Not all genetic resources seem to have the same immediate utility. Public and private breeders use germplasm mostly from adapted and productive commercial varieties. A survey conducted by the International Plant Genetic Resources Institute (IPGRI) indicated that only 6.5% of germplasm used by breeders came directly from an exotic source. Of this small portion, two-thirds were sourced from landraces conserved ex situ in genebanks. One-third was sourced from germplasm found in situ, with a predominant usage of landraces over wild species. Another survey in Germany gave similar percentages (6.9% of commercial varieties containing materials from genebanks). Surveys of maize germplasm in the United States have given even lower figures for use of exotic germplasm resources. (http://www.worldseed.org/-assinsel/osl-ass.htm).
At the global level, world food security depends, to a large extent, on the 30 or so crop species that provide most of the dietary energy or protein, and in particular on the three crops—rice, wheat and maize—which together provide more than half of it. Efficient utilization of plant genetic resources is a key to improving agricultural productivity and sustainability and can contribute to socioeconomic development, food security and the alleviation of poverty.14 These vital resources are seriously threatened by genetic vulnerability and erosion. Genetic vulnerability is defined as the condition that results when a widely planted crop becomes uniformly susceptible to a pest, pathogen or environmental hazard as a result of its genetic constitution, thereby creating a potential for widespread losses.15 One of the main causes of genetic vulnerability is the widespread replacement of diverse varieties by homogeneous varieties. Examples include the growing of the rice variety "IR 36." In 1982 this variety was grown on 11 million hectares in Asia.16 Similarly, over 67% of the wheat fields in Bangladesh were planted to a single wheat cultivar, "Sonalika", in 1983, and 30% of Indian wheat fields to the same cultivar in 1984.17 Reports from the USA in 1972 and 1991 indicate that, for each of the eight major crops, fewer than nine varieties made up between 50% and 75% of the total acreage grown.18 Ireland's Country Report cites 90% of its total wheat area sown to just six varieties. The dangers of planting large areas to a few genetically uniform crop varieties must be recognized, as these varieties could suddenly become uniformly susceptible to new pathogen races and be wiped out.
The most famous example of this is the potato famine of 1845-1848, when a epidemic of late blight (Phytophthora infestans) wiped out the potato crop in Europe and North America (for additional details see section on the CEEM project). Some other examples include:
1. Severe epidemics of Shoot fly and Karnal bunt in the 1970s in modern wheat varieties in India;17
2. The new race of corn leaf blight in the US which destroyed more than 15% of the crop in 1970;18 and
3. In 1975, white clover varieties in the UK had to be totally abandoned for several years when a new pathogen, Scerotinia trifoliorum, killed off white clover populations throughout much of Britain—all recommended varieties were susceptible.1
Similar examples are cited for the loss of wheat in Russia due to the severe winter of 1972 and the rust attack on sugar cane during 1979/80 in Cuba. This attack on the sugar cane, which covered 40% of the country, resulted in the loss of more than a million tons of sugar, worth about US $500 million.
Genetic erosion also leads to the loss of genetic diversity. This includes the loss of individual genes as well as the loss of particular combinations of genes present in locally-adapted landraces. The main causes of genetic erosion are: replacement of local varieties; changing agricultural systems; overexploitation of species; reduced fallow; overgrazing; land clearing; environmental degradation; pests/weeds/diseases; population pressure; and legislation policies .1 It is now clear that these forces need to be better understood so that the problem can be addressed more effectively. Further, the relationship between genetic uniformity and genetic vulnerability needs to be better understood. Both genetic vulnerability and genetic erosion are poorly documented, and more research needs to be done for the development of better indicators and measurements.
At the global level, there is now a consensus to develop an integrated approach to the conservation and utilization of PGRFA in major crops. This includes the need to:
1. Reduce genetic erosion in the field and the need to promote in situ conservation;
2. Ensure that the genetic diversity of major crops is adequately represented in ex situ collections, and that these collections are secure and available for use;
3. Use genetic diversity effectively, inter alia through improvement programs; and
4. Promote sufficient levels of genetic diversity in crops and breeding lines. These issues are examined on a crop by crop basis in a recent publication.1
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