A number of fungal, bacterial and viral diseases are found in soybean. Major diseases of soybean include charcoal rot, Fusarium wilt, Rhizoctonia root rot, sudden-death syndrome, anthracnose, brown stem rot, phomopsis, Sclerotonia stem rot, stem canker, soybean cyst nematode (SCN) and Asian soybean rust, to name just a few of a long list (Grau et al., 2004; Tulin and Lacy, 2004). Significant yield loss is caused by these diseases. It has been reported that SCN or Asian soybean rust can completely wipe out a crop under severe conditions. Considering the magnitude of the problem, the development of resistant soybean varieties has been the breeding objective of a number of breeding programmes. Some of the diseases are epidemic in nature, while others are endemic with more severe problems in one particular location than others. Therefore, which disease gets more priority in a soybean breeding programme is the matter of the magnitude of the problem. However, the development of disease resistance in soybean varieties is a major objective in soybean breeding for two reasons: cost of production and environmental protection. The use of a resistant variety helps to reduce production costs by minimizing the use of fungicides. Reduced fungicide use in turn helps to protect the environment.
Resistance can be introgressed from wild relatives or exotic germplasms. For example, Concibido et al. (1996) mapped a major partial-resistance locus on linkage group 'G' near restriction fragment length polymorphism marker C006V in plant introduction (PI) 209332, which was effective against three SCN race isolates tested. On the basis of this finding they were able to perform MAS. Soybean rust resistance has been identified in G. tomentella and introduced into cultivated species (Schoen et al., 1992; Singh et al., 1998). There is a long list of such developments, but resistance breeding continues as new pathogen races evolve and the host-pathogen response continues to change.
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