Glossary: Pi-Pz

Picea spp.
Spruce. Some five species of spruce are grown as softwood plantation trees. Not recommended for amateur breeders.
Pigeon pea
See: Cajanus cajan.
Pimenta dioica
Pimento or allspice. The fruits of this Central American, functionally dioecious tree were mistaken for black pepper by Columbus who thought he had reached India. Not an easy crop to breed.
Pimento
See: Pimenta dioica.
Pine
See: Pinus spp.
Pineapple
See: Ananas comosus.
Pinnate
A compound leaf that has leaflets arranged on either side of a stalk.
Pinus spp.
The pine trees, which are members of the family Pinaceae, in the order Coniferae, which is one of the five orders of the Gymnosperms. These are particularly important timber trees that provide much of the world’s soft wood.
Selection within existing populations, particularly in North America, looking for horizontal resistance to white pine blister rust (Cronartium ribicola) would be an appropriate activity for a university breeding club made up of forestry students.
Piper nigrum
Black pepper. A vegetatively propagated, tropical crop that is difficult to breed, and one that is not recommended for amateur breeders. This species is a good example of ancient clones that demonstrate the value and durability of horizontal resistance.
Piperonyl butoxide
A chemical used as a synergist to improve the insecticidal effectiveness of natural pyrethrins. Sesame oil is a natural alternative.
Piricularia oryzae
This is the fungus that causes the very important disease called rice blast. There is vertical resistance to this disease and amateur breeders looking for horizontal resistance would have to consider using the one-pathotype technique, particularly as some of the vertical resistances are quantitative.
Pistachio
See: Pistacia vera.
Pistacia vera
Pistachio nuts. A dioecious and evergreen tree native to the Near East, these nuts have been cultivated for 3-4 millennia. Not recommended for amateur breeders.
Pistil
The female part of a flower consisting of the ovary, style, and stigma.
Pisum sativum
The garden pea, or green pea. Peas have been found in the oldest agricultural sites in Europe and the Middle East.
Traditionally, peas were harvested mature, and the dried peas were used to make pea soup and peas pudding. Believed to be the fourth most important grain legume in terms of human nutrition, this pulse is now grown mainly for harvesting the immature seeds for freezing as a green vegetable.
The production of improved horticultural varieties is a possibility for amateur breeders, but they should not attempt to compete with professional breeders in the production of cultivars for the frozen food market.
The wild progenitor of Pisum sativum is extinct.
Pith
Parenchymatous tissue that stiffens the inside of a stem.
Plagiotropic branches
In a plant with dimorphic branching, the plagiotropic branches are the side branches that tend to grow horizontally and that bear the flowers and fruit. The orthotropic branch is the vertical stem that carries the apical meristem, and this is the branch that must be used for cuttings in crops such as coffee, cotton, and black pepper.
Plant
The plant kingdom includes all multi-cellular organisms that contain chlorophyll. These are the multi-cellular algae, mosses, ferns, gymnosperms, and angiosperms.
With the exception of some forest trees, all cultivated plants are angiosperms.
Note that a few angiosperms that are parasitic (e.g., dodder, Striga, broomrape) do not contain chlorophyll. However, these parasites originally possessed chlorophyll.
Plant breeders association
An association of amateur plant breeders who are breeding crops for horizontal resistance, with the goals of reducing the losses from crop parasites, reducing the use of crop protection chemicals, reducing the environmental and human health hazards caused by crop protection chemicals, and/or earning plant breeders’ royalties by breeding for comprehensive horizontal resistance.
See also: University breeding clubs.
Plant breeders' rights
Plant breeders’ rights are the equivalent of authors’ copyrights. A registered cultivar will earn royalties for its breeder on all licensed sales of seed. Amateur breeders should check the legislation and regulations of their own country.
Plant breeding
The scientific discipline concerned with crop improvement by genetic methods. See also: Pedigree breeding, population breeding, Genetic engineering.
Plant breeding institutes
Plant breeding institutes, often with a large staff of specialists, were deemed necessary because of the problems associated with breeding for single-gene, vertical resistance that were part of a gene-for-gene relationship.
The problems associated with this kind of plant breeding are the overall cost, the relatively few cultivar produced, and the short agricultural life of most of the cultivars which have ephemeral resistance.
See also: Professional plant breeding.
Plant disease
A plant disease may be infectious, and caused by a parasite, or it may be physiological, and caused by an environmental factor such as frost, a nutrient deficiency, or a toxin.
The parasites that cause plant disease are usually called pathogens, and they include fungi, bacteria, phytoplasmas, viruses, and viroids. Parasitic nematodes and angiosperms are often considered plant pathogens also.
Plant diseases are studied by plant pathologists, who are sometimes called phytopathologists.
Plant growth chambers
These are research chambers, which may even be an entire room, in which all variables contributing to plant growth can be controlled. These variables include light intensity, light quality, day-length, temperature, humidity, nutrients, presence or absence of parasites, and so on.
Plant hoppers
Homopterous insects characterised by antennae located on the sides of the head, below the eyes. Closely related to the cicadas, whiteflies, aphids, and scale insects.
Plant pathology
The scientific discipline concerned with the study and control of plant diseases, which are usually caused by micro-organisms called pathogens, such as fungi, bacteria, phytoplasmas, viruses, and viroids.
Plant quarantine
The isolation of newly imported plants to ensure that they are not introducing any foreign parasites. The term quarantine may refer to the quarantine station itself, or to the process of testing and purifying the plant material.
Plantain
In the tropics, a plantain is a starchy banana that is eaten cooked. In the industrial countries a plantain is a large sweet banana that is eaten raw.
Plantation forest
Man-made forest in the sense that the tree species used, their planting densities, and other factors, are controlled in much the same way as an agricultural crop is controlled.
Plants as food
Humankind evolved as a hunter-gatherer and an omnivore, and our most natural diet consists of both meat and plant foods. In general, meat provides up to twenty times more nutrition than most plant foods.
Consider the food supply of hunter-gatherers. Our centre of origin is in East and Southern Africa. This is an area of savannah that carries up to 20,000 kilograms of herbivore game animals per square kilometre, and these convert inedible grass into edible meat.
At the other extreme, tropical rain forest carries only 5-10 kg/sq.km. It is no accident that rain forests have the fewest archaeological remains of hunter-gatherers, or that our hominid ancestors favoured open grasslands.
Plant foods are also essential in the human diet as they provide various vitamins, fibres, etc. Vegans prefer a diet made up exclusively of plant foods, while vegetarians also consume dairy products, eggs, and sometimes fish.
Plasmopora viticola
The microscopic fungus that causes downy mildew of grapes. This was a new encounter disease, as it originated in the New World and was taken to Europe on rootstocks of wild American grapes intended for grafting to control Phylloxera.
This was the disease in which Millardet discovered Bordeaux mixture. In 1822, he found that vines next to the public road at the Chậteau Beaucaillon, in the Médoc district of Bordeaux were free of the disease, and he discovered that they had been spattered with a poisonous-looking substance to discourage passersby from eating the grapes.
This substance was the mixture of copper sulphate and lime that we now call Bordeaux mixture.
Plum
See: Prunus spp.
Pod
A poorly-defined term for a dry dehiscent fruit, typically in the family Leguminoseae.
Podosphaera
One of the six genera of the powdery mildews (Erysiphales). The cleistothecia have a single ascus and dichotomously branched appendages. Various species cause powdery mildews of the stone and pome fruits.
Polhill
A farmer in Kenya who, in the 1950’s, bred a famous cultivar of pyrethrum, now named after him, proving that plant breeding of many crops is within the capacity of amateur breeders.
Pollen
The male cells of higher plants, produced in the anthers of Angiosperms, or the male cones of Gymnosperms. Plants have many and varied mechanisms for transferring pollen to the female organs for fertilisation. The most common are pollination by wind or insects.
Pollen mother cell
The cell which, as a result of meiosis, becomes the mother of pollen cells in an anther. The pollen mother cell of some crops can be used to produce a haploid plantlet for later doubling of the chromosome number into a doubled monploid. Not a technique for amateur breeders.
Pollinating insects
Insects that pollinate plants. These are usually bees which are attracted to flowers by the offer of honey, but many other species of insects are involved in a wide variety of specialised flowers, such as those that stink of rotten meat to attract flies.
Pollination
The placing of pollen on a stigma for the purpose of sexual fertilisation. There are a variety of methods of natural pollination, of which wind and insects are the most common. Artificial pollination is usually done by hand, but a male gametocide may also be used. See also: Allogamy, Autogamy, cross-pollination, Inbreeder, Outbreeder, Self-pollination.
Pollution
Any form of contamination. In a modern context, the word is usually used to mean environmental pollution with cropprotection chemicals, factory exhausts, and other forms of agricultural or industrial waste.
Polycross
A system of mating in which a number of parents are represented in various combinations. Thus, full diallel cross, half diallel cross, random polycross.
Polycyclic parasites
Parasites which have several life cycles in the course of one epidemic cycle, or one season. See also: Monocyclic parasite; Oligocyclic parasite.
Polyethylene
Also called ‘polythene’. A thermoplastic, translucent polymer of ethylene that is impermeable to water vapour but permeable to oxygen and carbon dioxide. It makes a valuable protection for delicate seedlings, cuttings, etc. It is also used in the construction of plastic greenhouses.
Polygene
The combined effect of many genes which each have a small effect. Polygenes control the inheritance of a quantitatively variable characteristic.
Polygenic inheritance
Any inheritance that is genetically controlled by many genes of small effect, called polygenes.
Polygenic inheritance is quantitative in its expression, and it exhibits every degree of difference between a minimum and a maximum, usually with a normal distribution.
Polyphyletic
A species that originated by hybridisation from more than one wild progenitor.
Polyploid
An organism, usually a plant, which has more than two basic sets of chromosomes. Thus triploid, tetraploid, etc. See also: Allopolyploid, autopolyploid.
Pome fruits
Fruits of the botanical family Rosaceae which contain several seeds in a so-called 'core'. The term includes apples, pears, quince, and medlar. See also: Stone fruits.
Pomegranate
See: Punica granatum.
Poplar
See: Populus spp.
Poppy
See: Papaver somniferum.
POPs
Persistent organic pollutants.
Population
A group of individuals of one species occupying a particular area. A population may be either homogeneous or heterogeneous; or either homogenous or heterogenous. (Check these curiously similar words for differences of meaning and pronounciation).
Population breeding
The breeding method of the Biometricians, which is concerned with small improvements in quantitative characters that are genetically controlled by polygenes.
Population breeding usually involves recurrent mass selection. Population breeding is easy while Pedigree breeding is technical.
During the twentieth century, population breeding has rarely been used in most crops, particularly the autogamous crops. This leaves the field wide open for amateur breeders.
Population explosion
The very rapid population growth that can occur with an r-strategists species during a favourable season. Many crop parasites are r‑strategists, and it is their population explosions that are can be so alarming, and so difficult to control.
The function of the gene-for-gene relationship and the vertical subsystem in a wild plant pathosystem is to control the population explosion of a parasite, but it can do this only if it functions as a system of locking based on genetic diversity.
Horizontal resistance can also reduce the rate of population growth of the parasite to the point where the epidemic can no longer develop, and this is called population immunity.
See also: Population extinction.
Population extinction
The death of most of the individuals of an r-strategists population that occurs at the end of a favourable season.
With plant parasites, this happens typically in a discontinuous pathosystem, with the loss of host tissue that occurs with leaf-fall in a deciduous host species, or with the death of all plant parts, except the seeds, in an annual host species.
With crop parasites, it often occurs with harvest, such as the digging of potatoes, or the combine harvesting of cereals.
See also: Population explosion.
Population growth
Unlike an individual, a population can have growth that is positive, static, or negative.
Positive population growth occurs when each individual, on average, spawns more than one progeny. Static (or zero) growth occurs when each individual, on average, spawns exactly one progeny. Negative growth occurs when each individual, on average, spawns less than one progeny.
See also: Population immunity.
Population immunity
A host population that is less than immune, but which does not suffer an epidemic. Each host individual may be carrying the parasite, but the level of horizontal resistance is such that the population growth of the parasite is zero or negative.
Populus spp.
Poplar trees, used in plantation forests to produce hardwoods. Not recommended for amateur breeders.
Positive feedback
See: Feedback.
Positive screening
A plant breeding technique in which the best individuals in a genetically diverse population are preserved to become the parents, either of the next screening generation, or of new cultivar.
See also: Negative screening.
Post-harvest losses
Crop losses due to parasites that occur after harvest, usually in the store.
These losses can be reduced or prevented by ensuring (i) that the stored product is dry, to prevent moulds developing, and (ii) that the product is in an airtight container that lacks oxygen, to prevent various animal pests from eating it.
Potassium
A major nutrient of plants, represented chemically by the letter ‘K’, as in NPK, which stands for nitrogen, phosphate, and potassium. The deficiency symptoms are complex. The older leaves show browning of the tips and margins, with numerous brown spots close to the margins. There may also be dieback of the shoots.
Potato
See: Solanum tuberosum.
Potato blight
See: Phytophthora infestans.
Potato viruses
In the eighteenth century, in England, a group of farmers decided to breed potatoes for resistance to the ‘decline’ of potato stocks.
At this time, it was discovered that seed tubers coming from the Yorkshire Moors did not suffer this decline. This was a crucial parting of the ways. It was decided that importing clean seed tubers was easier than breeding for resistance.
From that day to this, we have been controlling potato viruses by certifying seed tubers free of them. The potato viruses spread rather slowly and, for that reason, they rarely appear in plant breeders’ screening populations.
If one seedling became infected, it was thrown out on the grounds of susceptibility. But the clones that were kept were escapes from infection and they were just as susceptible.
We have been losing horizontal resistance to these viruses during more than two centuries of potato breeding. As a consequence, this is a wonderful opportunity for amateur breeders.
Powdery mildews
See: Erysphales.
ppm
Parts per million; a measure of concentration. On the same basis, percentage is parts per hundred, and ppb is part per billion.
Predator
In the context of crop parasites, a predator is any animal, usually an insect or a nematode, that eats the parasites, and thereby contributes to biological control. See also: Hyper-parasite.
Predator-prey relationship
The category of parasitism in which there is a very low frequency of parasitism, but a very high injury from parasitism.
For example, lions parasitise zebras. They only parasitise one zebra at a time, so the frequency of parasitism is minimal. But they consume that one zebra entirely, so the injury from parasitism is maximal.
See also: Hostparasite relationship.
Pre-harvest losses
Crop losses from parasites that occur in the field, as opposed to post-harvest losses that occur in the store.
Princess pea
See: Psophocarpus tetragonobolus.
Procaryote
A taxonomic category of primitive, mainly one-celled organisms, that lack a true nucleus and other specialised organelles. The DNA occurs as a loop in the cytoplasm.
These are among the most primitive of living organisms and include all bacteria and cyano-bacteria. See also: Eucaryote.
Producers
In an evolutionary sense, producers are those organisms that convert solar energy into dietary calories. They do this by using chlorophyll as a catalyst to combine carbon dioxide and water to form carbohydrates. See also: Reducers, Consumers.
In an agricultural sense, producers are farmers, because they produce food, as opposed to consumers who buy and eat it.
Proefstation Oost Java
This was the Dutch sugarcane breeding station in Java where the famous POJ 2878 cane cultivar was produced. This cultivar has subsequently entered into the pedigree of just about every modern cane cultivar.
Professional plant breeding
There were no professional plant breeders before 1900. Plant breeding was undertaken by farmers, and it was often a hobby undertaken by amateurs, even clergymen (who often had time on their hands).
Mendel’s laws of inheritance and single-gene characters, such as vertical resistance, were unknown, and all this breeding involved quantitative, many-gene characters and horizontal resistance. It was unscientific but effective.
During the whole of the twentieth century, the great majority of professional plant breeders were in love with Mendelian genetics, and single-gene characters. This tradition continues today with genetic engineering which, of necessity, can handle only single-gene characters.
See also: Amateur plant breeders.
Progenitor
In a plant breeding context, a progenitor is the wild ancestor of a crop species. Many crop species, such as maize and wheat, have been changed so much by domestication that their progenitors are difficult to identify.
Many other progenitors became extinct because of hunter-gathering, while their domesticated cousins survived in the hands of farmers.
Progeny
In a plant breeding context, a progeny is the offspring of a controlled cross-pollination.
Prokaryote
See: Procaryote.
Propagation
Plant propagation may be by true seed (sexual) or it may be vegetative (asexual).
Seed propagation may involve segregating seed, which does not ‘breed true’, a pure line, which does ‘breed true’, or a hybrid variety, which has hybrid vigour.
Vegetative propagation is achieved with tubers, rhizomes, cuttings, grafts, bulbs, corms, etc.
Protective fungicide
See: Fungicide.
Protein
A nitrogenous organic compound that is an essential part of a living organism.
Structural proteins form exoskeletons, hair, hoof and horn, muscles, etc. Functional proteins include most enzymes, antibodies, etc.
Protein molecules are built up from about twenty different amino acids that are arranged in different orders in polypeptide chains.
The grain legumes, or pulses, are the main source of plant protein in the human diet. Some plant proteins are not digestible by humans and these are liable to ferment in the lower gut causing flatulence.
Proto-Indo-European (PIE)
The ancestor of all Indo-European languages, believed to have been the language of early wheat farmers in southern Turkey. See also: Renfrew hypothesis.
Protoplasm
The living contents of a cell, including the nucleus. See also: Cytoplasm.
Proximal
That part of a plant organ that is closest to its point of attachment. See also: Distal.
Prunus americana
The North American plum, which is a diploid. See also: Prunus domestica. Of rather specialised interest to amateur breeders.
Prunus amygdalus
The almond, which is closely related to the peach and which originated in central and western Asia. It is self-incompatible and cross-pollination is essential for fruit formation.
Almonds are cultivated for the seeds, known as nuts, mainly in Turkey and the Mediterranean, as well as in California. In the Old World, almonds are normally grown from seed, while in North America, they are propagated vegetatively.
Selection within the Old World crops could be profitable for amateur breeders but a more formal breeding program is not recommended.
Prunus armeniaca
The apricot, which originated in western China and, like the peach, is normally self-pollinated. Apricots can be hybridised with plums to produce ‘plumcots’. Not recommended for amateur breeders.
Prunus avium
The sweet cherry, which is a diploid. There are also a number of other cherry species, some of which are tetraploid, including the sour cherry, Prunus cerasus. The commercial importance of cherries has declined with rising labour costs and the rewards for amateur breeders are unlikely to be great.
Prunus domestica
The European plum, which is a hexaploid. Of rather specialised interest to amateur breeders. See also: Prunus americana.
Prunus persica
The peach, which is the most important of the stone fruits. It originated in China and is a self-pollinating diploid. Amateur breeders would face stiff competition from professional breeders.
Pseudo-cereals
The grain amaranths, mainly Quinoa (Chenopodium quinoa), and buckwheat (Fagopyrum spp), are cereal-like grains that are not of grass origin and are often called pseudo-cereals.
Pseudomonas solanacearum
A tropical and subtropical, bacterial plant pathogen with an extraordinarily wide host range.
The most serious diseases caused by it are bacterial wilt of potatoes and tomatoes, Granville wilt of tobacco, and Moko disease of bananas. It is a pathogen of minor importance on a wide range of other crops.
No vertical resistances are known and all resistance breeding must be for horizontal resistance.
Pseudostem
A false stem. Bananas have pseudostems which look like tree trunks but are not. Each banana stem consists of layers of leaf sheaths, with the flower peduncle growing up through the centre and emerging at the centre of the crown.
Pseudostuga menziesii
Douglas fir. An important softwood for plantation forests in Northwest America. Not recommended for amateur breeders.
Psidium guajava
Guava. This fruit is widely grown throughout the tropics and offers scope to amateur breeders, mainly by selecting within existing populations, which are very variable.
Psophocarpus tetragonolobus
The winged bean; also known as asparagus pea, four-angled bean, Manila bean, and princess pea.
Pteridophytes
The ferns.
Public domain
Free of copyright. Intellectual property that is in the public domain may be utilised by anyone for any purpose without restriction.
Puccinia coronata
Rust of oats. It seems that all breeding for resistance to this disease has involved vertical resistance. However some interesting experiments have been undertaken with multilines. There is much scope for work with horizontal resistance.
Puccinia erianthi
Sugarcane rust which is of interest because no vertical resistance occurs against it (although a few cases have been falsely reported). This is because sugarcane is derived from a continuous wild pathosystem.
The disease has occasionally been damaging when it appeared in an area of susceptible cane, as happened recently in Cuba. In general, however, the disease is quite unimportant, as it has been completely and permanently controlled with horizontal resistance.
Puccinia graminis tritici
Stem rust of wheat. There are three rusts of wheat, the others being yellow rust (Puccinia striiformis) and leaf rust (Puccinia recondita). Stem rust has probably attracted more research than any other plant disease and, unfortunately, virtually all of it has been associated with vertical resistance.
There is now great scope for work with horizontal resistance, and this is within the capacity for amateur breeders who are willing to tackle some of the more technical aspects of plant breeding.
Stem rust is a heteroecious parasite, and its winter host is the barberry (Berberis spp). It has long been known that wheat that was growing near barberry bushes was more quickly and more severely diseased with rust.
In Britain, it proved possible to eradicate all the barberries and stem rust is no longer a serious problem. However, the eradication of barberry in larger areas, such as North America, is impractical.
Puccinia hordei
Rust of barley. It seems that all breeding for resistance to this disease has involved vertical resistance and there is much scope for work with horizontal resistance.
Puccinia polysora
The tropical rust of maize which was accidentally taken to Africa some four centuries after maize itself. This was consequently a re-encounter disease and, at low altitudes near the equator, it was extremely damaging.
Attempts to breed for vertical resistance proved futile and, in the course of some 10-15 maize generations (i.e., 5-7 years) the disease declined to unimportance, as a result of a natural accumulation of horizontal resistance.
This phenomenon has been largely ignored by most plant pathologists but it is, in fact, one of the most important plant pathological events of the twentieth century, as it has taught us exactly how to breed for horizontal resistance.
Puccinia purpurea
Rust of sorghum. This is generally an unimportant disease because sorghum is open-pollinated, and it responds to selection pressures for horizontal resistance during cultivation.
Puccinia recondita
Leaf rust of wheat and rye. On rye, this disease is unimportant because rye is open-pollinated and, because it responds to selection pressures during cultivation, it has adequate horizontal resistance.
On wheat, however, it is an important disease, mainly because all breeding during the twentieth century has involved vertical resistance. Amateur breeders working with wheat should definitely aim at horizontal resistance to this disease.
Puccinia sorghi
The common rust of maize. Unlike Puccinia polysora, which has epidemiological competence only in the lowland tropics, the common rust occurs wherever maize is cultivated.
It is rarely important because most maize cultivars have adequate horizontal resistance to it, because they are open-pollinated and can respond to selection pressures during cultivation.
Puccinia striiformis
Yellow or stripe rust of wheat. This rust also attacks barley, and rye. As a general rule, stem rust is a serious disease in the warmer wheat areas, while stripe rust is serious in the cooler areas.
They are rarely both serious in one area. Amateur breeders working with wheat should consequently be concerned about one or the other but not both. And they should pay strict attention to on-site selection.
Pulses
Crops of the family Leguminoseae in which the harvestable product is the seed, otherwise known as grain legumes. Includes various categories of beans, peas, lentils, and grams.
Pumpkin
See: Cucurbita pepo.
Punica granatum
The pomegranite, a native of Iran but well known to the ancient Romans.
Pure line
A cultivar of a seed-propagated, inbreeding species in which all the individuals are effectively identical and are almost homozygous. A pure line thus ‘breeds true to type’. It is produced by self-pollinating the best heterozygous individual in a mixed breeding population for several generations.
In each generation, the progeny show a reduced variability, and the process is repeated 4-6 times until no further variability is apparent.
See also: Single seed descent.
Pyrenophora
This is the Latin name of the perfect (i.e., sexual) stage of many species of Helminthosporium.
Pyrethrins
Natural insecticides extracted from the flowers of Chrysanthemum cinerariifolium, which is a native of Dalmatia. This plant is now cultivated in a number of countries and the extracted pyrethrins are used mainly in household aerosols.
This insecticide is completely non-toxic to mammals, and it leaves no toxic residues. It also has a very rapid ‘knockdown’ effect.
It has been used for centuries by local people in Dalmatia, who put dried pyrethrum flowers in their bedding to control fleas and bed bugs, and no resistance has been known to develop to it in any species of insect. It is thus a stable insecticide.
At present, this very safe insecticide is too expensive to be used routinely on food crops. However, with improved yields of both flowers and pyrethrin content, and the development of a mechanical system of harvesting, the price might be brought down sufficiently to allow its use on food crops.
If the price of natural pyrethrins could be brought down sufficiently, by high yields and high pyrethrin content, and mechanical harvesting, the market for crop protection is virtually unlimited. This insecticide can be widely used both by organic farmers, and the producers of those fruits and vegetables in which the actual sprayed surface is eaten by people.
Pyrethrum is also a potential replacement crop for tobacco farmers whose crops are in lower demand.
Natural pyrethrins have several advantages over synthetic insecticides. First, as already mentioned, they are stable; they do not break down to new races of the insect. Second, their mammalian toxicity is extremely low and this is one of the safest insecticides available. Third, they break down to carbon dioxide and water after twenty-four hours of exposure to sunlight. Fourth, they leave no residues whatever. Finally, they are a very powerful insecticide.
Their chief disadvantage is their cost, and the overall objective of amateur breeders should be cost reductions sufficient to make natural pyrethrins competitive with synthetic insecticides for crop protection. A second disadvantage is that pyrethrum paralyses insects, but they are likely to recover, unless the insecticide is formulated with a synergist such as sesame oil.
In order to make pyrethrum a commercial success, a simple machine for mechanical harvesting will have to be developed. The harvested flowers must be carefully dried without overheating, and then sold to an extraction factory.
Pyrethroids
Synthetic pyrethrins. Unlike natural pyrethrins, pyrethroids are unstable and are liable to break down to new strains of an insect pest.
Pyrethrum
See: Chrysanthemum cineriifolium.
Pyricularia oryzae
Blast disease of rice. This is probably the most damaging disease of rice, causing a seedling blight, leaf blight, and neck rot. There is an urgent need for work on horizontal resistance, but this is difficult as vertical resistance complicate the situation, particularly as some of them are apparently quantitative vertical resistances.
Pyrus communis
The pear, which is one of the pome fruits. Pears and apples are antique fruits, and both Homer and Pliny the Elder recorded the names of ancient cultivars of each. Pears are easy to breed but amateur breeders should be aware of the fairly extreme difficulties associated with establishing a new cultivar.
Pythium
A genus of the downy mildews (Peronosporales). Some species cause stem and root rots, as well as damping-off of seedlings.