National Bureau of Plant Genetic Resources, Pusa Campus, New Delhi 110 012, India

Background and objectives
The Indian sub-continent is one of the world's mega-centres of crop origin and crop plant diversity. More than 20 major agri-horticultural crops originated here. Nearly 160 domesticated species of economic importance and over 320 species and their wild forms and close relatives are native to this region. These are distributed in eight diverse phytogeographical agroecological regions of India. The Indian region is also well known for its native plant wealth with over 800 species of ethnobotanical interest.The diversity exhibits a preponderance of variable landrace forms, primitive types belonging to different crops such as cereals, millets, legumes, vegetables, fruits, fibres, sugar-yielding types, spices, condiments, medicinal plants and others. Crops in which rich diversity occurs in India include rice, wheat, barley, pigeonpea, chickpea, small millets, greengram, blackgram, horsegram, moth bean, rice bean, cluster bean, amaranth, buckwheat, rapeseed and mustard, sesame, forage grasses, okra, eggplant, jackfruit, mango, tamarind, jamun, jute, cotton, ginger, turmeric, pepper, cinnamon and cardamom. The concentration of genetic diversity, comprising native species and landraces, is higher in the Himalayas, the peninsular region and the Andaman and Nicobar Islands. The richness of plant diversity is largely due to ecological diversity superimposed with tribal and ethnic diversification, plant usages and religious rituals.

The genetic variation in primitive landraces and wild relatives of crop plants is of immense value to breeders in improving crops through incorporating useful genes from these gene pools, particularly for resistance/tolerance to diseases. Whenever new, virulent forms of pathogens attack crops, researchers look towards germplasm collections for donors of resistance. These resources are, however, increasingly threatened now because of continuing degradation of their habitat and due to rapid replacement of locally adapted indigenous cultivars by modern high-yielding varieties. The fast-shrinking genetic diversity of our commercially grown crops means their increasing vulnerability to widespread epidemics, and pest rampage. Hence scientific management of these valuable resources, especially the wild relatives of crop plants, is of prime importance today.

Results and conclusions
The genetic diversity of primitive landraces and related wild species has provided useful genes as sources of disease resistance. Important examples have included an annual wild rice Oryza nivara providing resistance to tungro virus in cultivated rice; wild blackgram Vigna mungovar. silvestris and wild greengram Vigna radiata var. sublobata exhibiting tolerance to yellow mosaic virus; wild Abelmoschus species to yellow vein mosaic virus in cultivated A. esculentus, and wild species in the gene pools of sesame, cucumber and eggplant have been used for disease resistance. Wild sugarcane and related species in the gene pools have provided sources of resistance to stem rot. Several wild species are also used as rootstocks and breeding materials (primarily for disease resistance) for many fruits, including citrus, mango, jujube, and pome, stone and soft fruits.

Realization of the huge economic potential of India's plant genetic wealth, specifically the wild relatives of crop plants, will require increased awareness of the need to effectively conserve and use this diversity. Fortunately these efforts are addressed by the Indian Government's programmes and are conducted and coordinated by the National Bureau of Plant Genetic Resources (NBPGR), New Delhi, being the premier organization with a national mandate for collecting, exchanging, characterizing, evaluating and conserving plant genetic resources. The conservation programmes are implemented at three levels, namely genotype, species and ecosystem, and various approaches such as in situ, ex situ, in vitro and others are adopted. Undoubtedly with more genetic diversity available now and with better means to exploit it, facilitated by advances in biotechnology, India's plant genetic diversity holds great promise for contributing to the future welfare of humankind, particularly in its sustainable use as a source of disease resistance.