Plant diseases do not arise spontaneously. There must be a causal agent (the pathogen) in the vicinity of a susceptible plant (the host). The pathogen must be in a form that will grow and develop when transferred to the host. The infectious form of the pathogen is termed inoculum. The inoculum is transferred to the host by an agent of dissemination. There it must arrive at an appropriate site of infection with environmental conditions favorable to the pathogen if a parasitic relationship between the pathogen and the host is to develop. Following are examples of these new terms.
Some agents of dissemination are:
• splashing water and rainfall
• animals, including humans
• equipment and tools
Some forms of inoculum are:
• spores of fungi
• bacterial ooze (a concentrated mass of bacteria)
• virus particles
• strands of hyphae
Some sites of infection are:
The precise nature of the relationship that develops between the host and the pathogen is determined by the type of parasitism.
• Obligate parasites: these pathogens can exist only on a living host.
• Obligate saprophytes: these irritants cannot survive on a living host, only on nonliving matter. An obligate saprophyte cannot be considered a pathogen.
• Facultative saprophyte: this pathogen is normally a parasite, growing on a living host. However, it can survive on nonliving matter and, as such, can be grown on nonliving agar for laboratory study.
• Faculative parasite: this pathogen is normally a saprophyte, growing on nonliving matter. However, it can grow as a parasite on plants. In so doing, it must kill the host cells with secreted enzymes before it is able to feed on them. Faculative parasites can survive in the soil for long periods of time feeding on nonliving organic matter.
As long as the inoculum is only on the host, the plant is termed infested. Once the pathogen penetrates the host’s tissues, the plant is termed infected. Disease begins when the host responds to the injurious presence of the pathogen.