In The Southease Basic Facts of Plant Diseases Many fungi, bacteria, viruses, and nematodes are plant pathogens. A plant pathogen is an organism or agent that incites ifectious disease. An example of primary inoculum is the cells of the fire blight bacterium, Erwinia amylovora , that overwinter in infected bark tissue along the margins of cankers. In spring, the bacteria multiply and are spread by rain or insects to suculent tissue of apples or pears where primary infection occurs. In gerneral, the greater the amount of inoculum and the nearer it is to its host, the greater the potential for a disease epidemic.
Dissemination refers to the spread or dispersal of the pathogen from an inoculum source to a host.
Dissemination can occur by wind, splashing rain, insects, infested pruning tools, infected or infested transplants, and other means. Spread can occur over short distances within the tree canopy or from distant sources. Some pathogens have multiple means of dispersal. Erwinia amylovora can be spread by rain, wind, insects, and infested pruning equipment. Primary infection occurs when the pathogen comes into contact with a susceptible host under favorable environmental conditions.