Glossary: U-V-W

A large family that includes carrots, parsnips, and celery. The flowers are borne in umbels, in which all the flower stalks arise at the end of a stem, giving it the look of an umbrella.
One of the genera of the powdery mildews (Erysiphales) in which the cleistothecium contains several asci, and has curled ends to the appendages. The chief plant pathogen is U. necator, which causes a powdery mildew of grapes and other hosts.
United Nations Development Program, located in New York, USA.
United Nations Environmental Program, located in Nairobi, Kenya.
United Nations Educational, Scientific, and Cultural Organisation, located in Paris, France.
Uniform distribution
The converse of a patchy distribution. With a uniform distribution of parasitism, every individual in the host population is more or less equally exposed to the parasite.
A uniform distribution of parasitism is very desirable when screening plants for horizontal resistance because differences in the level of parasitism then represent differences in the level of resistance.
See also: Frequency, Injury.
“What happens when every door in the town has the same lock, and every householder has the same key that fits every lock?” This type of uniformity is a poor system of locking, yet this is the way plants were bred for vertical resistance during the twentieth century. See also: n/2 model.
University breeding clubs
University breeding clubs are in a halfway position between the true amateurs and the professionals. Their function should be to concentrate on population breeding and horizontal resistance. They have a number of prominent advantages:
Plant breeding is somewhat intimidating for beginners. The ambience of a university breeding club is undoubtedly the best way of overcoming this intimidation.
The techniques of breeding for horizontal resistance require ‘hands-on’ experience and a breeding club is the best means of providing such experience. The students themselves would do all the work of breeding and they would gain practical experience in every aspect of the breeding process.
As one of the inducements to join, students should earn course credits from their breeding club membership and participation. The professor in charge of a breeding club would earn teaching credits for this activity.
On graduation, students should be given life membership in their club or clubs. This would entitle them to consult the university experts, and to receive, test, report on, and utilise new lines coming out of their club(s) for the rest of their lives.
Graduates would be encouraged to start one or more new breeding clubs among farmers and other interested parties, in their new place of work. This would lead to a proliferation of breeding activity.
Breeding clubs would provide a new approach to teaching in which the students themselves are involved in the actual achievements of both demonstrating the value of horizontal resistance, and of producing new resistant cultivars.
Short-term research grants have no guarantee of renewal and our current system of short-term financing of agricultural research discourages long-term research projects, such as breeding for horizontal resistance. Because the breeding club work would be a teaching activity, its continuation would be secure, and the professor in charge could undertake long-term research in this topic.
The production of an assortment of valuable new cultivars in a range of locally important crops could provide valuable prestige for a university.
One of the chief criticisms of institutional and corporate plant breeding is that their work is so expensive, and that they are so specialised, and so technical, that their total breeding output is severely limited. Having many plant breeding clubs would greatly increase the output of new crop varieties.
University plant breeding clubs could provide an entirely new technique for overseas aid in agriculture. Overseas aid organisations could initiate these clubs in Third World universities, and support them with technical and financial assistance until they could stand on their own feet. If successful, these clubs could easily prove to be the most effective agricultural assistance technique of them all, as well as being one of the most inexpensive techniques of overseas aid.
University clubs should be encouraged to ‘twin’ with a secondary school club in order to assist school children, as was done with cassava in Nigeria. A similar ‘twinning’ with plant breeders associations should also be encouraged.
Unstable protection mechanisms
Any mechanism that protects a host from a parasite can be divided into one of two classes: stable and unstable.
Unstable protection mechanisms are those that protect a host only until a new strain of the parasite appears that is unaffected by that mechanism. This is because the mechanism is within the capacity for micro-evolutionary change of the parasite.
Unstable mechanisms include vertical resistance, and most modern synthetic fungicides and insecticides. Any genetically engineered resistances are also highly likely to be unstable.
See also: Stable protection mechanisms.
The order of Basidiomycete fungi that cause rust disease.
A genus of fungi in which various species cause rust diseases of Phaseolus vulgaris, Vicia faba, Pisum sativium, Trifolium spp., and Beta vulgaris.
The order of Basidiomycete fungi that cause smut diseases.
A genus of fungi that cause smut diseases in maize, wheat, barley, and oats.
Vaccinium spp.
The blueberry and cranberry. There are several species with edible berries, and these are the only cultivated members of the heather family, the Ericaceae. Scope for amateur breeders.
Possibly the greatest plant pathologist who ever lived, J.E. Vanderplank both developed the concepts, and coined the terms, of vertical resistance and horizontal resistance. He can be said to have transformed plant pathology by his development of theoretical aspects of this discipline. He died in 1997.
See: Vanilla fragrans.
Vanilla fragrans
This species is the only orchid grown for purposes other than ornamental. It originated in Mexico and the vanilla is extracted by fermenting the unripe pods and infusing them in alcohol. The main producer is now the Malagasy Republic (Madagascar).
The crop is propagated vegetatively and few clones are known to exist. There may be scope for selection by amateur breeders in the centre of origin.
Variable ranking
See: Differential interaction.
Differences displayed by individuals within a species .
In a botanical context, this term means a subdivision of a species. An agricultural or horticultural variety is called a cultivar.
Vascular tissue is plant tissue which transports water or nutrients. Vascular plants are those that contain vascular tissue, and are the ferns and seed-bearing plants (Angiosperms and Gymnosperms).
In a plant pathological context, a vector is an insect that transits a virus disease.
A person who does not eat or use any animal products whatsoever. See also: Vegetarian.
Any part of a plant, other than the fruit or seed, that is used for food. Culinary usage is often different from horticultural usage. In the kitchen, tomatoes and cucumbers are vegetables but, botanically, they are fruits. Similarly, rhubarb is technically a vegetable, but is called a fruit in the kitchen.
A person who generally avoids eating animal foods, particularly meat, but who may eat eggs and dairy products, and possibly fish. See also: Vegan.
Vegetative bud
A bud that grows into stems and leaves, as opposed to a flower bud.
Vegetative propagation
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.
When biotite (black mica) is roasted, it expands into vermiculite that is a useful addition to potting soils.
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.
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 what is used by 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 does not provide protection against a matching allo-infection.
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 used by 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, Racespecific 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.
Vertical resistance, ultimate function
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 subsystem of a pathosystem that is defined by the presence of a gene-for-gene relationship.
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 J.E. 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
Until recently, there was significant opposition to the use of horizontal resistance techniques, and the reason probably lay in various vested interests.
For example, a major increase in horizontal resistance will lead to a severe decline in the demand for crop protection chemicals. Much of the research funding for plant breeding programs comes from the big chemical corporations whose vested interests prevent them from funding research into horizontal resistance.
Similarly, many senior scientists have devoted their entire careers to vertical resistance and crop protection chemicals. Their vested interest in their scientific knowledge can make it difficult for them to accept alternatives. See also: Mindset.
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 blackeye pea, blackeye bean, China pea, Kaffir pea, marble pea, and southern bean. Some cultivars 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.
A slender climbing stem, this term is often used as a name for grapes.
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.
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.
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.
Giving birth to live young, as opposed to laying eggs. Many aphids, and other insects, are viviparous, and this increases their rate of population growth considerably, making them r-strategists.
Voandezia subterranea
Bambara groundnut. This is a minor indigenous African crop with edible seeds that are formed underground like peanuts. There is probably scope for improvement by local plant breeders associations.
Crop vulnerability is defined as susceptibility to an absent, foreign parasite that has epidemiological competence in the area in question. Should that parasite be imported, the vulnerability will be revealed and potential damage will become actual damage.
Some crop vulnerabilities are particularly severe, the most famous example being the vulnerability of potatoes in Ireland to blight.
Wallace, A. R.
An English naturalist who formulated the theory of evolution independently of Darwin. He lived 1823-1913.
See: Juglans regia.
Although the common wasp is much disliked, because of its sting, many wasps are hyperparasites of crop pests and they make a significant contribution to biological control.
Water-borne parasites
Any parasite that is disseminated in water. Relatively few crop parasites are water-borne, and those are mainly bacterial.
See: Rorippa nasturtiumaquaticum.
See: Citrullus lanatus.
A plant growing where it is not wanted. Weeds can cause serious damage to crops by competing for light, space and nutrients. In the old days, weeds were controlled largely by ploughing and handtilling.
These days, it is common to use selective herbicides which kill the weeds, but not the crop plants. Organic farmers, however, use good farming practices to minimize weeds and accept the presence of the few that remain.
Note that weeds are competitors, not parasites.
Weed suppression
Some crops, such as potatoes, are good at suppressing weeds. This is a useful alternative for organic farmers who do not use herbicides.
The process of controlling or removing weeds.
See: Triticum spp.
Small plant parasitic insects of the Order Homoptera, which also includes aphids. So-called because their wings and bodies are covered with white scales. Whiteflies like a warm climate and they are mostly tropical, subtropical, or greenhouse pests.
Wild oats
See: Avena fatua.
Wild plant pathosystem
An entirely autonomous (i.e., self-organising) pathosystem in which people have not interfered, either directly or indirectly.
It is characterised by its stability, genetic diversity, and genetic flexibility. It may be either continuous or discontinuous.
A vertical subsystem can evolve only in a discontinuous wild pathosystem.
Research into plant parasitism has been confined almost entirely to crop pathosystems, and there is an urgent need for research into wild plant pathosystems.
Wild progenitor
In crop science, the term ‘progenitor’ usually means the original wild ancestor. Every cultivated species of plant has one or more wild progenitors, some of which are extinct.
Wilt disease
A plant disease in which the principle symptom is wilting, in spite of an adequate moisture in the soil. Wilts are usually caused by microscopic fungi such as Verticillium spp., or Fusarium spp., or by bacteria such as Pseudomonas spp.
The wilting results from the fact that the water conducting vessels of the plant are occupied by the parasite, and are partly blocked. The parasite may also produce toxins that induce wilting.
Wind-borne parasites
Any parasite that is dispersed by wind. Most fungi and many insects are wind-borne. Wind dispersal can carry parasites for hundreds of miles. There is even evidence of aphids being carried across oceans on the jet stream.
A hedge or line of trees planted to protect crops from persistent winds.
Winged bean
See: Psophocarpus tetragonobolus.
Winter cereal
As their name suggests, winter cereals are sown in the autumn, and are able to continue growing, quite slowly, throughout the winter. Spring cereals, on the other hand, cannot survive a winter and must be sown in the spring.
Winter cereals have the advantage that they can begin active growth in the spring as soon as the thaw sets in, probably several weeks before tractors can get onto wet land in order to sow spring cereals.
This permits either (i) a longer growing season with a correspondingly higher yield, or (ii) an early harvest that escapes the full development of pests and disease epidemics.
Winter hardiness
The ability of a crop to withstand winter. Winter cereals are a typical example of winter hardiness.
Witches’ broom
A cluster of proliferating twigs, usually at the end of a branch, that resembles an old-fashioned broom made from a bundle of rushes tied to a stick. Witches’ broom is usually cause by a parasite, the most important being the fungus Crinipellis perniciosis that causes witches’ broom of cocoa.
See: Striga spp.
See: Isatis tinctora
World food problem
The real possibility that the human population will exceed the world food supply.