Glossary: So-Sz

Soap insecticides
A solution of soft soap (i.e., potassium soap) has been traditionally used to control aphids and similar insect pests. These days, water with a small content of dish detergent can be used. The affected insects are unable to breathe, and these insecticides are stable.
Sociability scale
A five-point scale used to indicate the sociability (i.e., the degree of gregariousness or clumping) of a plant species. The scale runs from 1 (solitary) to 5 (pure stand). The most successful plant domestications have involved species with a high sociability.
Social
A species is described as social when the individuals remain together in social groups. The converse is described as a ‘solitary’ species, in which individuals usually come together only purposes of mating.
Social animals are easier to domesticate and dogs, horses, cattle, sheep, and goats are all social species. Social insects include the ants, termites, and bees.
It is also possible to speak of ‘social’ plants that tend to grow in pure stands in the wild. These too are easier to domesticate than solitary species.
See also: Sociability scale.
Softwoods
Timber trees that are Conifers. The timber of these trees is able to store large amounts of water and this permits growth in areas that have dry summers. Softwood lumber is used mainly for building, and it is generally unsuitable for furniture.
Soil
The upper-most layer of the ground, soil is a complex mixture of minerals and micro-organisms, which has been described as both a micro-ecosystem, and the most complex of any ecosystem.
Soil is essential for most plant growth, providing both nutrients and a substrate to anchor the roots. Depleted soils are those in which the microbiological and/or nutritional components have been seriously reduced.
Depleted soils can best be restored to good condition with organic manures.
Soil conservation
Practices which are designed to prevent or reduce soil erosion. The principle methods are minimum tillage, contour ploughing, terracing, and mulching. The planting of trees to make windbreaks is also helpful.
Soil conservation is one of the more important aspects of sustainable agriculture.
Soil erosion
The loss of soil to either wind or water. During the 1930s drought in the American midwest, the wind erosion was so severe that this area became known as the ‘dust bowl’. Water erosion in the Tigris-Euphrates Valley over the last five millennia has extended the mouth of the combined rivers about 100 miles into the Arabian Gulf.
Soil erosion can be prevented by suitable soil conservation practices and this is one of the more important concerns of sustainable agriculture.
It is notable that rice paddies conserve soil very well, and that ancient rice‑based cultures still thrive while ancient wheat-based cultures are often extinct, as in the Tigris-Euphrates Valley..
Soil inoculation
Soil can be inoculated with either beneficial or parasitic micro-organisms. The most commonly used beneficial inoculants involve Rhizobium to encourage nitrogen fixation by leguminous plants.
Inoculation with soil-borne parasites is undertaken for the purpose of plant breeding and screening a heterogeneous plant population for horizontal resistance. The most effective method of soil inoculation is to transplant inoculated seedlings.
Soil microbes
Soil microbes are a crucial component of the soil ecosystem. They include bacteria, algae, fungi, and protozoa, as well as the more complex nematodes and micro-arthropods.
The overall health of a soil depends on the action of soil microbes, which break down organic matter, digest insoluble minerals, and interact symbiotically with plant roots. Soils that are treated with chemical fertilisers generally lose microbial diversity, leading to a slow decay in soil health.
Soil nutrients
Except for iron, plants absorb all their nutrients as inorganic chemicals, and most of them are extracted from the soil. Carbon dioxide, oxygen, and some water are extracted from the atmosphere, but all others come from the soil.
The three major nutrients are nitrogen, phosphates, and potash (NPK).
Minor nutrients are calcium, sulphur, iron, boron, and magnesium.
Trace elements include zinc, copper, manganese, etc.
See also: Deficiency diseases; Mobile and immobile nutrients.
Soil pasteurisation
Soil pasteurisation means that the soil has been heated to only 80°C (175°F). This kills most pests and pathogens without a complete sterilisation.
Pasteurised soil can be used as soon as it is cool, whereas sterilised soil must be kept for some three weeks for its microbiological activity to be restored.
Soil science
See: Pedology.
Soil sieves
Soil sieves are designed for the mechanical analysis of soils, separating the soil particles on the basis of their size.
The sieves come in sets which fit securely into each other, with the coarsest at the top, and the finest at the bottom. A set of soil sieves can be very useful for separating small seeds, which may be either wet or dry, from the debris of their extraction.
Soil structure
This term usually refers to the particle size of the various mineral components of the soil, and the proportions of those minerals.
A heavy soil has a high proportion of clay particles, which are very small. Such a soil is heavy to work, and has poor drainage.
A light soil has a high proportion of sand and silt particles. It is easy to work, and is freedraining.
A soil with the optimum structure for plant growth is often called a loam.
Soil-borne parasites
Plant parasites that are carried in the soil and are immobile. They are usually dormant until a suitable plant root grows close to them. They include fungi, nematodes, bacteria, and insects.
Rotation is the most common method of controlling soil-borne parasites, but they should be taken into account when breeding for comprehensive horizontal resistance to all locally important parasites.
Solanaceae
The potato botanical family (Solanaceae) includes the cultivated species eggplant, peppers, potato, tobacco, and tomato.
Except for the eggplant, which originated in India, all these crops are of a New World origin.
Solanum melongena
Eggplant, also known as aubergine or brinjal, originated in India and is the only important Old World cultivated species of the family Solanaceae. The crop is cultivated for its fruit which is eaten as a vegetable. It is open-pollinated and hybrid varieties are useful. Quite an easy crop for amateur breeders.
Solanum tuberosum
The potato, which originated in the equatorial highlands of South America.
The potato of commerce is a tetraploid that is self-compatible but somewhat intolerant of inbreeding. In addition, there are a number of both wild and cultivated diploid species that are self-incompatible and allogamous.
The original imports to Europe involved short-day potatoes in which tuber initiation was delayed until the autumn equinox in late September. This meant that the crop was likely to be destroyed by frost before it was mature.
During the eighteenth century, day-neutral potatoes were developed in Europe, partly by deliberate breeding, and partly by natural selection.
Nearly all potato breeding over the last century has involved disease-free certification of seed tubers of very susceptible cultivars, as well as breeding under conditions that have created a vertifolia effect.
As a result, modern potato cultivars have very low levels of horizontal resistance to many pests and diseases, and this crop is one of the most heavily treated with crop protection chemicals.
This problem has been greatly aggravated by the spread of the second mating type of Phytophthora infestans.
This is an excellent crop for amateur breeders working with horizontal resistance, particularly as even small increases in the levels of quantitative resistance will greatly assist organic farmers.
See also: Neo-tuberosum, Sweet potato.
Sorghum
See: Sorghum bicolor.
Sorghum bicolor
Sorghum, also known as: milo, kafir, durra, feterita, kaoliang, mtata, sorgo, jola, jawa, guinea corn, and cholam. It is grown mainly in Africa, India, China, and USA.
Sorghum is the fourth most important cereal in the world, after wheat, rice, and maize. It is very drought-resistant and this makes it an important food crop in arid and semi-arid areas.
Grain sorghums, as their name implies, are grown for grain which is used either for food or brewing. Sorgos and sweet sorghums are used mainly as fodder and for syrup production. Broom corn is used for making brooms. Relatively new dwarf hybrids allow harvesting by combine, and they have led to an increased cultivation of sorghum.
Sorghum is an open-pollinated, short-day plant. Hybrid varieties have been produced in USA. It is an excellent crop for plant breeders associations that are located in an appropriate area.
Sorghum vulgare
Synonym of Sorghum bicolor.
Sorgo
See: Sorghum bicolor.
Source of resistance
Mendelian breeders always need a source of resistance in order to undertake breeding for resistance to crop parasites. This source of resistance is usually controlled genetically by a single gene which is part of a gene-for-gene relationship, and which confers vertical resistance. If a source of resistance cannot be found, the breeding cannot be started.
Biometricians working with polygenically controlled horizontal resistance do not need a source of resistance. This is because they only have to change the frequency of polygenes that are already present, at a low frequency, in the screening population, using recurrent mass selection. However, this general rule does not prohibit the use of resistant parents in a breeding program.
Sowing
The process of planting seed (c.f., sewing).
Soybean
See: Glycine max.
sp. & spp.
Single and plural taxonomic abbreviations for species.
Specialists
Amateur breeders can often benefit by consulting a specialist when they need factual information. However, you should be aware that many specialists are not educated in the principles of horizontal resistance and may even be against it because of their lack of understanding.
Species
A term that has never been satisfactorily defined. Its most usual definition is a group of individuals that are sexually compatible with each other, but not with other members of the same genus or family.
A crop species is usually made up of different commercial varieties, agro-ecotypes, or pathodemes which are all sexually compatible with each other, but not with other crop species in the same genus of family.
However, both inter-specific and inter‑generic hybridisation are often possible. For example, wheat and rye are different genera, but they have been hybridised to produce an inter-specific hybrid called triti­cale.
Sphaerotheca
A member of the Erysiphales (powdery mildews) characterised by cleistothecia with simple, unbranched appendages and a single ascus.
Sphaerotheca macularis causes powdery mildew of strawberry; S. mors‑uvae attacks currants and gooseberries; S. pannosa attacks peaches and roses.
Spinach
See: Spinacia oleracea.
Spinacia oleracea
Spinach, a member of the Chenopodiacea family. A pot-herb crop with complex genetics that is not generally recommended for amateur breeders
Spine
See: Thorn.
Spore
A microscopic, reproductive body of fungi, bacteria, and other organisms. Spores may be produced either sexually or asexually. They have the same reproductive, dissemination, and survival functions as the seeds of higher plants, except that asexually produced spores do not exhibit sexual recombination and variation.
Sport
Mutants within a clone are often called sports. Many ornamental plants with variegated foliage are sports.
Sprayers
Any machine, whether hand held or tractor-mounted, and used for applying liquid pesticides to a crop, is known as a sprayer. Most sprayers use pressure to force the liquid through a nozzle in order to produce a fine mist.
Various nozzle designs produce solid cone, hollow cone, flat fan, and flooding applications, with different flow rates and different droplet sizes.
Airblast sprayers run the liquid into an air blast that breaks it into small droplets and, at the same time, tosses the leaves around in the blast to ensure that all plant surfaces are covered.
Ultra-low volume (ULV) sprayers produce extremely small droplets and are used in areas where water is scarce.
Spreader rows
Spreader rows are rows of a susceptible cultivar that run through a screening population in order to provide inoculum of appropriate pests and diseases. The purpose of spreader rows is to reduce parasite gradients as much as possible.
Great care must be taken with open-pollinated crops to ensure that no pollen from the susceptible cultivar enters the gene pool of the recurrent mass selection. This undesirable pollination can be avoided by either by decapitating or cutting down the spreader rows, or by asynchronous planting so that the spreader rows produce pollen only before or after the screening population is receptive.
Spreader surrounds
Spreader surrounds differ from spreader rows in that they surround the screening population rather than run through it. They are used when parasite gradients are not a problem.
Spring and winter cereals
Spring cereals are sown in the spring, as soon as the land is dry enough to take a tractor. Winter cereals are sown in the previous Fall, and they have the advantage of being several weeks ahead of the spring cereals. Breeding for winter hardiness is very similar to breeding for horizontal resistance. A heterogeneous population is sown in the Fall, and only those individuals that survive the winter can be screened the following summer. Nature does the breeding work for us.
Spruce
See: Picea spp.
Squash
See: Cucurbita spp.
Stable protection mechanisms
Any mechanism that protects a host from a parasite can be divided into one of two classes: stable and unstable.
A stable mechanism is one that does not break down to new strains of the parasite, which are unaffected by that mechanism. This is because the mechanism is beyond the capacity for micro-evolutionary change of the parasite.
Stable mechanisms include horizontal resistance, natural pyrethins, nicotine sulphate, rotenones, mineral oil, Bordeaux mixture and other copper fungicides, dithiocarbamates.
See also: Unstable protection mechanisms.
Stamen
The male organ of a flower. When mature, the anthers release pollen for the fertilisation of a female ovule.
Staple
The main item of a diet, hence ‘staple crop’ and ‘staple food’. Staple crops can be divided in to major and minor staples.
Starch
Starch is a polysaccharide used by many plants as a method of storing carbohydrates. It is a major constituent of the human diet, being obtained mainly from cereals and potatoes.
Starvation
The word starvation is derived from the old English word meaning ‘to die’ but, in modern usage, it can also mean malnutrition. In either event, it is the result of a food deficiency.
Statistics
The phrase “Lies, damn lies, and statistics” is a reference to inaccuracies in the news media. Mathematically, the term is entirely respectable and it provides a technique for determining the accuracy of quantitatively variable data.
In this sense, it is widely used in field trials for comparing many different factors, such as choice of cultivar, fertiliser use, plant spacing, and time of sowing. However, the use of statistically controlled field trials in entomology and plant pathology has led to major errors in the past because of parasite interference.
Statistical analysis used to be the bane of agricultural students and agronomists, but it is now quite easy with modern computer software. Amateur breeders working with horizontal resistance will not normally need to use statistics.
Stem
The part of the plant that carries the leaves and flowers. It is usually vertical, and it may be branched. An underground stem is called a rhizome.
Stem borers
The larval stage of various species of insect, so called because they bore a tunnel up the stem of herbaceous plants, often killing the stem.
Stem pitting
See: Tristeza.
Sterile
A sterile organism is one that is unable to reproduce. A sterile container or environment is one that is completely devoid of life of any description.
Sterile males
An entomological technique for controlling certain insect pests. Large numbers of male insects are made sterile, usually by radioactive irradiation, and are then released. They mate with females which then lay infertile eggs. The technique works best with species in which the males mate many times but the females mate only once.
Stigma
The female part of a flower that receives pollen.
Stock
The plant or rootstock on to which a scion is grafted.
Stolon
An underground stem (e.g., the stalk by which a potato tuber is attached to its parent plant).
Stoma
(Plural: stomata). Microscopic pores in the leaf epidermis, which allow the passage of carbon dioxide, oxygen, and water vapour. The size of the pores is controlled by guard cells, which can close them completely, if necessary.
Stone fruits
Fruits of the botanical family Rosaceae that contain a single hard seed, called a stone, pit, or pip. The term includes plums, cherries, greengages, peaches, apricots, almonds, and sloes.
See also: Pome fruits.
Strain
In a taxonomic sense, a strain is a subdivision of a species, often defined in terms of a physiological or parasitism criterion.
Straw
The dried stems and leaves of a cereal or pseudo-cereal crop. Straw is used mainly as bedding for farm animals and it is a major component of farmyard manure. Mechanisation has greatly reduced the demand for straw bedding, and dwarf varieties have greatly reduced the supply.
Strawberry
See: Fragaria ananassa.
Striga spp.
These tropical and subtropical species, known as witch weed, are members of the family Scrophulariaceae, and they are semi-parasitic, often causing considerable damage to maize, sorghum, and other crops.
The plants contain some chlorophyll and they damage their hosts mainly by robbing them of water. The seeds are minute, and are produced in huge numbers. They can remain dormant in the soil for years, and they germinate only in the presence of roots of a suitable host.
Scientists at IITA have bred maize with horizontal resistance to Striga.
Style
The stalk that supports the stigma.
Stylet
In a zoological context, a stylet is the piercing mouthpart of an insect.
Suboptimisation
The process in which a system is either damaged or misunderstood because of working at too low a systems level.
The problems arise either because emergents at higher systems levels remain unobserved, and/or because other subsystems are not taken into account.
An example of suboptimisation is when Mendelian plant breeders attempted to control various crop pathosystems using (i) the vertical subsystem only, (ii) only one vertical resistance employed on a basis of uniformity, and (iii) a vertical resistance controlled by only one gene.
See also: Holistic approach, Merological approach.
Subsistence crops
Crop species that are more or less confined to subsistence farming (e.g., taro).
Subsistence farmers
Farmers who grow crops primarily to feed themselves and their family. They may sell subsistence surpluses, but the demand for such produce is usually low. They may also grow one of more cash crops, such as coffee or pyrethrum.
Subsistence farming
Farming which feeds the farmer and family but produces few surpluses for sale. The converse is usually called commercial farming. Similarly, there are subsistence crops and cash crops. Almost all subsistence farming is now in the tropics.
Subsistence crops are often mixtures of several different species, and each species is usually cultivated as a landrace. Subsistence crops are thus genetically diverse, and genetically flexible.
Because they are cultivated without any use of pesticides, they have high levels of horizontal resistance. However, their yields and quality are usually considerably less than the modern cultivars of commercial farming. Subsistence crops are thus about halfway between a commercial crop pathosystem and a wild pathosystem.
Subsystem
A lower systems level. Thus, a functioning gene-for-gene relationship constitutes the vertical subsystem of a plant pathosystem, which is itself a subsystem of an ecosystem.
Succulent
In a botanical context, a succulent is a drought-resistant plant that usually contains a considerable store of water. This store is usually protected with very sharp spines, or some form of toxin or deterrent taste. Cacti are typical succulents.
Sucker
An underground shoot arising from either from the roots or the subsurface stem of a tree or shrub. Suckers are often an important means (occasionally the only means, e.g., date palm) of vegetative propagation.
Sucking bugs
See: Miridae.
Sugar
The sugars are sweet-tasting, soluble, crystalline carbohydrates. There are a number of different sugars in the human diet, the most common being sucrose, extracted from either sugarcane or sugar beet.
Sugar beet
See: Beta vulgaris.
Sugarcane
See: Saccharum officinarum.
Sulphur
Sulphur, in the form of sulphates, is a plant nutrient. It is an immobile element and deficiency symptoms, which resemble those of nitrogen deficiency, appear in the young leaves first.
Sulphuric acid
Often known as ‘battery acid’, this acid used to be sprayed on to potato crops to kill the haulms, in order to prevent spores of blight penetrating the soil and reaching the tubers. Other chemicals can also be used but, now that the second mating type is present, with functional oospores in the soil, there is no point in this practice.
Sunflower
See: Helianthus annuus.
Survival advantage
This term refers to both macro- and micro-evolution. That is, it applies to both evolutionary competition and ecological competition. It refers to any characteristic (e.g., horizontal resistance) that enables an individual to reproduce more effectively then its competitors.
Survival of the fittest
A phrase often taken to mean natural selection.
Susceptibility
The converse of resistance. A host is described as being susceptible to a parasite when that parasite is able to parasitise it, and extract nutrients from it. Agriculturally, a susceptible cultivar is likely to be destroyed by its parasites if it is not protected with pesticides.
Sustainable agriculture
A system of farming in which each generation inherits the family farm in as good, or better, condition than the last. No soil is eroded, no groundwater is depleted, no weed seeds have accumulated, no pests or diseases flourish, and no toxic chemicals lurk.
Swedes
See: Brassica napus.
Sweet potato
See: Ipomea batatas.
Sweet sorghum
See: Sorghum bicolor.
Symbiosis
A form of ‘living together’ in which each organism benefits the other. For example, a lichen is a symbiosis between a fungus and an alga. And the nitrogen-fixing root nodules of legumes are a symbiosis between the legume and the Rhizobium.
Symbiosystem
A subsystem of an ecosystem that involves symbiosis, or cooperation, between two entirely different species of organism.
Probably the most important example of symbiosis in crop husbandry is the nitrogen fixing root nodules on the roots of pulses and fodder legumes, formed by species of a bacterium called Rhizobium.
Symptom
A change in the nature or appearance of a plant that is indicative of disease.
Symptomless carrier
A plant, or cultivar, that has so much horizontal resistance to a virus disease that it exhibits no symptoms of that disease, in spite of being infected with it. Symptomless carriers can be a source of infection for nearby susceptible crops. However, if all cultivars were symptomless carriers, the virus disease would no longer be important.
Synchytrium endobioticum
The fungus that causes wart disease of potatoes.
Synergism
The effect produced when two or more factors, operating jointly, is greater then the sum of their effects when operating independently.
Synergist
A chemical which improves the effectiveness of another chemical. Thus sesame oil is a synergist of natural pyrethrins. The pyrethrins alone have a ‘knock down’ effect, but the insects are likely to recover. When the pyrethrins are used with an appropriate synergist, however, the insects are killed.
Synonym
In taxonomy, an alternative name.
Synthetic chemicals
The artificial production of chemical compounds. The definition of organic farming is that it uses no synthetic chemicals. That is, it uses no artificial fertilisers, no synthetic herbicides, no synthetic insecticides, and no synthetic fungicides. However, it may use natural chemicals, such as rock phosphate, natural pyrethrins, etc.
Synthetic variety
An improved variety of an outbreeding species, such as maize, sorghum, or alfalfa, which is a genetically diverse population.
Although this population consists of a cross-pollinating, seed-propagated species, most of the plants within it are high yielding, high quality, with high resistances, and agronomic suitability. These qualities may be preserved, or even enhanced, by careful selection of the individual plants destined for seed.
Alternatively, if this selection is not practised, the qualities may decline after a few generations, and the seed stocks must then be renewed.
System of locking
In a wild plant pathosystem, the gene-for-gene relationship acts as a system of locking which controls allo-infections, and reduces the population explosion of an r-strategists parasite. This system of locking is an emergent, which can be observed only at the population level of the system.
See also: Vertical subsystem, n/2 model.
Systemic
In a wide sense, ‘systemic’ means involving the entire system. In the context of crop protection chemicals, it means an insecticide or fungicide that penetrates the entire plant, and which kills a parasite inside that plant.
Systemic pesticides can thus cure parasitism, while protective pesticides merely prevent parasitism, but cannot cure it once the parasite has penetrated inside the host tissues. The practical difference is that protective pesticides must be applied before an epidemic or infestation starts, as a form of insurance premium. Systemic pesticides can be applied after the epidemic or infestation has started.
There is also a toxicological difference in that protective chemicals can be washed off, but systemic chemicals can linger.
Systems level
Most systems may be considered in terms of a hierarchical organisation, and each rank in the hierarchy is called a systems level. Thus, a system consists of various ranks of subsystems, and is itself part of a super-system.
A pathosystem is a subsystem of an ecosystem, and it has subsystems such as the vertical subsystem, and the horizontal subsystem.
Systems theory
See: General systems theory.
Systems thinking
The basis of systems thinking is the holistic approach, of seeing the system as a whole, of seeing the forest rather than the trees.
A system cannot be understood by an analysis of its parts. Systems thinking concerns the organisation of those parts, as a single system, and the emergent properties that emanate from that organisation.