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.[1]
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.
In a wild plant pathosystem, both the host and the parasite populations exhibit genetic diversity and genetic flexibility. Conversely, in a crop pathosystem, the host population normally exhibits genetic uniformity and genetic inflexibility (i.e., clones, pure lines, hybrid varieties), and the parasite population assumes a comparable uniformity. This distinction means that a wild pathosystem can respond to selection pressures, but that a crop pathosystem does not. It also means that a system of locking (see below) can function in a wild plant pathosystem but not in a crop pathosystem.
Pathosystem balance means that the parasite does not endanger the survival of the host; and that the resistance in the host does not endanger the survival of the parasite. This is self-evident from the evolutionary survival of wild plant pathosystems, as systems, during periods of geological time.[2]
The gene-for-gene relationship[3] is an approximate botanical equivalent of antigens and antibodies in mammals. For each resistance gene in the host, there is a corresponding, or matching, gene in the parasite. When the genes of the parasite match those of the host, the resistance does not operate.
There are two kinds of resistance to parasites in plants:
- Vertical resistance[4] involves a gene-for-gene relationship. This kind of resistance is genetically controlled by single genes, although several such genes may be present in a single host or parasite individual. Vertical resistance is ephemeral resistance because it operates against some strains of the parasite but not others,[4] depending on whether or not a match occurs. In agriculture, vertical resistance requires pedigree breeding and back-crossing. It has been the resistance of choice during the twentieth century.
- Horizontal resistance[4] does not involve a gene-for-gene relationship. It is the resistance that invariably remains after vertical resistance has been matched. It is genetically controlled by polygenes and it is durable resistance as many ancient clones testify. Its use in agriculture requires population breeding and recurrent mass selection.
Infection is the contact made by one parasite individual with one host individual for the purposes of parasitism. There are two kinds of infection:
- Allo-infection[5] means that the parasite originates away from its host and has to travel to that host. The first infection of any individual host must be an allo-infection. Vertical resistance can control allo-infection only. It normally does this with a system of locking (see below) which reduces the proportion of allo-infections that are matching infections.
- Auto-infection[5] means that the parasite originates on, or in, the host that it is infecting. Auto-infection and all the consequences of a matching allo-infection, can be controlled only by horizontal resistance. This is because the parasite individual reproduces asexually to produce a clone (or else reproduces sexually and quickly reaches homogeneity of matching individuals) and auto-infection is thus matching infection.
An epidemic is the growth of a parasite population which is made at the expense of the host population. There are two kinds of epidemic:
- Continuous epidemics[1] have no break in the parasitism; they have no gene-for-gene relationships; they involve evergreen trees, and some perennial herbs.
- Discontinuous epidemics[1] have regular breaks in the parasitism, due to an absence of host tissue during an adverse season, such as a temperate winter or tropical dry season; they often have a gene-for-gene relationship against some of their parasites; they involve annual plants, some perennial herbs, and the leaves and fruits of deciduous trees and shrubs.
- ^ a b c Robinson, Raoul A. (1987) Host Management in Crop Pathosystems. Macmillan, New York, Collier-Macmillan, London, 263pp
- ^ Robinson, R.A.(2010) Self-Organizing Agro-Ecosystems; Sharebooks Publishing http://www.sharebookspublishing.com
- ^ Flor, H.H. (1942); “Inheritance of pathogenicity in Melampsora lini.” Phytopath., 32; 653-669.
- ^ a b c Vanderplank, J.E. (1963); “Plant Diseases; Epidemics and Control.” Academic Press, New York & London, 349pp.
- ^ a b Robinson, R.A. (1976); “Plant Pathosystems.” Springer-Verlag, Berlin, Heidelberg, New York, 184pp.
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