded from 5 million ha in 1979 to 15 million ha in 1990. So far the Wild Abortive cytoplasm remain the most stable source of male sterility (Yuan and Lin, 1989). Rice faces special pressures from narrowly based diversity for major traits, continuous use of the same variety or close relatives in multiple cropping, dangerously intensive cultivation practices, reliance on monogenic resistance to important insect pests, and warm and humid environments that sustain the multiplication of pests and rapid shifts within insect populations. The genetic base has also narrowed in temperate regions, such as in Japan (Kimura et al., 1986) and the United State (Dilday, 1990). The inclusion of wheat and rice in continuous cropping systems (Bangladesh, the People's Republic of China, and India) poses new risks of developing new pathogens common to the two hosts (Chang, 1988). The custom of growing traditional varieties in rotation with HYVs in different crop seasons, as is practiced in Thailand, can reduce potential crop losses. Minor Crops In 1988, the Agricultural Research Service (ARS) of USDA requested that the Crop Advisory Committees (CACs) 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. agriculture (National Research Council, 1991a). For apples, two cultivars account for more than 50 percent of the entire U.S. crop. The CAC for apples concluded "commercially cultivated apples are so lacking in genetic diversity that they could easily become victim to a catastrophe" (Apple Crop Advisory Committee, 1987:16). Walnuts are represented by a few cultivars, all of which come from a small gene pool of progenitors. Three diseases and two pests do economic damage to the walnut crop. Prunus cultivars in the United States— peach, plum, cherry, almond, and apricot— have a limited genetic base. Most commercial peach cultivars can be traced back to fewer than a dozen parents, with the variety Chinese Cling represented in the majority of them. Commercial nectarine cultivars can be traced back to four parents. The entire tart cherry industry is based on a single heirloom cultivar, Montmorency. The sweet cherry industry uses the Bing cultivar on

OCR for page 47
Managing Global Genetic Resources: Agricultural Crop Issues and Policies A crop of sunflowers is grown for the production of sunflower oil. Credit: Food and Agriculture Organization of the United Nations. 75 percent of its acreage in the northwestern United States. Apricots have a narrow gene base, whereas plums have the broadest of any of the Prunus crops in the United States. The Prunus CAC considered all of these stone fruit tree crops in the United States to be susceptible to insect and disease pests. Because the ecologic risks associated with certain chemical pesticides limit their use in pest and disease control, vulnerability will likely increase. The Vitis CAC said that grape is a prime example of genetic vulnerability because of uniformity. It cites example of "the narrow genetic base upon which most of the world's commercial grape production rests" (Grape Commodity Advisory Committee, 1987:10). It suggested that a majority of vineyards in the United States contained grapevines susceptible to a wide range of pests. Pests and diseases are being introduced into formerly "clean environments" (Grape Commodity Advisory Committee, 1987:11). 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. They are vulnerable to four diseases and two major pests. Between 1974 and 1976, the sugar beet crop in the central and western parts of the United States suffered from a powdery mildew epidemic (Erysiphe betae) . Another root crop, the sweet potato, is considered to be in a critical state. One cultivar makes up 75 percent

OCR for page 47
Managing Global Genetic Resources: Agricultural Crop Issues and Policies of sweet potato production in the United States, and 79 percent of all cultivars are derived from just three parents. Sunflower hybrids grown in the United States also have a very narrow genetic base, with most being derived from a inbred lines produced by USDA. Only one source of cytoplasmic male sterility is used throughout the entire world. Although genes for resistance to some major insect and disease stresses can be found in the remarkably complete assemblage of wild North American sunflower (Heiser, 1975) now 1975) now maintained by USDA, the wild sunflower program has been severely curtailed because of a lack of funds. Although the CAC reports identify other crops for which the genetic bases have been broadened (cowpeas, peanuts, tomatoes, clover), on the whole, genetic diversity in the minor crops appears to be seriously restricted and to lag behind gains that have been made for major crops. The level of resources it has taken to build in early warnings of potential epidemics and alternate protection strategies for major crops is proportionately reduced or absent for minor crops. Yet, the costs in loss of employment, closure of farms, or disappearance of industries and businesses resulting from an epidemic would be severe, both economically and socially. Genetic Vulnerability Today For major crops in developed countries, varietal turnover and the number of varieties planted have increased since 1972, indicating that the overall level of genetic vulnerability may have decreased. 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. In the United States most minor crops have extremely narrow genetic bases, and progress in diversifying them lags far behind that in major crops. The relatively small industries associated with many of these crops do not have the resources to incorporate rapidly needed genes for resistance into new cultivars. The fruit crops are particularly vulnerable to epidemics in the near future, because many of the chemicals that have buffered them from disaster over the past 3 decades are now being withdrawn from the market. Small farmers and hand laborers may be adversely affected by the lack of genetic protection bred into these crops. In the developing world, high-yielding hybrids and modern varieties of the major crops (especially rice and wheat) have come into

OCR for page 47
Managing Global Genetic Resources: Agricultural Crop Issues and Policies dominance within the past 15 to 25 years. The immediate effect of these introductions is to make available novel genes for resistance to widespread diseases to which landraces may have been susceptible. Simultaneously, rapid losses of native diversity are occurring. As a few releases from the same breeding programs come to dominate a developing country's production, increased vulnerability will almost certainly emerge. It is doubtful whether breeding programs in developing countries can react as rapidly to future epidemics as U.S. maize breeders did between 1970 and 1972. For the past 20 years, leadership in wheat and rice breeding for developing countries has come from CIMMYT and IRRI, respectively. There are indications that these institutes will devote smaller proportions of their effort to variety development in the future. 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. This is of particular significance if the IARCs move away from breeding in favor of other activities, such as biotechnology or development of farming systems for marginal environments before national breeding capabilities are more uniformly strengthened. At least 5 to 10 years of effort are needed to put strong national breeding programs into place or to improve ineffective ones. Those countries with undersized or ineffective programs that are now depending on the IARCs for new HYVs must begin as soon as possible to build their national breeding programs to the size and efficiency that will serve their countries properly. The pooling of several small programs can, in theory, provide a sufficient base for developing and using improved varieties at frequent intervals. Unfortunately, the histories of planned, government-led, intercountry cooperation show few successes. Remarkable skills in management and diplomacy and stable resources are needed to make such cooperation productive. However, commodity-based networks such as the Programa Regional Cooperativo de Papa (PRECODEPA, Cooperative Regional Potato Program) for potatoes and the Latin American Maize Project for maize promise to be effective in disseminating germplasm and promoting crop improvement. RECOMMENDATIONS Assessment of genetic vulnerability involves the synthesis of four components: the area devoted to major cultivars, trends in pest and

OCR for page 47
Managing Global Genetic Resources: Agricultural Crop Issues and Policies pathogen evolution, the extent of genetic uniformity for resistance or susceptibility to biotic and abiotic stresses, and the ability for development of replacement cultivars. Such data are essential to monitoring the potential for severe crop losses. Reductions in public sector funding for agricultural research and service agencies have seriously diminished capabilities for monitoring and responding to crop vulnerability in the United States and other nations. The potential for crop vulnerability must be nationally and globally monitored. Information on potential threats should be exchanged among cooperating institutions (for example, the Food and Agriculture Organization of the United Nation, IARCs, USDA, and other national agencies). Responsibility for gathering data lies in national authorities that oversee varietal development and use, and in international and multinational organizations that distribute seed widely. Genetic uniformity per se does not cause crop susceptibility or epidemics; indeed, uniformity for many traits is demanded by farmers, processors, and consumers. When susceptibility to diseases, pests, or environmental stresses exists, then uniformity for that trait (regardless of whether the variety is otherwise homogeneous or heterogeneous) enhances the risk of crop damage. A wide range of genetic and agronomic strategies should be employed to minimize crop uniformity and consequent susceptibility. Plant breeders and agronomists should balance the need for uniformity in agronomic traits with the need for diversity in resistance to biotic and abiotic stresses. Extension specialists should work with processor and consumer groups to win acceptance of greater diversity in crop varieties. Complex interrelationships among plant hosts, pathogens, pests, vectors, and the environment are evolutionary in nature and determine the crop response— whether it will be resistant or susceptible. Pyramiding of resistance genes provides more durable resistance; single-gene resistance is often more easily overcome by evolving pest populations. 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. Agricultural systems that mimic natural ecosystems through use of, for example, multiple lines, varietal mixtures, relay cropping, and integrated pest management, are significantly more stable than monocultures of a few genetically close varieties and, in many cases, provide yields at least as good as those of monoculture systems.

OCR for page 47
Managing Global Genetic Resources: Agricultural Crop Issues and Policies 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.

OCR for page 47
Managing Global Genetic Resources: Agricultural Crop Issues and Policies This page in the original is blank.