The role of various chemicals present in soil, and rhizosphere and phyllosphere of plants, which serve as specific signals and are recognized by the plant pathogenic bacteria, is discussed in plant pathogenesis. The role of bacterial secretion systems and quorum sensing in causing plant diseases is highlighted. How plant-associated microbial patterns trigger innate immunity defenses and their circumvention by plant pathogenic bacteria is dealt. The production of reactive oxygen species by plants and their neutralization by plant pathogenic bacteria is dealt. The production of phytoalexins by plant and their detoxification by plant pathogenic bacteria is given. The role of peptide- or protein-containing toxins, such as tabtoxin, phaseolotoxin, coronatine, syringomycin, tagetitoxin, and rhizobitoxine produced by many bacterial plant pathogens in causing plant diseases is dealt. The role of stomatal defense, a plant defense mechanism and molecular action of coronatine in countering this defense is discussed.