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.
The gene-for-gene relationship 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 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, 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 have no break in the parasitism; they have no gene-for-gene relationships; they involve evergreen trees, and some perennial herbs.
- Discontinuous epidemics It apparently functions as a system of locking in which every host and parasite individual has half of the genes in the gene-for-gene relationship (i.e., n/2 genes, where n is the total number of pairs of genes in that relationship). Each gene in the host is the equivalent of a tumbler in a mechanical lock, and each gene in the parasite is the equivalent of a notch on a mechanical key. Provided that each n/2 combination of genes occurs with an equal frequency, and with a random distribution, in both the host and parasite populations, the frequency of matching allo-infections will be reduced to the minimum. For example, with six pairs of genes, each host and parasite individual would have three genes, and there would be twenty different locks and keys; with a twelve-gene system, there would be 924 six-gene locks and keys. Given an equal frequency and a random distribution of every lock and key, the frequency of matching allo-infection would be 1/20 and 1/924, respectively. These figures are obtained from the binomial expansion illustrated by Pascal's triangle. who called it the cryptic error in field trials. Parasite interference does not affect the demonstration of vertical resistance, but it can totally destroy the evidence for high levels of horizontal resistance.