It appears that the ultimate function of vertical resistance is to reduce the population explosion of an r-strategist parasite, which is usually a parasite with a very rapid asexual reproduction. The reduction is normally achieved by reducing the frequency of allo-infections that are matching infections. However, in some pathosystems, the reduction is achieved by preventing or reducing the reproduction of the parasite. See also: Quantitative vertical resistance. Vertical subsystem
A fungus that causes wilt diseases in an exceptionally wide host range. The symptoms are a wilt that happens in spite of an adequate soil moisture. These symptoms are identical to the Fusarium wilts, except that they tend to occur at somewhat lower temperatures. Vertifolia effect
The effect, first recognised by Vanderplank, in which horizontal resistance is lost during breeding for vertical resistance or during breeding under protection from pesticides . The effect is named after the potato cultivar 'Vertifolia' because of its very low level of horizontal resistance to blight, revealed when its vertical resistance was matched. The mechanism of this effect is that the level of horizontal resistance is concealed in the absence of parasitism, if there is a functioning vertical resistance, or protection from pesticides . Plants with high levels of horizontal resistance are relatively rare in a screening population, and plants with lower levels of horizontal resistance tend to be selected on the basis of their other attributes. In the course of decades of breeding, the level of horizontal resistance can reach dangerously low levels. Vested interests
There is a widespread antipathy to horizontal resistance and the reason probably lies in various vested interests. For example, a major effort to increase horizontal resistance would lead to a severe decline in the demand for crop protection chemicals. Much of the research funding comes from the big chemical corporations whose vested interests prevent them from funding research into horizontal resistance. Similarly, many senior scientists have devoted their careers to crop protection chemicals or to vertical resistance. Their vested interest in their scientific reputations prevents them from admitting that there may have been a better approach. See also: Mindset. Vetch
See: Vicia sativa. Vicia faba
The broad bean, also known as horse, field, tick, or Windsor bean. Vicia sativa.
Vetch. This and other species are useful fodder legumes but their use is declining in favour of alfalfa. Vigna unguiculata
Synonym: Vigna sinensis. Cowpea; also known as black-eye pea, black-eye bean, China pea, Kaffir pea, marble pea, and southern bean. Some cultivar are grown for their green pods which are known as yard-long bean, asparagus bean, snake bean, and Bodi bean. Cowpeas are one of the main pulses of Africa and many other tropical and subtropical areas. They are utilised as dried beans and as pot herbs. The plants are normally self-pollinating, but considerable cross-pollination by large insects occurs in the wetter areas.
There is scope for accumulating horizontal resistance by amateur breeders . Vine
A slender climbing stem, this term is often used as a name for grapes. Viroid
A primitive form of virus which consists of little more than genetic code. Viroids cause a few plant diseases and they differ from virus diseases mainly in that they are seed-transmitted. Spindle tuber disease of potatoes is caused by a viroid. Virus
Viruses are too small to be seen with an optical microscope, and require an electron microscope. They are obligate parasites that cause a wide range of diseases in most crop species . Many viruses are transmitted from plant to plant by insects, mostly aphids and leaf hoppers. However, others can be transmitted by simple contact, or by soil-inhabiting organisms such as nematodes and fungi. See also: Viroid, Vector. Vitamins
Organic compounds that are essential in small quantities for human nutrition, but which cannot be manufactured in the human body. They occur in a wide variety of foods, and they are one of the main justifications for a healthy and balanced diet. Vitis vinifera
The grape vine, cultivated primarily for the manufacture of red and white wines, but also for table grapes and raisins. Grapes have been cultivated for millennia in Europe without any use of crop protection chemicals, and their levels of horizontal resistance to all their old-encounter diseases was entirely adequate. However, they had little resistance to Phylloxera, downy mildew, and other new encounter parasites that originated in the New World. It is probably impossible to improve the horizontal resistance of the classic wine grapes to these new encounter parasites without an unacceptable loss of wine quality. However, Phylloxera is controlled by grafting classic vines on to wild vine rootstocks that are resistant, but this leads to a significant loss of yield. There is scope for amateur breeders to breed Vitis vinifera rootstocks with horizontal resistance to Phylloxera.
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