In plant pathosystems there may be discontinuity in either space (spatial discontinuity) or time (sequential discontinuity). Spatial discontinuity generally refers to genetic diversity. Because a diversity of vertical resistances in the host population usually compels a comparable diversity in the parasite population, the two populations of host and parasite can usually be expected to have a similar degree of genetic diversity.
There are two relevant levels of genetic diversity. Mixing of the host among many other different species of plant means that many parasite individuals will make contact with a non-host. Such a contact is usually fatal or seriously debilitating. In a pathosystem context, diversity within a single species of host refers to a diversity of vertical resistances. A parasite individual that makes contact with a non-matching host is either killed, if a rust spore, or debilitated, if an aphid.
In a pathosystem context, diversity in the host population leads to problems for the parasite. The parasite may find a suitable host by employing relatively few but complex individuals, such as insects, which make use of sense organs and tactical mobility. Or the parasite may employ very many, very cheap and very small, randomly distributed individuals, such as fungal spores. These entirely lack any tactical mobility or control of direction, but there are very many more of them.
To avoid confusion with the term 'sequential discontinuity', spatial discontinuity and continuity are referred to as genetic diversity and uniformity respectively. Genetic diversity is the norm in wild plant pathosystems. This is true even in host populations that exhibit a natural self-pollination, or an apomictic seed production, or a vegetative reproduction, because natural pure lines and clones are usually small. The biomass of such a genetically uniform plant population is unlikely to exceed that of a large tree. The population as a whole would then have a diversity comparable to a forest of mature trees.
Genetic uniformity is the norm in crop pathosystems, where a host population (i.e., one crop in one field) is normally a single pure line, clone, or hybrid variety. This genetic uniformity can extend to many crops in many fields when a single cultivar is being cultivated over a large area.
It is probable that the landraces that were cultivated prior to the use of pure lines were protected in part by a diversity of vertical resistances. It is likely, therefore, that the use of pure lines increased the severity of parasites in which a gene-for-gene relationship occurs. Such an increase is perhaps at its most probable in the rusts and aphids of the small grain cereals and the grain legumes.
One of the most important differences between commercial agriculture and subsistence farming is that subsistence farmers usually plant mixtures of crop species, with mixtures of genetic lines within each species. This genetic diversity is comparable to that of many wild ecosystems. However, the use of such mixtures is very labour-intensive and it cannot be recommended for commercial agriculture.
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