An organism which lives in or on other living organisms and derives its nutrients from the latter is called parasite. The relationship between a parasite and its hosts is known as parasitism. Many fungi and most bacteria grow on a non-living substrate within a living plant. The organism of this type of mode of nutrition is called saprophyte. Based on the different types of modes or nature of nutrition, the relationship between the host and parasite or saprophyte is termed in many ways viz., obligate parasite (biotroph), obligate saprophyte, facultative parasite, facultative saprophyte, hemibiotroph and necrotroph (perthotrophs or perthophyte).
Parasitism in cultivated crops is common phenomenon. Any agent that can cause suffering or damage or disease is called a pathogen. In plant pathology, the term ‘pathogen’ is usually used to the living or infectious organisms. The ability of a pathogen or parasite to cause disease is known as pathogenicity.
It is obvious that a plant becomes diseased when it is attacked by a pathogen or parasite. The ultimate condition i.e. disease occurs by passing through some distinct events. Thus, the genesis or chain of events or stages of disease development are called pathogenesis. This is also called as disease cycle.
The events that occur in specific order are namely inoculation, penetration, establishment of infection, invasion or colonization, growth and reproduction, dissemination and survival of the pathogen (over wintering or over summering in absence of the host). The events will continue to repeat in the same order in presence of both the host and pathogen/parasite that may lead to severe disease condition.