Pasture

Land covered with fodder grasses and legumes, and used for grazing farm animals such as cattle and sheep. Pasture grasses

See: Fodder grasses. Pasture legumes

See: Fodder legumes. Patchy distribution

The converse of a uniform distribution. With a patchy distribution of parasites, some individuals in the host population may be heavily parasitised, while others may escape entirely. Patchy distributions occur typically with soil-borne parasites, and gregarious insects (e.g., the leaf hoppers of maize streak virus). A patchy distribution is a nuisance when screening plants for horizontal resistance because it produces escapes from parasitism, and these provide a false indication of resistance. A patchy distribution can be overcome during screening for resistance by using a grid screening. That is, the screening population is divided into a grid of perhaps one-metre squares, and the best individual in each square is kept, provided parasites are present in that square.

A patchy distribution can also occur over time. For example, swarms of the desert locust occur only once in 10-15 years. This period is long enough for a population of an annual host to lose most of its resistance to these insects.

A patchy distribution is an evolutionary survival advantage for the parasite, because it prevents the host from accumulating resistance. See also: Frequency, injury. Pathodeme

A sub-population of a host that is defined by a stated characteristic of resistance. For example, many different cultivars may possess vertical resistance gene 2, but no others) even though they differ in many other respects. Horizontal pathodemes differ in their levels of horizontal resistance. Pathogen

A category of plant parasite that causes disease and is studied by plant pathologists. The term includes fungi, bacteria, phytoplasmas, viruses, and viroids. When nematodes are studied by plant pathologists, they too are called pathogens.

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