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Glossary: M

Macadamia nut
See: Macadamia spp.
Macadamia spp.
There are three cultivated species of macadamia nuts. They all originate in Northern Australia but are now cultivated in Hawaii and California also. The species are of doubtful taxonomic rank, and they interbreed freely to produce fertile hybrids. Some scope for local amateur breeders.
Mace
See: Myristica fragrans.
Macro-evolution
Evolution above the species level, as opposed to micro-evolution, which is evolution below the species level. Macro-evolution operates during periods of geological time, it produces changes that are new, it produces an increase in complexity, it is irreversible, it produces new species, and it produces new genetic code. Micro-evolution is the exact converse in all of these attributes.
It is now thought that the mechanism of macro-evolution is natural selection operating on emergent properties, at all systems levels.
Macroscopic
Visible to the naked eye, c.f. microscopic.
Magnesium
An essential plant nutrient. Magnesium is a mobile element and, consequently, the older leaves show symptoms first. Deficiency is easily recognised by a necrotic area between the main veins of the older leaves. It can be cured by an application magnesium sulphate (bath salts).
Maize
See: Zea mais.
Maize streak virus
This African virus is transmitted by leaf hoppers that are gregarious. As a consequence, the spread of the virus within a crop is limited and, only a low percentage of plants are diseased. This low frequency of disease exerts no selection pressure for horizontal resistance. The few diseased plants are so susceptible that they are usually killed, and the population as a whole remains susceptible. Occasionally, a much higher proportion of plants become infected, and the disease is then very destructive.
This disease has two important lessons for breeding for horizontal resistance. First, it is essential to select plants that have few symptoms but that are known to be infected, otherwise chance escapes will be chosen without any genetic advance in resistance. Second, inoculation is desirable to ensure as uniform a distribution of parasitism as possible. With this virus, disturbing the leaf hoppers every day, so that they eventually inhabit every plant, achieves such a uniformity.
Major staple
A major staple is a crop that has a high yield per person-hour, and per unit area; that is reliable from season to season; that produces a food that can be stored; and a food that is easily cooked. A major staple liberates a significant proportion of the population from food production, and they become available for other specialised activities, such as arts and crafts, medicine, architecture, and all those attributes of a sophisticated civilisation, which can be defined as the growth of cities. There are only three major staples in the world. These are wheat, rice, and maize. Every ancient and modern civilisation was based on one of these three crops, and any area or society that lacked them failed to produce a major civilisation. See also: Minor staple.
Malarial mosquitoes
These mosquitoes provide good examples of unstable insecticides, such as DDT.
Male gametocide
Any substance that kills the male reproductive cells (i.e., pollen, or pollen mother cells) of a plant, rendering it male-sterile. Male gametocides can be used to convert an inbreeder (e.g. wheat) into an outbreeder, for purposes of recurrent mass selection. Treated plants become the female parents, and untreated plants become the male parents.
There is also considerable interest in using male gametocides for the commercial production of seed of hybrid varieties but, so far, the available substances are not efficient enough.
Male sterility
A male sterile plant is one that has fertile ovules but sterile anthers and/or pollen. Male sterility can be induced with a male gametocide, or it may be genetically controlled. Male sterility can be useful in plant breeding by forcing inbreeding plants to cross-pollinate.
Malus pumila
The apple. Apples are members of the rose family (Rosaceae) and are of very ancient origin in Eurasia. Pliny the Elder (23-79AD), listed twenty two varieties of apple known to the ancient Romans.
Today, apples are probably the most popular fruit, with oranges or, perhaps, bananas being second. The apple is self-incompatible and bees are necessary for pollination. Hand pollination is easy, but the main difficulty in breeding is the very large number of seedlings that have to be screened in order to produce one new cultivar.
Cultivated apples are normally grafted on to seedling rootstocks. An old apple orchard can be useful for testing promising scions in a breeding program, because an old tree can carry some fifty or more grafts.
The story of Johnny Appleseed suggests a technique for amateur plant breeders. His real name was John Chapman, and he travelled westward, in the early 1800s, into Ohio, Indiana, and Illinois. As he went, he planted hundreds of apple seeds that he had obtained from cider presses in Pennsylvania. Most of his seedlings would have produced aberrant types, but some were very useful. In any event, these early settlers wanted apples mainly for making applejack, as this was the only source of alcohol they had. His activities helped to make the Ohio Valley a major apple producing area, and North America soon had a greater variability in apples than Europe. He was ultimately responsible for the phrase “As American as apple pie”.
In Canada (and elsewhere, no doubt) passengers eating an apple on a train would often throw the core out of the window. Many of these train tracks are now abandoned, and have been converted into hiking trails. Numerous apple trees grown from those unwanted apple cores line these tracks and they merit investigation as a possibly useful, and readily available, population for selection purposes. It should be remembered that each core would normally produce several trees that are genetically very different from each other, even though they seem to grow as one tree that is apparently branched near the ground.
Mammalian toxicity
Before being released to growers, new crop protection chemicals have to be tested for their mammalian toxicity. This is usually measured in milligrams of the chemical, per kilogram of mammalian body weight, required to kill 50% of the test population. This lethal dose is called the LD50. These tests, of course, are made on laboratory animals, usually rats or mice.
Manchineel
See: Hippomane manchinella.
Mandarin orange
See: Citrus reticulata.
Manganese
A trace element nutrient of plants, manganese is a component of many enzymes. A specialist should be consulted if manganese deficiency is suspected.
Mangifera indica
The mango. The most popular of the tropical fruits, mangoes are to tropical region peoples what apples are to temperate region peoples. Mangoes vary widely in their fruit quality and the best are probably the finest fruit of all. Unfortunately, the best do not reach temperate markets, and most people in the industrial countries have not experienced a really good mango. Mango is a member of the family Anacardiaceae, which also includes the cashew nut (Anacardium occidentale).
Many mango fruits have two or more embryos, of which one or more is a zygote and does not ‘breed true’, while the other produces a nucellar seedling, that does ‘breed true’. Many mango trees that are derived from a casually discarded seed have two or more trunks, joined at the base by anastomosis, but differing genetically because of the two types of embryo. Pollination is usually by insects and is essential for fruit set, even when all the embryos are apomictic. Self-pollination is possible.
The best approach for amateur breeders is selection within local populations.
Mango
See: Mangifera indica.
Mangolds
See: Beta vulgaris.
Mangosteen
See: Garcinia mangostana.
Manioc
See: Manihot esculenta.
Manihot esculenta
Cassava, or manioc. This important tropical food crop originated in Central and South America and was taken to Africa by the Portuguese at an early date. The edible tubers are divided into sweet and bitter types, the latter containing toxic amounts of hydrocyanic acid, which is liberated when the enzyme linase acts on a glucoside called linamarin. These bitter types are cultivated in areas where wild pigs, baboons, and porcupines are serious pests. The hydrocyanic acid can be eliminated by washing, boiling, or roasting.
Cassava is an important famine reserve in areas where desert locusts are serious. The leaves are also used as a pot herb. High-yielding cassava, producing up to 70 tonnes/hectare, can be cultivated commercially for starch production. The crop is propagated vegetatively, and true seeds are very variable. Its wild progenitors are extinct.
Scientists at IITA, in Nigeria, launched an innovative program in which true seed of cassava was given to school children to grow in the school garden, with a view to doing their own selection work. This was both a valuable education, and a means of farmerparticipation in breeding. This is an example that should be copied with many crops, in many schools, in many countries.
Manila bean
See: Psophocarpus tetragonobolus.
Manila hemp
See: Musa textilis.
Manilkara zapota
Chiclé, a tree native to Central America, with cultivars that are propagated vegetatively. The bark is tapped for latex which is boiled to produce chiclé gum, the basis of chewing gum. However, demand outstrips supply and synthetics are now used. There is need for cultivars suitable for plantations. A possible long-term project for amateur breeders in appropriate areas.
Manure
This word is usually taken to mean organic fertiliser, in the form of excrement of farm animals (farmyard manure), night soil, sewage solids, bone meal, dried blood from abbatoires, or guano.
Manygene characters
See: polygenic characters.
Maranta arundinacea
Arrowroot. A South American crop grown primarily for its high quality starch used for invalid foods and face powder.
Marker gene
A Mendelian gene that is used to identify the progeny of cross-pollination in an inbreeding species of crop.
Marrow
See: Cucurbita pepo.
Mass selection
Often called population breeding, or recurrent mass selection this is the converse of Mendelian or Pedigree breeding. Mass selection requires a population, as large and as genetically diverse as possible, which is screened for the best individuals that are to become the parents of the next screening generation. It is the technique of choice for many-gene characters, and for amateur breeders working with horizontal resistance. The selection criteria can include all aspects of yield, quality, agronomic suitability, and horizontal resistance to all locally important parasites. It is necessary to screen for all the desirable characteristics in each breeding cycle and a holistic approach is recommended.. The assessment of each individual must be relative to the neighbouring individuals and the population as a whole.
Matching
In terms of the gene-for-gene relationship, an allo-infection is described as matching when the vertical parasitism gene(s) of the parasite match the vertical resistance gene(s) of the host (i.e., the biochemical key of the parasite fits the biochemical lock of the host). The vertical resistance then fails to operate and the allo-infection is successful. See also: Non-matching.
Maté
See: Ilex paraguariensis.
Maximum
The highest possible value of a quantitative variable.
Mayetiola destructor
The Hessian fly. A stem borer of wheat. This parasite is interesting in that vertical resistance against it is quantitative. It is thought that the evolutionary function of this resistance is to reduce or even prevent reproduction of the parasite, rather than to reduce the frequency of matching allo-infections. In either event, it appears that the evolutionary function of the gene-for-gene relationship is to stabilise the population explosion of an r‑strategist parasite.
Mean
An alternative term for average. See also: Gaussian curve, Normal distribution, Mode, Skewed distribution.
Mechanisms of resistance
The mechanisms of resistance in plants to their parasites are many and varied. As a general rule, they are of little interest to amateur breeders working with horizontal resistance, who should use the holistic approach. It is a great mistake to breed plants for a single, prominent resistance mechanism, such as hairy leaves that resist certain insects.
Medicago sativa
Alfalfa, known as lucerne in Britain. This is probably the most important of the fodder legumes and it is used for grazing, hay, and silage. Its origins are ancient and are apparently linked to the domestication of the horse. The plant is pollinated by special bees, although some self-pollination does occur. This is one of the relatively rare examples, during the twentieth century, of recurrent mass selection being used by professional breeders to accumulate horizontal resistance.
Meiosis
Reduction division. This is the process in which the two sets of chromosomes in a diploid nucleus separate to form two haploid nuclei that become gametes. See also: Mitosis.
Melampsora lini
Flax rust. This is the disease in which H.H. Flor discovered the gene-for-gene relationship.
Melinis minutiflora
A tropical fodder grass, called molasses grass, and native to Africa.
Meloidogyne spp.
A widespread genus of root feeding nematodes, that do not form cysts and which can be serious pests of crops.
Melon
See: Cucumis melo.
Melon, water
See: Citrullus lanatus.
Melongene
See: Solanum melongena.
Mendel, Gregor
Gregor Johann Mendel (1822-84) is often regarded as the father of genetics. More accurately, he is the originator of single-gene (or Mendelian) genetics, although his work was later used to explain the action of polygenes. This is an example of the time lag in science. Mendel’s work was ignored for thirty-five years. After its recognition, in 1900, its importance was greatly over-emphasised in plant breeding for the next century. Today, single-gene genetics still dominate plant breeding and, of necessity, genetic engineering.
Mendelian
Pertaining to Mendel’s laws of inheritance.
Mendel's laws of inheritance
Mendel’s laws of inheritance were based on his work, but were formulated only after his death. They are not of great interest to amateur breeders working with many-gene characters. The first law states that when two homozygous individuals are crossed, the F1 individuals are phenotypically identical. The second law states that recessive characters that are masked in the F1 of a cross between two homozygous individuals, will reappear in a specific proportion in the F2. The third law states that members of different allele pairs (i.e., Aa and Bb) will assort independently of each other when gametes are formed, provided that the genes are not linked.
Mentha spp.
Mint, peppermint, spearmint, and Japanese mint (menthol). These species hybridise freely and are very variable. There is scope for amateur breeders looking for both highly specialised crops of exceptional quality, and useful levels of horizontal resistance.
Mercury
Mercury compounds are mostly very toxic and their use as crop protection chemicals, particularly as fungicidal seed dressings, is now banned.
Meristem
The undifferentiated tissue of a plant growing point. Meristem cells are capable of dividing into various different tissues and organs. They are the equivalent of the human stem cells in medical terminology.
Meristem culture
A technique for freeing vegetative propagating material from virus and other diseases. The meristem is the part of the plant that is undergoing active cell division to produce new tissues. These new tissues remain free of all parasites for a short period. By removing the meristem, and culturing it with tissue culture techniques, it is possible to produce a new plant that is free of parasites. Not a suitable technique for amateurs.
Merological approach
A systems term meaning systems analysis, or systems management, that is being conducted at the lower systems level. The converse, in which the system is studied at the higher systems levels, is called the holostic approach. The holistic approach is essential if suboptimisation is to be avoided.
Metabolism
The chemical processes that take place in any living organism. Anabolism is constructive metabolism, and is concerned with the synthesis of proteins, carbohydrates, and other substances. Catabolism is destructive metabolism, and is concerned with the breakdown of chemical substances to produce energy.
Metamorphosis
Metamorphosis in insects usually occurs in the final instar and it results in an organism that is markedly different from that of the earlier instars. For example, a caterpillar metamorphoses into a butterfly.
Metaxenia
The phenomenon in which plant tissues outside the embryo sac (i.e., edible fruit tissues) are influenced by the pollen. This phenomenon is seen, for example, with dates. It is often called zenia.
Metroxylon spp.
Sago palm. Several species are used in S.E. Asia and Polynesia for the production of sago, a starch extracted from the pith of a palm stem that is about fifteen years old.
Microcyclus ulei
This is the fungus that causes South American Leaf Blight (SALB) of Para rubber. It is of interest in that rubber grows in the Amazon valley, which is permanently warm and wet, and it has a gene-for-gene relationship in spite of apparently having a continuous pathosystem.
In fact, the rubber tree is deciduous, in spite of its continuously warm and wet tropical environment, and this demonstrates the evolutionary value of the deciduous habit in producing a discontinuous pathosystem for the control of parasites.
Micro-evolution
Evolution within species. Unlike macro-evolution, micro-evolution operates during periods of historical time, it produces changes that are not intrinsically new, it produces no increase in complexity, it is reversible, it produces new ecotypes, and it does not produce new genetic code.
Micro-evolution in a wild ecosystem produces differing ecotypes as a result of different selection pressures in different parts of the ecosystem. It is the result of natural selection.
Micro-evolution in an agro-ecosystem system produces differing agro-ecotypes as a result of different selection pressures in parts of the agro-ecosystem. It is the result of an artificial selection called domestication, or plant or animal breeding.
The classic example of micro-evolution was industrial melanism.
Micro-organism
Any organism that is microscopic or ultra-microscopic (i.e., viruses, which are too small to be visible with a light microscope, and can be seen only with an electron microscope).
Most plant pathogens are microscopic, but most plant pests are macroscopic.
Microscope
A magnifying instrument. A high power optical microscope will discern organisms as small as bacteria, but smaller organisms (e.g., viruses) require the considerably greater magnifying power of an electron microscope. See also: Dissecting microscope.
Microscopic
Too small to be seen with the naked eye.
Microsphaera
A genus of the Erysiphales (powdery mildews) characterised by cleistothecia that contain several asci, and appendages that dichotomise several times at the tip.
The main species of economic importance are M. alphitoides (oak mildew), M. berberis, (barberry mildew) and M. grossulariae (Gooseberry mildew).
Mildew
Plant pathologists recognise two kinds of parasitic mildew. Powdery mildews occur on the external surface of a plant, and they belong to the Erysiphales. Downy mildews penetrate the internal tissues of the host, they belong to the Peronosporales, and they include potato blight (Phytophthora infestans) and grape downy mildew (Peronospora viticola).
Millardet
In 1882, Pierre Marie Alexis Millardet discovered the first fungicide, which he called Bouillie Bordelaise or Bordeaux mixture. See also: Plasmopora viticola.
Millet
Any of the cereals belonging to the genera Echinochloa, Eleusine, Panicum, Paspalum, Pennisetum, and Setaria.
Milo
See: Sorghum bicolor.
Mindset
A state of belief that resists change, despite opposing evidence. Scientists are supposed to be open-minded, but there are many examples of scientific mindset through history.
When Joseph Lister first introduced his concept of antiseptic (now called aseptic) surgery, there was furious opposition from many of his medical colleagues.
When Gregor Mendel discovered single-gene genetics, he was ignored for over thirty years.
When Alfred Wegener introduced his geological concept of continental drift in 1915, most geologists vigorously denied its very possibility for about half a century, until plate tectonics proved him right.
For an even longer period, geologists also denied the possibility of catastrophic change, until Walter Alvarez showed that the extinction of the dinosaurs was due to a major extraterrestrial impact.
And when J.E. Vanderplank introduced his concept of horizontal resistance, in 1963, it was met with comparable resistance in a mindset which continues to this day.
See also: Vested interests.
Mineral oil
Mineral oil can be sprayed on to the surface of water where it makes an impervious film that prevents mosquito larvae from breathing. This is an example of a stable insecticide.
Minimum
The lowest possible value of a quantitative variable.
Minimum tillage
A cultivation technique that makes some use of herbicides in order to disturb the soil as little as possible. The main objective is one of soil conservation but other advantages accrue, such as a reduction in cultivation costs, etc.
Minor staple
A minor staple is the principle food crop of an agricultural people, but one that is not sufficiently productive to become a major staple. Minor staples permit the development of village societies only. They do not liberate a sufficient proportion of the people from agriculture to permit the growth of cities, and the development of a sophisticated civilisation. Examples of minor staples include yams in West Africa, sorghum and millets in East Africa, potatoes in the high Andes, and taro in Papua New Guinea.
Miracle rices
The dwarf rice cultivars of the Green Revolution. See also: IRRI.
Miracle wheats
The dwarf wheat cultivars of the Green Revolution. See also: CIMMYT.
Miridae
The plant bugs, many of which are serious pests of crops.
Mist propagator
A transparent chamber for rooting cuttings in a nutritionally and biologically inert rooting medium that discourages rotting. The cuttings are left with as much leaf as possible, in order to maximise photosynthesis, and water loss is prevented by keeping the leaves permanently wet with an automatically controlled, fine mist of water. High light intensities are recommended, even at the risk of relatively high temperatures in the chamber. Many crops, in which vegetative propagation was previously difficult or impossible on a commercial scale, can now be vegetatively propagated in mist propagators.
Mites
Small arthropods of the Order Acarina, and important parasites of both plants and animals. They differ from insects principally in that they have eight legs. The plant parasitic mites are often called spider mites, and are often coloured red. They can cause considerable damage to plants by feeding on the surface cells of stems and leaves, causing severe lesions resembling ‘burn’.
Miticide
A pesticide that kills mites.
Mitosis
A dividing of a nucleus to produce two daughter nuclei that are genetically identical to each other. See also: Meiosis.
Mobile nutrients
Plant nutrients that can be moved internally from one part of a plant to another. When there is a deficiency of a mobile nutrient, the deficiency symptoms occur in the older leaves. Mobile nutrients include: Nitrogen, Phosphorus, Potassium, and Magnesium.
See also: Immobile nutrients.
Mode
The most frequent quantitative variable within a mixed population. For example, height in people ranges from the minimum to the maximum, and this character has a normal distribution. Very short people are rare. So are very tall people. The most frequent height is the mode. This distribution is represented by the ‘bell-shaped’ or Gaussian curve, and it is typical of other quantitative variables such as horizontal resistance. With a normal distribution, the mode is also the mean, or the average. With a skewed distribution, the mode and the mean are different.
Moko disease
A wilt disease of banana caused by the bacterium Pseudomonas solanacearum.
Molasses
The sweet syrup of non-crystallising sugars left over from the refining of cane sugar. This syrup is utilised in cattle feed, and in the fermentation of rum. It is also refined into treacle for human consumption.
Mold
The American spelling of mould.
Molecular biology
A modern branch of biology in which biological phenomena are studied at the systems level of the molecule. It includes the study of single genes and genetic engineering.
Molecule
The smallest part of a chemical compound that can take part in a chemical reaction. A molecule consists of a group of atoms,.
Momordica charantia
The bitter gourd, bitter cucumber, or balsam pear. This monoecious vine originated in the Old World and is now common throughout the tropics. The young fruits are peeled and steeped in water to remove the bitterness before cooking. This crop is amenable to improvement by amateur breeders.
Monkey nut
See: Arachis hypogea.
Monocotyledon
Any Angiosperm that has only one cotyledon in each seed. They are often called the narrow-leaved plants, and the leaf veins are usually more or less parallel. The flower parts are in multiples of three. Seeds of monocotyledons cannot be split into two halves like split peas. Cultivated monocotyledons include all the cereals and other grasses (Gramineae), onion family (Alliaceae), palm family (Palmae), banana family (Musaceae), ginger family (Zingerberaceae), yam family (Dioscoreaceae), and pineapple family (Bromeliaceae).
Monoculture
The cultivation of a single crop, without any crop rotation. Monoculture greatly increases the chances of serious epidemics, particularly of soil-borne parasites. Monoculture is most dangerous when it is continued for a long period of time, when it involves very large acreages, and when the entire crop consists of a single, genetically uniform cultivar, and when that cultivar is protected by vertical resistance.
Possibly the largest and longest monoculture consisted of the United Fruit Company banana plantations of the Gros Michel cultivar in various countries of the Caribbean. It was eventually ruined by the soil-borne diseases called Panama disease and Moko disease.
Monocyclic parasites
Parasites that have only one life cycle in each season or crop cycle. See also: Oligocyclic, Polycylic.
Monoecious
Greek = one house. The occurrence of separate male and female flowers on one plant. See also: dioecious, hermaphrodite.
Monogenic characters
Characters whose inheritance is controlled by a single gene. For example, vertical resistances are monogenic characters.
Monogerm
Sugar beet in which each fruit contains only one seed. This is an important commercial advantage as it removes the necessity of thinning out the young seedlings in the field by hand.
Monolock
In the crop pathosystem, we have misused the gene-for-gene relationship by employing it on a basis of crop uniformity called monolock. For this reason, vertical resistance is temporary resistance in agriculture. Monolock is a hostparasite system of locking that has been ruined by uniformity.
“What happens when every door in the town has the same lock, and every householder has the same key, which fits every door?”
This kind of uniformity occurs in cultivars that are genetically uniform, and in which every plant has the same biochemical lock (i.e., vertical resistance). Such a cultivar is likely to be cultivated in crop populations that total millions, probably billions, and possibly even trillions, of plants, all with the same lock.
Monoploid
A plant possessing only one basic set of chromosomes. See also: Doubled monoploid.
Monozygotic
Monozygotic twins are produced from a single fertilised egg, which then divides into two separate but genetically identical embryos. See also: Dizygotic.
Monsoon
Seasonal winds in India and S.E. Asia. The wet monsoon blows from the southwest, from May to September, and brings rain from the Indian Ocean. The dry monsoon blows from the northeast from October to April, and brings dry conditions.
Morning glory
See: Ipomea.
Morus spp.
The mulberry. Morus nigra is the black mulberry, an ancient crop native to the Middle East and cultivated for its fruit for many centuries in the Mediterranean area. Morus alba is the white mulberry which originiated in China and is used for feeding silkworms.
Mosaic
In plant pathology, many virus diseases are called ‘mosaic’. This term is also applied to the symptoms of these viruses, which produce a leaf mottling of normal and abnormal colouration. There is little difference between a mosaic and a mottle.
Mosquitoes
Flies of the family Culicidae, in the Order Diptera. These insects are vectors of several serious, tropical, human diseases, such as malaria, yellow fever, dengue fever, filariasis, and encephalitis. They are relevant here because of their resistance to unstable insecticides.
Moth
Adult insects of the Order Lepidoptera, which have large membranous wings, covered in scales that often confer colours that constitute a superb camouflage. At rest, the wings are folded over the body, with the upper surfaces outward, for purposes of concealment. Unlike butterflies, moths normally use scents (called pheromones), rather than wing colours, as sex attractants.
The fore-wings are larger than the hind wings. The long, slender antennae are often feather-like. The young stages are known as caterpillars or grubs, and many are serious parasites of crops. The sucking mouth part (proboscis) of the adult moth is usually a coiled tube, and is used for extracting nectar from flowers.
Mottle
In plant pathology, many virus diseases are called ‘mottle’. This term is also applied to the symptoms of these viruses, which produce a leaf mottling of normal and abnormal colouration. There is little difference between a mottle and a mosaic.
Mould
The term has three meanings in agriculture. First, in the sense of ‘mouldy’, meaning stored products damaged by fungi, usually resulting from too high a moisture content. Second, some fungal plant diseases are called ‘mould’, particularly when the fungus is visible as a furry growth on the diseased tissues. Third, soil that is high in organic matter is often called ‘mould’.
Mtata
See: Sorghum bicolor.
Muck
1. Muck soils, consisting entirely of highly decomposed plant material. 2. Farmyard manure.
Mulberry
See: Morus spp.
Mulch
A covering spread over soil with a view to conserving soil moisture, protecting crop roots, controlling weeds, encouraging beneficial soil organisms, and adding nutrients to the soil. Mulch usually consists of dead plant material such as straw, old leaves, bark, or cereal husks and chaff.
An ornamental mulch of crushed stone is now fashionable for flower beds and potted plants. A plastic mulch, consisting of polyethylene film, can be useful for weed control, or as a means of heating the soil, with the greenhouse effect, in order to kill soil-borne parasites of crops. However, stone and plastic mulches do not add nutrients to the soil.
Mulching can also help soil conservation.
Multiline
A crop population which consists of a mixture of several pure lines that are morphologically very similar, but each of which has a different vertical resistance. The idea of the multiline is to introduce a diversity of vertical resistances into an otherwise genetically uniform cultivar.
In practice, a multiline is normally useful only if there is a single species of parasite to be controlled, because a multiplicity of different parasites cannot easily be controlled in this way.
Multi-locational testing
The testing of cultivar in a wide range of agro-ecosystems with a view to identifying those with a wide environmental adaptability. This approach is useful with vertical resistance, but is inappropriate with the concept of comprehensive horizontal resistance, which usually limits a cultivar to a single agro-ecosystem.
Mung bean
See: Phaseolus aureus.
Musa fehi
The Fe’i banana. A close but unimportant relative of the true banana, which occurs in the South Pacific. The fruiting bunch is erect, unlike the true banana in which the bunch is pendant. Not a crop for amateur breeders.
Musa sapientum
The Latin name usually given to the edible bananas and plantains (but not the Fe’i banana). Both the taxonomy and the common usage terms are confused. A banana is a sweet fruit that is eaten raw and ripe. A plantain is a starchy fruit that is usually eaten cooked and either ripe or unripe. As a fruit, bananas are second only to grapes in commercial importance.
The bananas are Old World monocotyledons and they are the largest of herbs. It is incorrect to speak of the banana ‘tree’, as it has no woody tissues. The so-called ‘trunk’ is a pseudostem made up a fibrous true stem surrounded and supported by leaf sheaths.
The fruits are sterile because the plant is a triploid and usually has both male and female gametic sterility as well. Definitely not a crop for amateur breeders.
For the first half of the twentieth century, bananas were cultivated in such large acreages, mainly by the United Fruit Company, in the countries surrounding the Caribbean, that these countries were known as ‘Banana Republics’.
The cultivation of one perennial clone (‘Gros Michel’) in huge acreages, in climatic conditions permanently favourable to epidemics, was probably the largest and most enduring monoculture ever achieved. This monoculture was eventually ruined by a number of different parasites, and this indicates that the very serious pests and diseases are damaging mainly because we cultivate susceptible crops, and not because of any inherent savagery of the parasite.
The first of these destructive parasites was Panama disease (Fusarium oxysporum f.sp., cubense). Many incorrectly blame the disease, when it was undoubtedly the monoculture that was at fault.
Another major disease was a bacterial wilt (Moko disease) caused by Pseudomonas solanacearum. More recently, Sigatoka disease (Mycosphaerella musicola), and Black Sigatoka disease (Mycosphaerella fijiensis) have become important.
The most important insect pest is the banana weevil (Cosmopolites sordidus). It seems that these are all new encounter parasites, as bananas are very ancient clones that have been cultivated for millennia without serious parasites in their centre of origin in S.E. Asia.
Musa textilis
Manila hemp, or abaca. This fibre was the finest of the plant fibres and was used extensively for the highest quality ropes in fishing and shipping. It has now been supplanted by plastic ropes, but it is still in demand for extra strong papers, such as tea bags. The fibres are retted out of the outer sheaths of the petioles that form the pseudo-stem.
Musaceae
The banana and ensete family.
Mushroom
The macroscopic sporing body of a fungus. Mushrooms usually have gills, while toadstools have pores. Both edible and poisonous mushrooms occur. The cultivation of edible mushrooms is economically important, but breeding of this crop by amateur breeders is not recommended.
Mustard
See: Brassica spp.
Mutagenic
Any substance or process (e.g., exposure to radioactivity) that induces mutations. Occasionally, induced mutations can be useful in crop plants, and the techniques of inducing them are usually considered to be plant breeding tools which, however, are not recommended for amateur breeders.
Mutant
An individual or clone that exhibits a mutation. Often called a sport.
Mutation
A mutation is a change that occurs in a single gene. A mutant is an individual, or a clone, that exhibits such a change. Mutants that occur within existing clones of cultivated plants are often called sports. Mutations are usually deleterious in wild plants, but crop mutations occasionally have agricultural value. The special features of many ornamental plants, such as variegated leaves, are often due to mutations.
Mycelium
The microscopic filaments of a fungus. When seen in the mass, macroscopic mycelium is often called mould.
Mycology
The study of fungi.
Mycoplasma
See: Phytoplasma.
Mycorrhiza
One of a group of fungi that form symbiotic associations with the roots of higher plants. The fungi are more effective than the root at extracting nutrients from the soil, which they provide to the plant in exchange for carbohydrates.
Mycosphaerella
An Ascomycete fungus that causes leaf spots and stem lesions on many crops, including banana (M. musicola), strawberry (M. fragariae), peas (M. pinoides), brassicas (M. brassicicola), flax (Pasmo disease, M. linorum) and cucurbits (M. citrullina).
Myristica fragrans
The nutmeg. This tree is a member of the family Myristicaceae, native to the Moluccas, and it produces two distinct spices. Nutmeg is the dried seed, and mace is the dried aril tissue that surrounds the seed. The former is normally used in sweet dishes, and the latter in savoury dishes.
Like cloves and cinnamon, this spice was part of the incredibly valuable spice monopolies, first held by the Arabs and Venetians, then by the Portuguese, then by the Dutch, and finally by the British, before being broken by worldwide competition.
The tree is dioecious and this makes the establishment of an orchard very difficult as the two sexes occur in equal proportions but few males are required. The sex of a tree cannot be determined until flowering, some 5-8 years after planting, and the excess males must then be removed. Half of the replacements are also males, and must later be removed, and so on.
There is room for considerable improvement by selecting within existing populations and then by vegetative propagation. This is within the scope of amateur breeders.
Myzus persicae
The green peach aphid. This aphid is the vector of many virus diseases, and is particularly important in potatoes.