Plant propagation without sexual reproduction, usually by means of cuttings, grafts, tubers, bulbs, or corms. The population derived by vegetative propagation from a single individual is known as a clone. All the individuals within a clone are genetically identical, apart from an occasional mutant or 'sport'. Vegetative propagation is thus a useful means of obtaining genetic uniformity, and of preserving agriculturally valuable characteristics. Venturia inaequalis
This Ascomycete fungus is the cause of apple scab. Vermiculite
When biotite (black mica) is roasted, it expands into vermiculite that is a useful addition to potting soils. Vernalisation
A treatment with low temperatures to induce flowering. Some winter cereals will not produce flowers if they are sown in the following spring, and they must be sown in the autumn if an entire summer is not to be wasted. To vernalise winter cereals, the seed is wetted to initiate germination, and then stored at just above freezing for several weeks. This seed can then be sown in the spring to produce a crop in the same year. This technique is useful for breeders working with winter cereals. Vertical
In a plant epidemiological context, this term is entirely abstract, and it means that a gene-for-gene relationship is present. Vertical resistance and vertical parasitic ability both result from a gene-for-gene relationship. A vertical subsystem of a plant pathosystem is defined by the presence of a gene-for-gene relationship. The individual genes of a gene-for-gene relationship are called vertical resistance genes, and vertical parasitism genes respectively, and are usually labelled with numbers, with matching genes being given the same number. Similarly, pathotypes and pathodemes that are defined by the presence of vertical genes, are labelled with the numbers of those genes. See also: Vertical parasitic ability, vertical resistance, n/2 model, System of locking, Habgood nomenclature. Vertical parasitic ability
Parasitic ability that results from a gene-for-gene relationship. Vertical parasitic ability is the parasitic ability of the Mendelians; its inheritance is normally controlled by single genes, each of which has a corresponding, or matching gene in the host. In the wild pathosystem, vertical parasitic ability is part of a system of locking which can control allo-infections only, and which depends on genetic diversity in the host population. See also: Horizontal parasitic ability, vertical resistance, n/2 model, Habgood nomeclature. Vertical parasitism genes
The gene-for-gene relationship involves pairs of genes, with one of each pair in the parasite and the other in the host. The genes in the parasite are called vertical parasitism genes and they confer vertical parasitic ability. Note that a single vertical parasitism may be conferred by more than one vertical parasitism gene. Vertical pathodeme
A population of a host in which all the individuals have a stated vertical resistance in common. Note that these individuals may differ in other respects (i.e., they may be different cultivars). Vertical pathotype
A population of a parasite in which all the individuals have a stated vertical parasitic ability in common. Note that these individuals may differ in other respects (i.e., they may be different varieties as defined by other criteria). Vertical resistance
Resistance that is conferred by a gene-for-gene relationship, but which provides no protection against a matching allo-infections. It is thought that the sole evolutionary function of all vertical resistances is to control the population explosions of r-strategist parasites. Vertical resistance normally achieves this with a system of locking that greatly reduces the proportion of allo-infections that are matching infections. This reduction is usually achieved by killing the non-matching, allo-infecting parasite. Quantitative vertical resistance does not kill non-matching parasites, but it does prevent them from reproducing, and this satisfies the evolutionary function. Alternatively, quantitative vertical resistance allows non-matching parasites (particularly fungi) to reproduce, but at such a low rate of reproduction that the population explosion is reduced to unimportance. Vertical resistance is the resistance of the Mendelians; its inheritance is normally controlled by single genes, each of which has a corresponding, or matching gene in the parasite. In the wild pathosystem, vertical resistance is part of a system of locking which can control allo-infection only, and which depends on genetic diversity in the host population. When employed on a basis of genetic uniformity in a crop pathosystem, vertical resistance is temporary resistance in the sense that a single matching allo-infection rapidly leads to the failure of the entire cultivar. See also: Horizontal resistance, Race-specific resistance, Vertical parasitic ability, n/2 model. Vertical resistance genes
The gene-for-gene relationship involves pairs of genes, with one of each pair in the parasite and the other in the host. The genes in the host are called vertical resistance genes and they confer vertical resistance. Note that a single vertical resistance may be conferred by more than one vertical resistance gene. Vertical resistance, inactivation
Breeding for horizontal resistance is possible only if all vertical resistance are either absent or inactivated during the screening process. In some crops (e.g., wheat) it is impossible to find parents that lack vertical resistance genes entirely. The vertical resistance must then be inactivated. One method of doing this is to use the one-pathotype technique. Another is to use the saturation technique.
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