Glossary: Pa-Ph

Paddy
See: Oryza sativa.
Palmae
The palm family. All palms are tropical or sub-tropical. The most important crop palms are coconut, oil palm, and date palm. Palms are difficult to breed and, with exception of the coconut in certain circumstances, they are not recommended for amateur breeders.
Panama disease of banana
See: Musa sapientum.
Panicum maximum
Guinea grass. A tropical fodder grass native to East Africa.
Panicum miliaceum
This is the common millet. It is a cereal of ancient domestication, originating in eastern Asia. It was cultivated by the Chinese before the introduction of rice, by the prehistoric Lake Dwellers, and by the ancient Greeks and Romans.
Its wild progenitors are extinct. It has a short growing season (60-90 days) and it is drought-resistant. Some cross-pollination occurs, and this crop is suitable for amateur breeders.
Papaver somniferum
This is the opium poppy. Its cultivation is illegal in most countries.
Papaya
See: Carica papaya.
Paprika
See: Capsicum spp.
Parameter
A measurable or quantifiable characteristic which is often definitive.
Parasite
Any organism in which the individual spends a major part of its life cycle inhabiting, and obtaining nutrients from, a single host individual. The term may be applied to a species, a population, or an individual. Plant parasites include insects, mites, nematodes, Angiosperms, fungi, bacteria, phytoplasmas, viruses, and viroids.
Parasite gradients
The distribution of a parasite within a crop is usually uneven, with a gradual variation from a high density to a low density. This variation is known as a parasite gradient. Unless it is taken into account during recurrent mass selection, it can cause serious errors in the assessing of the level of horizontal resistance in individual selections. This problem can be overcome by the use of grid screening. See also: Patchy distribution.
Parasite identification
See: CABI.
Parasite interference
When the levels of parasitism are being measured in small test plots, the movement of parasites from one plot to another can cause measurement errors of several hundred-fold. This phenomenon is called parasite interference, or interplot interference.
Because it involves allo-infections, the effects of unmatched vertical resistance are greatly enhanced in small plots, in comparison with the effects of horizontal resistance which are greatly diminished.
More than any other, this phenomenon has misled crop scientists over the relative values of the two kinds of resistance. Parasite interference has also caused serious errors in field trials that have led to unnecessarily high rates of pesticide use.
Parasitic ability
The ability of a parasite to cause parasitism, and to inhabit and obtain nutrients from a living host, in spite of the resistance of that host. There are two kinds of parasitic ability called vertical and horizontal parasitic ability respectively.
Parasitism
The process in which a parasite inhabits, and obtains nutrients from, its host.
Parenchyma
Plant tissue consisting of unspecialised cells, usually with air spaces between them. Many plant organs, such as the inside of many stems (e.g., pith), are made up mainly of parenchyma.
Paris Green
A very nasty pesticide containing copper and arsenic, which was widely used until replaced with DDT.
Parsley
See: Petroselinum crispum.
Parsnips
See: Pastinaca sativa.
Parthenocarpic
The production of fruit without pollination, as with bananas.
Parthenogenetic
The development of an individual from a gamete without fertilisation.
Partial resistance
This term, meaning ‘incomplete’, was originally used to describe horizontal resistance. Unfortunately, ‘partial’ also means biased, and the term would better describe verticalresistance. Horizontal resistance would then be impartial resistance. These terms are best avoided.
Pascal’s triangle
A mathematical device for calculating the possibilities of ‘either‑or’ events, such as ‘boy or girl’ in single-child births, or the presence or absence of vertical genes.
For example, with three births, there are one possibility of three boys, three possibilities of two boys and a girl (i.e., ‘boy-boy-girl’, ‘boy-girl-boy’, and ‘girl-boy-boy’), three possibilities of two girls and a boy, and one possibility of three girls.
These possibilities are called the binomial coefficients. They are important for calculating the numbers of biochemical locks and keys that there will be in the n/2 model of the gene-for-gene relationship and the vertical subsystem.
Paspalum spp.
Tropical fodder grasses from South America.
Passiflora edulis
The passion flower, which is cultivated for its fruit that are used to add flavour to fruit salads and drinks. This crop is open-pollinated and offers scope for amateur breeders who should aim at horizontal resistance to locally important parasites, increased fruit size and juice content, and yield.
Passion fruit
See: Passiflora edulis.
Pasta wheat
See: Triticum durum.
Pasteurisation
Named after Louis Pasteur, this is a technique of heating wine, milk, food, or soil to about 80°C in order to destroy harmful micro-organisms. This level of heating does not lead to a complete sterilisation, for which a temperature of about 120°C is required. Pasteurised soil can be used as soon as it is cool, whereas sterilised soil usually needs about three weeks to recover its beneficial micro-biological activity.
Pastinaca sativa
The parsnip, which is an open-pollinated, biennial member of the Umbelliferae, and is cultivated for it large, yellow, tapering root, which is eaten as a vegetable. Amenable to breeding by amateurs.
Pasture
Land covered with fodder grasses and legumes, and used for grazing farm animals such as cattle and sheep.
Pasture grasses
See: Fodder grasses.
Pasture legumes
See: Fodder legumes.
Patchy distribution
The converse of a uniform distribution.
With a patchy distribution of parasites, some individuals in the host population may be heavily parasitised, while others may escape entirely. Patchy distributions occur typically with soil-borne parasites, and gregarious insects (e.g., the leaf hoppers of maize streak virus).
A patchy distribution is a nuisance when screening plants for horizontal resistance because it produces escapes from parasitism, and these provide a false indication of resistance.
A patchy distribution can be overcome during screening for resistance by using a grid screening. That is, the screening population is divided into a grid of perhaps one-metre squares, and the best individual in each square is kept, provided parasites are present in that square.
A patchy distribution can also occur over time. For example, swarms of the desert locust occur only once in 10-15 years. This period is long enough for a population of an annual host to lose most of its resistance to these insects.
A patchy distribution is an evolutionary survival advantage for the parasite, because it prevents the host from accumulating resistance. See also: Frequency, injury.
Pathodeme
A sub-population of a host that is defined by a stated characteristic of resistance. For example, many different cultivars may possess vertical resistance gene 2, but no others, even though they differ in many other respects. Horizontal pathodemes differ in their levels of horizontal resistance.
Pathogen
A category of plant parasite that causes disease and is studied by plant pathologists. The term includes fungi, bacteria, phytoplasmas, viruses, and viroids. When nematodes are studied by plant pathologists, they too are called pathogens.
Pathogenic
An organism, called the pathogen, is described as pathogenic when it is able to induce disease in another organism, called the host.
Pathologic race
An obsolete term meaning vertical pathotype. See also: Physiologic race.
Pathology
The study of diseases. Plant diseases are studied by plant pathologists, sometimes called phytopathologists.
Pathosystem
A subsystem of an ecosystem, and one that is defined by parasitism. A pathosystem normally involves the interaction between a population of one species of host, and a population of one species of parasite.
In a plant pathosystem, the host species is a plant. The parasite is any species which spends a major part of its life cycle inhabiting, and drawing nutrients from, one host individual.
The parasite may thus be an insect, mite, nematode, parasitic Angiosperm, fungus, bacterium, phytoplasma, virus, or viroid.
However, herbivores which graze populations of plants are usually regarded as belonging to the wider concept of the ecosystem.
See also: Continuous pathosystem, crop pathosystem, Discontinuous pathosystem, Heteroecious, and wild pathosystem.
Pathotype
A sub-population of a parasite that is defined by a stated characteristic of parasitic ability. Thus vertical pathotype and horizontal pathotype. See also: Pathodeme.
Pattern
The basic unit of a system. A word is a pattern of letters; a molecule is a pattern of atoms, a wall is a pattern of bricks, and so on. A system is a pattern of patterns, and each pattern of patterns is called a systems level.
Pea
See: Pisum sativum.
Pea, pigeon
See: Cajanus cajan.
Pea, winged
See: Psophocarpus tetragonobolus.
Peach
See: Prunus persica.
Peanut
See: Arachis hypogea.
Pear
See: Pyrus communis.
Pearl millet
See: Pennisetum typhoides.
Peat moss
Dead moss of the genus Sphagnum, commonly used in potting mixtures.
Pecan
See: Carya pecan.
Pedigree breeding
The breeding method of the Mendelians, also known as the gene-transfer breeding technique, which usually involves the transfer of a single gene from a wild plant to a cultivar.
In practice, this gene usually controls resistance to a parasite, and it confers vertical resistance. The wild plant and the cultivar are hybridised, and the progeny segregate into those which carry the gene and those which do not.
The progeny are mostly halfway between the two parents in their yield and crop qualities. The best of the individuals which are carrying the gene for resistance is back-crossed to the original cultivar, with further segregation for resistance.
The back-crossing is repeated until the progeny have all the desirable qualities of the original cultivar, as well as the gene for resistance from the wild plant.
See also: population breeding, recurrent mass selection.
Pedology
The science of soils, including their classification, formation, structure, and composition.
Peduncle
The stalk of an inflorescence.
Penicillin
The antibiotic obtained from the fungus Penicillium. This antibiotic is an example of an unstable protection mechanism.
Pennisetum clandestinum
Kikuyu grass. A fodder grass from Kenya that is now widespread throughout the tropics.
Pennisetum purpureum
Elephant grass, also known as Napier grass. This grass is so-called because it grows tall enough to hide an African elephant. It occurs wild in the general area of Uganda.
It is a highly productive fodder, and it provides an excellent mulch. It is usually propagated by stem cuttings of 3-4 nodes. Seed is produced abundantly but is difficult to collect. An appropriate target for amateur breeders.
Pennisetum typhoides
Pearl millet, also known as bulrush millet, spiked millet, and cattail millet, and as bajra in India.
This is an ancient crop and the most important of all the millets. It originated in Africa but was taken to India at an early date. Its value lies in its tolerance of poor soils and low rainfall.
The plant is open-pollinated and exhibits extreme variation. An attempt to produce hybrid varieties in India was highly successful until the breakdown of vertical resistance to downy mildew (Sclerospora graminicola).
A suitable crop for amateur breeders who should aim at horizontal resistance and purely local requirements.
Pentaploid
A plant with five sets of chromosomes. Pentaploids are usually sterile.
Pepper
For black pepper (also green and red peppercorns) see Piper nigrum. For red peppers (also sweet, green, Jalapeno, etc., peppers) see Capsicum spp.
Perennial
A plant that lives for several years, and usually flowers every year. See also: Annual, biennial.
Perithecium
A sexually produced fungal body of an Ascomycete that contains one or more asci.
Permeability
The ease with which a substance will cross a membrane. For example, polyethylene film is permeable to oxygen and carbon dioxide, but impermeable to water vapour. For this reason, it makes an excellent protective cover for delicate cuttings, etc.
Peronospora destructor
Downy mildew of onion.
Peronospora nicotianae
Blue mould of tobacco.
Peronospora parasitica
Downy mildew of Brassicas.
Peronospora schachtii
Downy mildew of sugar beet.
Peronospora viticola
Downy mildew of grapes. This fungus originated in North America and was accidentally introduced to Europe during the import of rootstocks resistant to Phylloxera. Bordeaux mixture was discovered in connection with this disease.
Peronosporales
An Order of the Phycomycetes which includes the downy mildews, including potato blight.
Persea americana
The avocado pear. A highly nutritious salad fruit containing up to 30% oil that has a composition similar to olive oil. The avocado originated in Central America (Mexico-Guatemala) but is now grown in most tropical and subtropical countries.
The seed is highly heterozygous and selected clones must be propagated vegetatively as grafts on seedling stocks. A good project for breeders associations which should aim at selection within local populations.
Persistent organic pollutants (POPs)
Hazardous synthetic chemicals that cause birth defects and fatalities among both humans and wild species. Many POPs are crop protection chemicals. See also: Dirty dozen.
Person-Habgood differential interaction
This differential interaction is the definitive characteristic of the gene-for-gene relationship. It was first described by Robinson (Plant Pathosystems, 1976, Springer-Verlag, Heidelberg, New York, & London) and the details are not necessary for amateur plant breeding.
Pest
In its widest sense, any organism that interferes with the activities of humankind. In the sense of pest control, or pest management, the term includes all agricultural, medical, veterinary, industrial, and domestic pests. However, our use of the term pesticide is more specific (see below).
Pesticide
Any substance that kills pests. On this website, the term pesticide refers exclusively to substances that kill crop parasites. Competitors, such as weeds, and the substances that kill them (i.e., weed killers, herbicides), are not included.
Insecticides, fungicides, bacteriocides, miticides, and nematocides are all pesticides that kill crop parasites. They may be applied as liquids, dusts, vapours, or pellets, and they may be applied to the crop itself, to the soil, or to the seed.
While most pesticides are manufactured synthetically, there are also some natural ones, such as rotenone and pyrethrin. However, in organic agriculture even their use is discouraged. See also: IPM.
Pesticide over-kill
This term describes any application of a pesticide that involves a greater dosage, or a higher frequency of application, than is necessary for a control of the pest in question.
Pesticide overload
This term is usually used to describe the long-term effects of an excessive use of pesticides.
Pesticide pollution
The pollution of food, fodder, fields, and the environment with pesticides
Petal
The components of a corolla of a flower. Each petal is a modified bract and is delicate. Petals are usually brightly coloured to attract pollinating insects, and even birds. Wind-pollinated plants do not need such attractants, and usually have inconspicuous flowers (e.g., grasses).
Petroselinum crispum
Parsley, a member of the family Umbelliferae. An ancient crop from the Mediterranean, known to the classical Greeks and Romans.
The leaves are rich in Vitamin C, and are used as a flavouring in soups and salads. Hybridisation with celery has produced new variants of both species.
Petiole
The stalk that joins a leaf to a stem.
pH
A system of measuring acidity on a scale of 1-14, with neutrality at pH7.0, with increasing acidity below, and increasing alkalinity above pH7.0.
The scale is logarithmic. This means that, say, pH4 is ten times more acid than pH5, which is ten times more acid than pH6, and so on.
Phaseolous acutifolius
The Tepary bean is of very ancient domestication in Mexico and was later replaced to a large extent by Phaseolus vulgaris. Of limited interest to amateur breeders.
Phaseolus aconitifolius
Moth, or mat bean. This is a very drought-resistant, self-pollinated grain legume that requires hot tropical temperatures. The green pods may be eaten as a vegetable, the seeds are eaten cooked, and the plant makes a useful forage crop. Of local interest to amateur breeders.
Phaseolus angularis
The Adzuki bean, probably a native of Japan, has been established since antiquity in China. The plants are self-fertile when bagged but cross-pollination is frequent. This makes it an easy crop for amateur breeders.
Phaseolus aureus
The green or golden gram, Mung bean. A popular bean in China and India because it causes little flatulence. In China it is used for making ‘bean sprouts’. This bean is self-pollinating and amateur breeders should commence by selection within existing populations.
This is a domesticated crop whose wild progenitors are extinct.
Phaseolus calcaratus
The rice bean. This is a self-pollinating Old World tropical bean that is eaten with rice, or in place of rice, in the Far East.
Phaseolus coccineus
The scarlet runner bean. A plant of the humid tropical uplands which originated in Central America. The young green pods are eaten sliced and boiled, and the dried seeds can also be cooked and eaten. Of local interest.
Phaseolus lunatus
The Lima or Sieva bean, also known as the butter bean. Named after the capital of Peru, archaeological remains of this bean have been found there dating from 6000BC. However, it is thought that this bean probably originated in the Guatemala area of Central America and was taken to South America by early travellers.
The green shelled beans are eaten as a vegetable, and the dried beans are also cooked and eaten. The plant is self-pollinating, but some natural cross-pollination occurs. Of interest to amateur breeders working with horizontal resistance.
Phaseolus mungo
Black gram. This is a highly prized pulse in India. The flowers are self-pollinating and cross-pollination is very rare. Of local interest to amateur breeders who should commence by selecting within existing populations.
The wild progenitors of this crop are extinct.
Phaseolus vulgaris
The haricot bean, also known as the French bean, common bean, kidney bean, salad bean, runner bean, snap bean, string bean, and frijoles.
The species originated in Mexico and it shows great variation, with beans ranging in size from the small ‘pea beans’ to the large ‘kidney beans’, and with colours ranging from white through yellow, pink, brown, and red to black. The so-called ‘pinto’ beans are speckled brown. Both determinate and indeterminate plants occur.
This is the most widely grown species of Phaseolus and it is the most important single source of vegetable protein in the human diet. It offers great scope for amateur breeders working with horizontal resistance. In non-industrial countries, selection within existing landraces should be the first step.
The plant is self-pollinated and, when hand-pollinating, some manual dexterity is required in emasculating the flowers. Late selection should be used.
An alternative approach is to use black beans as a genetic marker, using recurrent mass selection. If the breeding involves white beans, a mixture of white bean cultivars is planted in alternate rows with mixtures of black bean cultivar. About 1-5% cross-pollination will occur.
The white beans are harvested and grown as a crop whose harvest is separated into white and black beans. The black beans, which are the product of cross-pollination, are then grown and harvested, and the white beans of that harvest are kept for late selection and eventual use as parents in the second breeding cycle. The black beans of that harvest are kept for use as parents in the second breeding cycle.
Virtually all bean breeding during the twentieth century has involved Pedigree breeding and vertical resistance. The exception is a horizontal resistance breeding program in Mexico, which has revealed both the great potential for the development of horizontal resistance, with an average 18% genetic advance in the early breeding cycles, and the feasibility of amateur breeding (See Garcia Espinosa, et al., Chapter 25, in Broadening the Genetic Base of Crop Production, Eds H.D.Cooper, C.Spillane, T.Hodgkin, ISBN 0-85199-411-3, CABI Publishing, 2001).
The main diseases that will require use of the one-pathotype technique in order to inactivate all vertical resistances during screening are anthracnose (Colletotrichum lindemuthianum), and rust (Uromyces phaseoli).
Other major diseases are bacterial blight (Xanthomonas campestris f.sp. phaseoli), halo blight (Pseudomonas phaseolicola), and bean mosaic virus.
The insect pests include many aphids, white flies, leaf hoppers, and beetles. The Mexican bean beetle (Epilachna varivesta) is serious in Central America and the USA. The bean fly (Melanagromyza phaseoli) is important in Africa, Asia, and Australia.
Phenotype
The observable properties of an organism produced by the interaction of its genotype and the environment.
For example, recessive characters are part of the genotype, but they are not expressed phenotypically in the heterozygous state.
And the Person-Habgood differential interaction is a phenotypic demonstration of a gene-for-gene relationship, but a genotypic demonstration would require inheritance studies in both the host and the parasite.
Pheromone
A sex attractant chemical. These chemicals can now be synthesised, and used in insect traps to prevent sexual fertilisation, thus providing a control of some species of obnoxious insects.
Phleum pratense
Timothy grass. One of the relatively few fodder grasses sown for grazing.
Phloem
The food-conducting tissues of a plant, as opposed to the xylem, which conducts water. In general, the phloem carries carbohydrates downwards from the leaves, while the xylem carries water and minerals upwards from the roots.
Phoenix dactylifera
The date palm, which has a sub-tropical, semi-arid origin in the Middle East. This is possibly the oldest plant domestication in the world.
The plant is dioecious and breeding is exceptionally difficult. Propagation by seeds is a waste of time, because of the loss of fruit quality, and vegetative propagation with basal suckers is essential.
The quality of the date fruit is affected by metaxenia. A new-encounter killer disease, ‘Bayoud disease’ (Fusarium oxysporum f.sp.albidinis) is spreading inexorably from Morocco eastwards.
Breeding is exceptionally difficult, but local people should be on the lookout for high-quality seedling palms that are resistant to Bayoud disease.
Phosphate
Compounds of phosphorus, phosphates are an essential plant nutrient. Organic phosphate fertilisers are usually rock phosphate or bone meal. The artificial phosphate fertilisers have had their soluble phosphate content increased by industrial means. Phosphate deficiency symptoms include poor growth, and leaves with a bluish-green to purple coloration.
Photoperiod-sensitive
Photoperiod-sensitive plants depend on a particular day-length to initiate flowering or some other stage of development.
For example, short-day potatoes will initiate tuber production at any time of year in the tropics, where there is an approximately twelve-hour day throughout the year.
But when taken to temperate regions, these potatoes will start tuber formation only as the September equinox approaches, and the delayed crop will then be killed by frost before it is mature. This explains why potatoes could not be grown in Europe until day-neutral (or photoperiod-insensitive) cultivars were found.
Photosynthesis
All living organisms can be divided into three groups called producers, reducers, and consumers. Producers are the only organisms that can convert solar energy into the sugars and starches (carbohydrates) on which all life is based. This process is called photosynthesis and it converts solar energy, water, and carbon dioxide into carbohydrates, giving off oxygen as a waste product.
It occurs mainly in green plants that contain chlorophyll, but it can also occur in more primitive organisms such as the cyano-bacteria. Photosynthesising plants are thus at the bottom of the food chain and all life depends on them. They are also responsible for maintaining the world’s supply of oxygen.
Phycomycete
A fungus characterised by the absence of cross-walls in the mycelium and, when sexual reproduction occurs, it produces an oospore, and it does not involve an ascus or a basidium.
The most important plant pathogenic phycomycetes are the Peronosporales (downy mildews).
Phyllactinia
A genus of the powdery mildews in which the cleistothecia contain many asci, and the rigid appendages lift the cleistothecia from the leaf so that they fall to the ground.
Phyllotaxis
The arrangement of leaves on a stem. This arrangement may involve opposite pairs, whorls, alternates, etc.
Phylloxera vitifoliae
Now renamed Daktulosphaira vitifoliae, this is the famous Phylloxera root-inhabiting aphid that was taken from America to France, in the mid-nineteenth century, and threatened the European wine industry with ruin.
The problem was solved by grafting the classic wine grapes to wild American grape root-stocks that were resistant. However, importing these rootstocks introduced the American downy mildew of grapes, threatening ruin for the second time.
Phylogeny
The evolutionary relationship, based on evolutionary history, between any two taxonomic levels.
Physiologic disorder
A plant disease that is non-parasitic. Both nutrient deficiencies and toxicities, as well as frost damage, etc., are physiologic disorders.
Physiological race
An obsolete term meaning vertical pathotype. See also: Pathologic race.
Physiological source/sink
In a plant, a physiological source is tissue that generates nutrients. Thus the leaves generate carbohydrates by photosynthesis, and the roots generate water and minerals from the soil.
A physiological sink is tissue that assimilates those nutrients, usually taking precedence over other tissues. Thus the actively growing shoots, the flowers, and, above all, the seeds, are physiological sinks which grow at the expense of other parts of the plant.
Phytoalexin
A fungus-inhibiting substance produced in a plant.
Phytopathogenic
A term meaning ‘pathogenic to plants’.
Phytopathology
See: Plant pathology.
Phytophthora colocasiae
Downy mildew of taro, also known as taro leaf blight. Developing horizontal resistance to this disease is a useful and feasible project for amateur breeders.
Phytophthora infestans
This is the fungus that causes potato blight, which is historically the first and most important plant disease, and was responsible for the ‘Hungry Forties’ of the nineteenth century, and the great Irish famine. It was this disease that initiated the science of plant pathology.
Phytophthora infestans has two mating types, known as A1 and A2. Each mating type is hermaphrodite but self-sterile. This means that oospores can be formed only if both mating types are present, as happens in the centre of origin in Mexico.
When blight was accidentally taken to New York, and then to Europe, in the mid-nineteenth century, it was taken as A1 only and, for 150 years, it could reproduce asexually only. This meant that the fungus could overwinter only in potato tubers.
The initial inoculum was small, and the epidemic developed slowly. It was known as ‘late blight’ for this reason.
In the late twentieth century, A2 was taken to Europe, and it was spread all over the northern hemisphere in certified seed potatoes. This means that functional oospores are now being formed in most of the potato-growing areas of the world.
The initial inoculum is now be much greater, and late blight is becoming early blight. The variability of the fungus is likely to increase considerably, and unstable resistances (i.e., vertical resistances) and unstable fungicides will break down much more quickly.
Higher levels of horizontal resistance are now required to provide a full control of the disease.
Potatoes are an excellent crop for amateur breeders, and there is an acute need for horizontal resistance to blight and other pests and diseases.
Phytoplasma
A mycoplasma that is parasitic in plants. A mycoplasma is a micro-organism smaller than a bacterium and without a cell wall.
Phytosanitation
The national and international regulations that control the movement of plant propagating material around the world. The purpose of these regulations is to prevent the spread of dangerous crop parasites to those parts of the world that are still free of them.
Phytosanitation can be international, regional, or local. International phytosanitation is usually effective because of border controls. Regional phytosanitation within a country is usually ineffectual because of the lack of border controls. Local phytosanitation involves a single farm, and it can be very effective against soil-borne and seed-borne parasites.
Phytosociology
The study of plant communities. There is a loose parallel between animal and plant domestication. Just as herders found that social animals were the easiest to domesticate, so agriculturalists found that ‘social’ plants (i.e., those that tend to grow in pure stands), such as the wild progenitors of the cereals, were the easiest to domesticate.
Phytotoxin
This term means a substance that is toxic to plants; the adjective is phytotoxic.