ABSTRACT
Among the microbial plant pathogens, bacterial pathogens are mostly unicellular organisms, much smaller in size, compared to fungal pathogens. They are classified as prokaryotes which have less defined structural characteristics compared with eukaryotes which include fungal pathogens. The presence of fossil prokaryotes was found on rocks which were estimated to be about 3.5 billion years old, indicating their existence as the first recorded forms of life on Earth (Schumann 1991). Bacterial species pathogenic to plants have been estimated to be about 100 of the 1,600 species reported to be present in the world (Agrios 2005). The nuclear material present in bacterial cells is not separated by a membrane as in eurkaryotes. The genome of the bacteria is in the form of a single chromosome with double-stranded (ds) DNA as a closed circular form. Plasmids capable of self replication are present in bacterial cells and they are extrachromosmal DNA that determines certain characteristics, including pathogenicity, resistance to chemicals and antibiotics, and tumor production. The plasmids may be exchanged between bacterial species/strains, resulting in variations in characteristics governed by plasmids. The bacteria lack other organelles present in eukaryotic cells. Several bacterial species are strictly saprophytic and are involved in the decomposition of organic wastes produced from industries or by dead animals and plants. Several bacterial species are useful in crop production, because of their ability to fix atmospheric nitrogen and function as biological control agents that may be applied as an effective disease management strategy against crop diseases.
