Abstract
This chapter discusses the potential of PGPRs in protecting several crops such as cereals, vegetables, legumes and others from infection by different diseases caused by viruses, fungi, bacteria, phytoplasmas, nematodes and parasitic weeds, as well as ones caused by nutrient deficiencies. Though PGPR can use multiple modes of action in controlling plant pathogens, the entire process can be summarized in two basic mechanisms, i.e. direct and indirect. The mechanisms of direct effect on plant pathogens include production of antibiotics like pyocyanine, pyrrolnitrin, 2,4-diacetylphloroglucinol, production of siderophores, the synthesis of hydrogen cyanide, production enzymes that can hydrolyze the cell walls of plant pathogens, competition for colonization sites and for nutrients, as well as degradation of the pathogenicity factors of substances such as toxins and enzymes. On the other hand, the indirect effect includes induction of resistance and promotion of growth in plants against pathogens.