ABSTRACT
Biological control or biocontrol of crop diseases has been considered both in a narrow sense to indicate the control of one organism by another organisms and also in a broad sense to indicate the use of natural or modified organisms, genes or gene product to reduce the effects of pathogens and to favor development of crop plant species (Beirner 1967; Cook 1987). Biological disease management involves utilization of biotic and abiotic agents that act through one or more mechanisms to reduce the potential of the pathogen directly or indirectly by activating host defense systems to reduce the disease incidence and/intensity. Several investigations have established that the combination of biotic and abiotic agents results in synergism, improving the effectiveness of pathogen suppression. Biological control agents (BCAs) that effectively reduce incidence and spread of crop diseases caused by microbial pathogens may be divided into two major groups: (i) biotic agents and (ii) abiotic agents. They display varying degrees of biocontrol potential, depending on the host-pathogen combination and the environmental conditions in different geographical locations. Biotic BCAs are nonpathogenic, living organisms that exhibit antagonistic potential against microbial pathogens – oomycetes, fungi, bacteria and viruses that are present in a free-state or in vectors living in the soil environments. They are able to suppress pathogen development through one or more mechanisms. Abiotic BCAs are derived from inorganic or organic sources and they may also act directly on the pathogens or indirectly by activating host plant defense systems. They may be classified based on their chemical constitution or on the mode of action on the target pathogens. The biotic BCAs vary in their biocontrol efficiency due to their sensitivity to environmental conditions, capacity to adapt to new locations, rhizosphere competence, quantum of production of metabolites toxic to target microbial pathogen(s). Hence, it is necessary to establish the precise identity of the strains/isolates, based on the cultural, biochemical and genetic characteristics (Narayanasamy 2013).
