Bacterial plant pathogens are small unicellular rods from 1 to 3 p m in length. They do not have a well-defined nucleus, nor a nuclear membrane. Bacteria are spread by insects, air currents, splashing rain, and by mechanical means. Free moisture usually is necessary for infection, and penetration of host tissue occurs through wounds or stomatal openings. These pathogens invade the vascular system or intercellular spaces in host tissue, and necrosis results from toxins produced or enzymatic activity of the bacteria.