ABSTRACT
Improvement of host plant resistance to diseases caused by soilborne microbial plant pathogens has immense practical importance. Development of cultivars with acceptable level of resistance to soilborne microbial pathogens is considered as the best approach, because it can reduce or avoid the cost and effects of other chemical, physical, biological, cultural and regulatory control methods, in addition to being ecofriendly. Host resistance indicates the magnitude or capacity of a plant species or cultivar to reduce activity or harmful effects of a microbial pathogen. Plant disease resistance has been studied from different angles to integrate evolutionary, genetic, epidemiologic and economic conceptual framework. Plants are exposed to several microbial pathogens present in the soil environment. However, they are susceptible to only a small proportion of the pathogens, indicating the existence of effective defense mechanisms that prevent infection by large proportions of microbial pathogens. It appears that resistance of plants to microbial pathogens is the rule, while susceptibility is an exception. Plant defense systems operate at different stages of infection by microbial pathogens. The first line of defense is expressed at whole plant level, where cells containing thick cuticle, thickened walls and other physical structures may obstruct pathogen entry/further development. The second line of defense may be exhibited at cellular level, where a well-coordinated, controlled and dynamic regulation of gene activity, results in the synthesis of different kinds of proteins and antimicrobial compounds that arrest the progress of infection. Both passive (preformed) and active nature of defense processes are involved in the prevention of pathogen entry into the plant for deriving nutrition for its growth and reproduction. Two major forms of resistance have been differentiated and they are designated differently, as constitutive and inducible, qualitative and quantitative, race-specific and race-nonspecific and inoculum-reducing and rate-limiting resistance to describe similar phenomena, using overlapping and sometimes confusing terms (Narayanasamy 2002, 2008).
