Glossary: D

Dactylis glomerata
A pasture grass grown mainly in the temperate regions of the Old World.
Daktulosphaira vitifoliae
The new scientific name for Phylloxera vitifoliae of grapes.
Damping-off
A disease of very young seedlings, which rots the stem at the soil surface. Affected seedlings then fall over like miniature, felled trees.
The disease is caused by fungi such as Phytophthora, Pythium, and Rhizoctonia, and it is greatly aggravated by over-watering, which should be avoided. Otherwise, the best methods of controlling the disease are to use soils that have either been pasteurised with steam heat, or treated with a fungicidal soil drench.
Dandelion
See: Taraxacum.
Darwin, Charles
The English discoverer of evolution, Charles Robert Darwin (1809-82) was appointed to the post of naturalist on the scientific expedition of HMS Beagle (1831-6). In 1842, he bought Down House, in Kent, where he lived for the rest of his life, apparently suffering from Chagas disease, which he had contracted in South America.
Having a private income, he could investigate as he pleased and at his own slow pace. By 1844 he had developed his theory of evolution but he delayed publication until a note from Alfred Wallace revealed his independent discovery of the same idea.
In 1858, their joint paper was read to the Linnaean Society and, in 1859, Darwin published his famous book On the Origin of Species by Means of Natural Selection. The book was widely and quickly recognised, but opposition came from religious groups who preferred a literal interpretation of the Bible.
Dasheen
See: Colocasia esculenta.
Date palm
See: Phoenix dactylifera.
Daucus carota
The carrot. This is an open-pollinated member of the family Umbelliferae and the production of uniform lines is difficult. Some work has been done on hybrid varieties but there are technical problems with this approach.
There is plenty of scope for the accumulation of horizontal resistance, but amateur breeders should be a little wary of tackling this crop.
Day-length
A parameter that governs the initiation of flowering and other developments in plants.
Tropical plants are often short-day plants (e.g., potatoes that will not form tubers until the September equinox when grown in temperate regions) and temperate plants are often long-day plants (e.g., hops and olives which will not flower in the tropics).
Day-neutral
A day-neutral plant is one that is not affected by day-length (e.g., temperate cultivar of potato). See also: photoperiod-sensitive.
DDT (Dichloro-diphenyl-trichlor-ethane)
One of the dirty dozen chemicals called POPS (persistent organic pollutants). The first, most famous, most successful, and most notorious of the synthetic insecticides.
It was first synthesised chemically in 1873 but its insecticidal properties were not discovered until 1939, by the Swiss entomologist Paul Müller, who was awarded the 1948 Nobel Prize in Medicine.
DDT proved to be of enormous value in the control of insect vectors of human diseases, such as malaria, typhoid, and cholera. Scientists still speculate whether DDT or penicillin has saved the most human lives.
DDT also controlled the vectors of many animal diseases, as well as numerous crop pests. The latter included major pests such as Colorado beetle of potato, boll worms of cotton, and codling moth of apples. It should be remembered that these pests had previously been treated with compounds of lead, arsenic, mercury, and cyanide.
DDT was both cheap and persistent. The peak production in the USA was in 1961 when 175 million pounds were produced. However, its widespread abuse led to serious environmental damage and fears for human health.
Because DDT is water-insoluble, but fat-soluble, it accumulates in body fat, and a phenomenon called biomagnification leads to increasing concentrations of DDT up the food chain. Humans, as well as many carnivorous birds are at the top of their food chains and accumulate the highest levels.
A further problem was the development of DDT-resistance in target insects. This was the first example of an unstable insecticide.
Another problem concerned the killing of non-target and beneficial insects, such as pollinating bees, and the agents of biological control.
In 1973, its use in the USA was banned, and many other countries followed this example. It is now banned by international treaty except in areas where its use is essential for the control of malaria.
Death
In systems terminology, death is a loss of behaviour, while decay is a loss of structure. Life is an emergent property, and death is the irrevocable loss of that emergent.
Death rate
The rate at which a population is losing individuals.
When the death rate is constant, and equal to the birth rate, the population size does not change. When the death rate exceeds the birth rate, the population growth is negative, and the population size declines. But when the birth rate exceeds the death rate, the population growth is positive, and the population size increases.
When the positive population growth is very rapid, and it is called a population explosion. This rapid rate is typical of r‑strategists.
deBary
The German botanist Heinrich Anton deBary (1831-88) is considered the founder of modern mycology.
Deccan hemp
See: Hibiscus cannabis.
Deciduous
The habit of some trees and shrubs of shedding their leaves, by abscission, at the end of each growing season. The function of this habit is usually to escape an adverse season, such as a winter, or a tropical dry season.
The deciduous habit also has advantages in the control of leaf parasites by providing a discontinuous pathosystem in which a gene-for-gene relationship can operate as a system of biochemical locking.
Most deciduous trees are Angiosperms. See also: n/2 model, Seasonal tissue.
Deficiency diseases
Deficiency diseases are among the non-parasitic physiological disorders, which are due mainly to nutritional deficiencies or toxicities. Each nutritional element produces its own deficiency symptoms.
Within one plant, mobile elements can be taken from old tissues to feed the young tissues, and the symptoms then appear mainly in the older tissues. Conversely, immobile elements cannot be re-allocated in this way, and the main deficiency symptoms then appear in the youngest tissues.
Deficiency symptoms are easily confused with herbicide injury, and a specialist should usually be consulted.
For the symptoms of each mineral deficiency, see Nitrogen, Phosphorus, Potassium, Magnesium, Calcium, Boron, Sulphur, Iron, Zinc, Copper, and Manganese.
Defoliation
Loss of leaf. Defoliation can be:
1. natural, as with leaf-fall in a deciduous tree or shrub;
2. pathologically induced by the activities of parasites;
3. induced by the misuse of herbicides; or
4. from abnormal environmental conditions, such as severe drought.
Dehiscent
This term means that a seed capsule, anther, etc, opens spontaneously when mature.
Democratic plant breeding
The converse of autocratic plant breeding. With democratic plant breeding, as many breeders as possible are producing as many cultivars as possible so that the farmer has a wide choice of cultivars.
This approach is possible with the use of horizontal resistance because breeding for this kind of resistance is so easy. In many cases, farmers can do their own plant breeding.
Once there are enough amateur plant breeders, the whole system of crop improvement will become self-organising.
Density-dependent selection
The limiting of the size of a population (e.g., a vertical pathotype) by mechanisms that are also controlled by the size of population.
This is a probable genetic mechanism for controlling the system of locking of the n/2 model, ensuring that all the n/2 biochemical locks and keys occur with an equal frequency.
The rarity of a vertical pathotype or pathodeme is a reproductive advantage that leads to commonness. And commonness is a reproductive disadvantage that leads to rarity.
Derris elliptica
The powdered dried root of this leguminous plant contains rotenone and other toxic compounds that are used as an insecticide and a fish poison in S.E. Asia.
The insecticide is used mainly as a hair wash to control lice. This derris insecticide is stable, as no derris-resistant strains of insects have appeared during centuries of use. Derris dust can also be used as an insecticide on crops.
The pounded roots are soaked in water to produce a fish poison, and the poisoned fish can be eaten without risk.
Selected clones of the crop are propagated vegetatively by cuttings of mature stems. But seed set is common and improvement by amateurs is feasible. However, there are no serious parasites of derris, and breeding for horizontal resistance appears to be unnecessary.
Derris is a short-day plant that needs a tropical forest ecology, and there appears to be no possibility of producing temperate cultivars.
Desert locust
See: Schistocerca gregaria.
Designated host
A genetically stable host (i.e., a clone or pure line) which has been chosen for use in the one-pathotype technique in a horizontal resistance breeding program.
The designated host has a resistance that is matched by the designated pathotype, which is cultured on that host for the entire duration of the breeding program.
All the original parents of the breeding population are chosen on the basis of their susceptibility to the designated pathotype, which is used to inoculate every screening population. This will ensure that all vertical resistances are matched during the screening for horizontal resistance, regardless of how the vertical resistance genes may have recombined during the crossing process.
Only one designated pathotype may be chosen for each species of parasite. The one-pathotype technique is necessary only when vertical resistances occur in the host species. However, its use is not always necessary, even then, and alternative techniques exist.
Designated pathotype
A pathotype (i.e., strain, or race) of a parasite which has been chosen for use in the one-pathotype technique in a horizontal resistance breeding program.
The designated pathotype is cultured on the designated host for the entire duration of the breeding program. All the original parents of the breeding population are chosen on the basis of their susceptibility to the designated pathotype, which is used to inoculate every screening population. This will ensure that all vertical resistances are matched during the screening for horizontal resistance, regardless of how the vertical resistance genes may have recombined during the breeding process.
Only one designated pathotype may be chosen for each species of parasite. The one pathotype technique is necessary only when vertical resistances occur in the host species. However, its use is not always necessary, even then, and alternative techniques exist.
Designation
See: Designated host; Designated pathotype.
Dessicator
A glass jar with a air-tight lid that is used for drying out small quantities of plant tissue, such as seeds or root nodules, with a dessicating chemical.
Dry calcium chloride is a powerful desiccating chemical, but it is toxic and must be kept well separated from living tissues. Alternatively, silica gel is harmless, but it is less powerful in its drying action.
Determinate habit
The converse of the climbing habit in plants, i.e. a bushy plant rather than a vine.
A determinate plant remains relatively small and close to the ground, like dwarf beans or potatoes. The determinate habit results from the terminal flower of an inflorescence opening first, and the stem grows no further.
Dew
See: Guttation.
Diallel cross
A polycross in which each parent is mated with every other parent.
In a full diallel cross, each parent is represented twice, once as a male and once as a female. More commonly, a half diallel cross is used, in which each parent is represented only once, either as a male or a female, but not both.
A half diallel cross is usually used at the start of recurrent mass selection. The alternative is to use a random polycross.
Dichotomous
A dichotomous stem is one that forks regularly into two branches.
Dicotyledon
Any Angiosperm that has two cotyledons. They are often called the broad-leaved plants.
Seeds of dicotyledons can be split into two halves (e.g., split peas) and they include all the peas and beans, most of the temperate fruits and nuts, crops of the cabbage, cucumber, and potato families, cotton, tobacco, rubber, tea, coffee, cocoa, cassava, sweet potato, and many vegetables, herbs and spices.
Dieback
A plant disease symptom in which stems die backwards from the tip. Diebacks are usually caused either by a pathogen attacking the young tissue of the stem tip, or by a disease in another part of the plant producing toxins that kill the growing point. They can also have a physiological cause, such as a nutrient deficiency.
Differential interaction
A table of host and parasite interactions (i.e., responses of resistances to parasitic abilities and vice versa) in which several different pathodemes are necessary to identify a pathotype, and several different pathotypes are necessary to identify one pathodeme.
A differential interaction is also known as a variable ranking, as opposed to the constant ranking that is typical of horizontal resistance and horizontal parasitic ability. The Person-Habgood differential interaction is the definitive interaction of vertical resistance and vertical parasitic ability.
Digitaria decumbens
A subtropical fodder grass native to Southeast Africa.
Dikaryon
A fungus in which each cell has two haploid nuclei, which are usually genetically distinct. Dikaryotic mycelium is thus equivalent to diploid mycelium. It occurs mainly in the rust fungi.
Dimorphous branching
Some crop species (e.g., arabica coffee, cotton, black pepper) have two kinds of branches. The orthotropic branch is the branch that grows vertically, and it produces side branches, called plagiotropic branches, that tend to grow horizontally.
It is usually the plagiotropic branches that bear the flowers and seed. cuttings must be taken from the orthotropic branch, and this severely limits vegetative propagation.
Dioecious
Greek = two houses (pronounced dye-ee-shous). A plant species in which the male and female sexes are separated in different plants. See also: Hermaphrodite.
Dioscorea alata
This is the Asian yam, also known as the white yam, the greater yam, the winged yam, and the water yam. See Dioscorea spp., for a description of the genus.
This yam was of major importance to the seafaring Polynesians who took it to most of the tropical islands of the Old World. It is propagated vegetatively, because most cultivars never produce fertile seed, and some are completely sterile.
Not recommended for amateur breeders. This is a crop with extinct wild progenitors.
Dioscorea bulbifera
This is the aerial yam, also known as the potato yam. It is of minor importance as a food crop but was probably important in ancient times. It is the only species that occurs wild in both Africa and Asia.
See Dioscorea spp., for a description of the genus. Not recommended for amateur breeders.
Dioscorea cayenensis
This is the yellow yam, also known as the twelve-month yam, and the yellow guinea yam. In spite of its name, this is a West African species that still occurs wild. It was taken to the New World with the slave trade.
It is widely grown in West Africa but it is not as important as Dioscorea rotundata. Not recommended for amateur breeders. See Dioscorea spp., for a description of the genus.
Dioscorea rotundata
This is the white yam, also known as the Guinea yam, and the eight-months yam. It originated in West Africa and is the most important species agriculturally.
Many clones exist but most of them set fertile seed so rarely that breeding is all but impossible. Not recommended for amateur breeders. See Dioscorea spp., for a description of the genus.
Dioscorea spp.
These are the true yams, not to be confused with sweet potatoes, which are called yams in the southern USA.
Although generally considered a monocotyledon, this genus has many features of dicotyledons, including reticulate veining in the leaves and occasional seeds with two cotyledons, in which only one cotyledon develops. The genus is very old geologically, and it occurs, and has been domesticated, in both the Old and the New Worlds. The principle species are described under their specific names.
The true yams are monoecious. Most cultivars are propagated vegetatively because they produce fertile seed rarely or not at all. This makes breeding extremely difficult, and these crops are not recommended for amateur breeders.
There are few pests and diseases of yams and, because all the cultivated clones are ancient, they are a useful demonstration of both the durability and the efficacy of horizontal resistance.
Wild yams were recently in danger of extinction due to the demand for natural diosgenin in the manufacture of oral contraceptives. However, the development of synthetic diosgenins has eliminated this threat.
Dioscorea trifida
The cush-cush yam is the only cultivated yam that is indigenous to the New World. Not recommended for amateur breeders. See Dioscorea spp., for a description of the genus.
Dioscorides
Pedanus Dioscorides was a first century Greek physician who wrote a standard work called De Materia Medica that concerned plants and minerals of medical significance.
Diploid
A cell or a plant with two sets of chromosomes. One set comes from each parent. Diploidy is the normal state in most plants and animals.
See also: Doubled monoploid, Haploid, Tetraploid, Triploid, Dikaryon.
Diptera
The Order of insects called flies, characterised by having only one pair of wings. This is one of the largest orders of insects.
The Order includes biting insects such as black flies, mosquitoes, and sand flies. Houseflies are carriers of human diseases such as typhoid and cholera, and this was the first insect to develop resistance to DDT.
Dirty Dozen
The list of the twelve most persistent organic pollutants (POPs) banned at a United Nations convention in May 2001. Nine of the chemicals in this list are crop protection chemicals and, of these, eight are insecticides.
Discontinuity
See: Discontinuous pathosystem.
Discontinuous epidemic
See: Discontinuous pathosystem.
Discontinuous pathosystem
A pathosystem in which the parasitism is intermittent because there is a complete absence of host tissue at periodic intervals, such as during a tropical dry season, or a temperate winter.
Discontinuous pathosystems involve seasonal host tissue, and they occur typically with annual plants, and the leaf parasites of deciduous trees and shrubs.
Discontinuity confronts the parasite with three difficult problems, because it must survive the absence of host tissue, it must find a new host individual when tissue is again available and, if vertical resistance occurs, it must match the host that it does find.
Alloinfection is of primary importance in discontinuous pathosystems, and vertical resistance has a high survival value.
Note that discontinuity and vertical resistances can occur in the parasitism of the winter hosts of heteroecious rusts and aphids even though these hosts may be perennials. This is because the parasite is oblidged to migrate to its summer host.
See also: Continuous pathosystem.
Discontinuous variation
In genetic terms, variation among individuals may be continuous or discontinuous. Continuous variation means that there is every degree of difference between two extremes. Discontinuous variation means that a character is either present or absent, and there are no intermediates.
Continuous variation results from quantitative inheritance, while discontinuous variation results from qualitative inheritance.
Disease
Plant diseases usually have the most colourful names, such as blight, downy mildew, powdery mildew, blast, rust, smut, smudge, wart, streak, blister, and scorch.
Plant diseases are caused by parasitic organisms called pathogens, that are usually microscopic, and which include fungi, bacteria, phytoplasmas, viruses, and viroids.
The development of a disease within a host population is called an epidemic.
Deficiency diseases are due to nutritional inadequacies and are best described as physiological disorders.
Disease cycle
See: Epidemic cycle.
Disease escape
For a variety of reasons, some individuals in a screening population may remain free of pests or disease. Also known as chance escape, this phenomenon can be very misleading because it is so easily confused with resistance.
See also: inoculation, patchy distribution.
Disinfection
In a plant protection context, this term means destroying the initial inoculum in order to control the epidemic. Disinfection is undertaken most commonly with contaminated seed and infected seed. But it can also refer to storage containers, with a view to reducing post-harvest losses.
Disinfestation
This term means the same as disinfection except that it normally refers to insects.
Dissecting microscope
A low power, stereoscopic microscope with two optical systems that provide a three-dimensional view.
Dissemination
The geographical spread of pests or disease.
The natural dissemination of fungi is usually by wind-borne spores, while insects usually travel by flight, often assisted by wind. However, dissemination can also occur with irrigation water, contaminated or infected seed, muddy boots or tractor wheels, international trade, travellers, etc.
Distal
That part of a plant organ that is most distant from its point of attachment. See also: Proximal.
Distribution, normal
See: Normal distribution.
Dithiocarbamates
A group of synthetic fungicides developed during the 1920s and 1940s that are popular mainly because they are stable.
Diurnal
During daylight hours, as opposed to nocturnal.
Diversity
See: genetic diversity.
Dizygotic
Dizygotic twins develop as two separate embryos produced by two separate ova fertilised by two separate sperm. Also known as fraternal twins.
See also: Monozygotic.
DNA
Di-ribo-nucleic acid. The protein which encodes genetic information, and controls all things inherited. In plants and animals, the DNA is located in the chromosomes.
Dodder
See: Cuscuta spp.
Dolichos lablab
See: Lablab niger.
Domestication
The process by which ancient cultivators changed wild plants into crop plants by artificial selection.
Usually, domestication was a very gradual process in which cultivators tended to use their best plants as parents for the next crop, producing quantitative improvements. Occasionally, however, domestication would progress in sudden and dramatic developments, with qualitative changes, as when both the non-shattering and free-threshing forms of wheat were discovered.
These changes occurred thousands of years ago, and the descendants of those forms have been in continuous cultivation ever since. Ancient domesticators often achieved results that modern plant breeding cannot improve as, for example, with pineapples, bananas, olives, and the classic wine grapes.
A few plant species were domesticated quite recently. These include rubber (Hevea brasiliensis) and oil palm (Elaeis guineensis).
Dominant character
A genetic character is described as dominant when its controlling allele eclipses the recessive allele.
Dormancy
Inactive, as in sleep. Many seeds exhibit dormancy, which is a valuable ecological and evolutionary survival mechanism that ensures survival of the species in the event of some disaster that destroys all non-dormant individuals.
Dormancy can be a nuisance in agriculture, and in plant breeding. It can often be broken by mechanical or chemical reduction of the seed coat, or by temperature treatment of the seed.
Dottato
An ancient Roman cultivar of fig that was mentioned by Pliny the Elder (23-79 AD) and which is still being cultivated in Italy. This is an example of an ancient clone demonstrating the durability and efficacy of horizontal resistance.
Doubled monoploid
A monoploid (i.e., haploid) cell or plant that has undergone a doubling of its chromosomes to produce a functional diploid.
Doubled monoploids are produced artificially, usually by culturing a pollen mother cell, or a pollen cell, into a haploid plantlet, which is then stimulated chemically to double its chromosome number. Alternatively, an unfertilised ovule can sometimes be made to grow into a haploid plantlet by pollination with pollen from a different species.
Doubled monoploids are completely homozygous, and this can be very useful in various plant breeding procedures.
See also: Haploid, diploid, Tetraploid, Triploid.
Douglas fir
See: Pseudostuga menziesii.
Downy mildews
Plant parasitic fungi of the Order Peronosporales, so called because they produce a very light, white mildew on the external surfaces of the plant lesions, usually on the lower leaf surfaces.
The best known members are potato blight (Phytophthora infestans) and downy mildew of grapes (Plasmopora viticola). Downy mildews were originally controlled by Bordeaux mixture.
Dried blood
Obtained from slaughter houses, dried blood is often used as an organic fertiliser. However, the supply is limited.
Drought resistance
The ability of a plant to withstand drought. This property can be very valuable in areas of uncertain rainfall. For example, sorghum has greater drought resistance than maize, and is grown in many semi-arid areas for this reason.
Duram wheat
See: Triticum durum.
Durra
See: Sorghum bicolor.
Dwarf varieties
See: Wheat, and Rice.
Dysmicoccus brevipes
The mealy-bug that causes wilt of pineapples. There is great need for horizontal resistance to this pest but this is not a task for amateur breeders.