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Vaccine
A preparation of a pathogenic micro-organism or virus, which has been killed or attenuated so as to lose its virulence but which carries antigens. When injected into a living animal the immune system is stimulated to produce antibodies to counteract the antigens. The antibodies remain in the living system thus providing immunity against any subsequent potentially pathogenic infection by the same organism.

Vaccination
Treatment to render an individual resistant or immune to a particular infectious disease.

Vacuole
A cavity in the cytoplasm containing air, liquids, food, waste products etc., a vacuole is not rigid.

Vagabond's disease
Darkened, thickened skin caused by years of infestation with body lice, Pediculus humanus humanus.

Vaginitis
Inflammation of the vagina.

Validation
The process of assessing the accuracy for a given purpose of a simulation model by comparing the model's predictions with independent results.

Value chain
A concept used in the development of corporate strategy, whereby the firm's operations are divided into a series of activities.

Valvifers
Basal portions of the ovipositor in Hymenoptera.

Valvulae
Processes from the valvifers to form the body of the ovipositor (terebra) and the ovipositor sheath (third valvulae) in Hymenoptera.

Vapour action
On part of pesticidal material is similar to fumigant except that active ingredients may be released slowly over varying time periods.

Vapour drift
Movement of pesticide in the form of vapour away from the treated area after application (see drift). The movement of herbicidal vapours from the area of application.

Vapour pressure
That property which causes a chemical compound to evaporate.

Vapour releasing product
A formulated product containing one or more volatile ingredients, the vapours of which are released into the air. Evaporation rate normally is controlled by using suitable formulations and/or dispensers.

Variability
The property or ability of an organism to change its characteristics from one generation to the other.

Variable
A quantity able to assume different numerical values.

Variable costs (also direct costs)
Costs which change according to size of activity eg. use of fuel, seed.

Variable overhead absorption rate
A calculation that divides the total variable overheads of a cost centre by a measure of output such as units, volume or labour hours.

Variance
The amount actual time, real costs or final R & D results deviate from the values anticipated in a plan. Also in statistics the variance of set of n measurements x1, x2, ..., xn is the average of the squares of the deviations of the measurements about their mean.

Variance analysis
The analysis of differences between standard costs and actual costs into their causes.

Variant
Any unnamed variation within a species, variety, forma specialis or physiolgic race.

Variation
The differences occurring among individuals of a group, such as species, variety or strain.

Varietal mixture
A mixture of different cultivars planted in the same field; same as cultivar mixture.

Varietal stability
Degree of change and variation occurring in a cultivar or variety.

Variety
A group of individuals within a species that differs in certain characters from other groups of the species e.g. plants in a species that differ in form, colour, fruit size, fruit flavour etc.

Vascular
The conducting tissues (phloem, xylem) of plants. Term applied to a plant tissue or region consisting of conductive tissue (phloem and xylem); also, to a pathogen that grows primarily in the conductive tissues of a plant, and in animals pertaining to or having many blood vessels.

Vascular tissue
A general term referring to either or both xylem and phloem.

Vector
Literally 'a carrier'. An animal carrying a micro-organism pathogenic for members of another species; the vector may or may not be essential for the completion of the life cycle of the pathogenic micro-organism. Also, the vehicle for cloning, typically a DNA molecule (plasmid or bacterophage DNA) capable of self-replication in a host organism.

Vector resistance
Resistance of a host plant to the vector of a virus. Three basic types of vector resistance are recognised, antibiosis, antixenosis and tolerance.

Vegetative
Asexual; somatic.

Vegetative phase
Usually refers to a non-reproductive phase in fungi and plants. In virus infection, the period during which there is an actual multiplication of viral material; the phase preceding the final infective phase.

Vein
A vascular bundle forming the framework of fibrous tissue in a leaf, a blood vessel conducting blood towards the heart in any animal; also, more heavily sclerotised portions of wings of insects, which are remains of lacunae.

Vein banding or clearing
A symptom of virus-infected leaves in which tissues along the veins are darker green than other laminar tissue (the tissue between the veins becomes chlorotic).

Venereal disease
Any of a number of different sexually transmitted diseases.

Venn diagram
A diagram portraying graphically each sample event as a sample point in a sample space S.

Ventral
The upper or front side. Opposite of dorsal. Referring to the underside of an organism.

Verification
The process of checking that the model code in fact represents the model equations formulated.

Vermicle
Infective stage of Babesia in a tick.

Vermiform larva
A legless, headless, worm-like larva typical of some Diptera.

Verminous intoxication
Variable condition of systemic poisoning caused by absorbed metabolites produced by parasites.

Vernalisation
To treat seeds, bulbs or seedlings of a plant to shorten its vegetative period and thereby induce earlier flowering and fruiting.

Verruga peruana
Clinical form of Carrion's disease, caused by the bacterium Bartonella baclliformis and transmitted by sand flies.

Vertebrate
Having a vertebral column. A member of the animal kingdom known as the vertebrate and including the mammals, reptiles, birds and fishes.

Vertical resistance
In theory, resistance governed by one or more genes in a host, each of which corresponds to a matching gene for parasitic ability in the pest species; sometimes called gene-for-gene resistance.

Vertical transmission
Transmission to progeny.

Vertifolia effect
Proposed by Vanderplank it is the decline of horizontal resistance due to the absence of selection pressure, during breeding for vertical resistance.

Vertigo
A sensation of having the surroundings rotate, or of rotating in space. Similar to but not synonymous with dizziness.

Very fine spray
A dispersion of drops below 30 to 100 Ám volume median diameter.

Very low volume
See spray xii.

Venturi spreader (Air-scoop)
Device mounted on aircraft wing to replace blower for breaking up and discharging spray liquids.

Vermicide
Pesticide for worms.

Vesicant
Producing skin blisters.

Vesicle
In fungi a bubble-like structure produced by a zoosporangium and in which the zoospores are released or are differentiated. In animals a blister; a small circumscribed swelling of the epidermis containing a clear fluid. In general a body constructed as a bladder; a small, rigid, thin-walled structure.

Vesicular disease
Any disease of the urinary bladder, such as vesicular schistosomiasis.

Vessel
A xylem element or series of such elements whose function is to conduct water and mineral nutrients.

Vestigial
Poorly developed; degenerate; non-functional.

Viable
Able to live; of spores, able to germinate.

Viable count
The enumeration of living organisms in a given sample, usually by plating in agar.

Viability
The state of being alive. Capacity to germinate.

Vigour
Strength, or force, or power.

Viral
Pertaining to viruses.

Viremia
The presence of virus in the haemolymph or blood.

Virescence
A symptom in which green pigmentation occurs in plant tissues not normally green.

Virescent
A normally white or coloured tissue that develops chloroplasts and becomes green.

Virginogeniae
Progeny of parthogenetic females eg. aphids developing from unfertilised eggs.

Virginoparae
Female aphids giving birth to young by parthenogenesis.

Viricide
A substance that completely and permanently inactivates a virus.

Virion
The mature virus, the ultimate phase of viral development. The virion is either a naked or an enveloped nuclecapsid.

Viroid
A pathogenic agent that has virus-like properties composed only of a small amount of RNA and containing no protein; the RNA is reproduced during infection.

Virosis
Virus disease.

Virulence
Relative capacity to cause disease; degree or measure of pathogenicity of a pathogen. Sometimes restricted to cases in which races interact differentially with host cultivars.

Virulence factor
Virulence gene or genes in a pest which have not necessarily been identified but can be used for practical purposes in gene-for-gene relationships.

Virulent
Capable of causing a severe disease; strongly pathogenic.

Viruliferous (of vector)
Containing a virus; of an insect vector, containing virus and being capable of introducing it into a suscept.

Virus
An ultra-microscopic (one dimension less than 200 Ám) organism. Viruses cannot reproduce alone but must first infect a living cell and usurp its synthetic and reproductive facilities. Responsible for some diseases in plants and animals.

Virus cryptogram
A descriptive code summarising the main properties of a virus.

Virus inactivator
A chemical that inactivates a virus causing loss of infectivity.

Virus inhibitor
A chemical that prevents virus transmission without inactivating the virus. Inhibition of transmission may be temporary and can sometimes be avoided by dilution. Inhibitors frequently act on the recipient host rather than on the virus itself.

Virus-tested
Shown to be free of specific virus or viruses.

Virustatic
Pertaining to the action of a chemical that inhibits the multiplication of a virus.

Viscerotropic virus
A virus which attacks primarily the viscera; having a predilection for the abdominal or thoracic viscera.

Viscid
Sticky.

Vitamin
An organic compound which occurs in very small amounts in most vegetable matter without which normal functions may not proceed. There are many different ones and they were originally distinguished by letters of the alphabet, vitamin A, B, C, etc. Often referred to by a name indicating their chemical composition, eg. panthothenic acid, riboflavin, cyanocabalamine, etc. Absence of any one in the diet produces a specific syndrome. Continued complete absence can result in death. Some animals appear to be able to manufacture their own requirements of certain vitamins.

Vivaparous
Giving birth to live young, as distinct from oviparous, ie. laying eggs.

Viviparae
Female aphids giving birth to live young (opposite of oviparae). The young undergo embryonic development in the abdominal cavity of the female.

Viviparously
A method of a sexual reproduction occurring in aphids by which the young are born alive and active.

Vivotoxin
A toxic substance produced internally in the infected host of by the pathogen within it.

Voiding
Evacuating.

Volatile
A compound is said to be volatile when it evaporates or vaporises (changes from a liquid to a gas) at ordinary temperatures on exposure to the air.

Volatilisation
Formation of gases.

Volatility
Ability of a substance to volatilise or assume a gaseous state.

Volatilise
To vaporise.

Volume rate
The amount of liquid applied per unit area. Spray volume rates are often described as high, medium, low, very low or ultra-low but to avoid confusion they are best specified in litres per hectare (see spray volume).

Voluntary intake
The amount of feed consumed by an animal unrestricted by the quantity available. Maximum voluntary intake is used to describe the maximum quantity (usually per day) that a given animal can consume when neither quantity, quality nor time are limiting.

Volunteer
A plant from a previous season's crop that regenerates in a subsequent crop, eg. potato tuber.

Vulva
The external opening of the nematode female reproductive system.