A pathosystem is a subsystem of an ecosystem and is defined by the phenomenon of parasitism. A plant pathosystem is one in which the host species is a plant. The parasite is any species in which the individual spends a significant part of its lifespan inhabiting one host individual and obtaining nutrients from it. The parasite may thus be an insect, mite, nematode, parasitic Angiosperm, fungus, bacterium, mycoplasma, virus or viroid. Other consumers, however, such as mammalian and avian herbivores, which graze populations of plants, are normally considered to be outside the conceptual boundaries of the plant pathosystem.
A host has the property of resistance to a parasite. And a parasite has the property of parasitic ability on a host. Parasitism is the interaction of these two properties. The main feature of the pathosystem concept is that it concerns parasitism, and it is not concerned with the study of either the host or parasite on its own. Another feature of the pathosystem concept is that the parasitism is studied in terms of populations, at the higher levels and in ecologic aspects of the system. The pathosystem concept is also multidisciplinary. It brings together various crop science disciplines such as entomology, nematology, plant pathology, and plant breeding. It also applies to wild populations and to agricultural, horticultural, and forest crops, and to tropical, subtropical, as well as both subsistence and commercial farming.