A pest can be described as any organism capable of causing damage to crop plant. Types of Crop Pest Important pests of crop plants are grouped into the following classes: insects birds; rodents; monkeys; man; nematodes. Insect pests can be classified into various groups based on their mode of feeding. These groups of insect pests include: biting and chewing insects; piercing and sucking insects; boring insects. Biting and Chewing Insects: These insect pests possess strong mandible and maxillae (mouthparts) which enable them to bite and chew plant parts. Examples are: termites, grasshoppers, leaf worms, army worms, mantids, locusts and beetles. Piercing and Sucking Insects: These insect pests possess strong mouthparts called proboscis which enable them to pierce through plants and suck liquid materials from plants’ tissues. Examples are: aphids, cotton stainers, mealy bugs, scale insects, capsids or mirids and white flies. Burrowing Insects: These insects including their larvae are capable of burrowing into plant parts and destroying the tissues of the plant or fruits or seeds. Examples are: bean beetles, stem burrowers, maize weevils and rice weevils. EFFECTS OR ECONOMIC IMPORTANCE OF INSECT PESTS IN CROP PRODUCTION Insect pests destroy crops in the field through their biting, chewing, boring, sucking and defoliation activities. They cause reduction in viability of stored produce. Spot of injuries by insects may predispose crops to disease attack. They increase the cost of production during the course of controlling them. They render vegetables and fruits unattractive and unmarketable. Some are carriers or vectors of diseases. The profits of farmers are reduced. They reduce the quality of produce either in the store or in the field. They generally reduce the yield of crops. They can also cause total death of crop plants. PREVENTION AND CONTROL OF PESTS Pests of crops can be prevented or controlled through the following methods: physical control; cultural control; biological control; chemical control. a) Physical Control: This involves the physical removal of pests by: hand-picking of insects and larvae, setting traps to catch rodents, shooting rodents with gun, fencing round the farm with wire nets. b) Cultural Control: This method involves the use of farm practices to prevent or control pests, especially on the field. Examples of cultural control are: practising crop rotation; use of pest-resistant varieties of crops; appropriate tillage operations; use of insect traps; hand-picking and destruction of insects; burning crop residues; timely planting of crops; proper weeding or sanitation; timely harvesting; close-season practices, especially in cotton. c) Biological Control: This involves the introduction of the natural enemies of pest to control or keep the pests population under control. Such enemies eat up or feed on these pests, thereby reducing the population of the pests. d) Chemical Control: This involves the use of chemicals called insecticides or pesticides to control pests of crop plants. Definition of Insecticide: An insecticide is a chemical preparation used to control insect pests. Forms or Groups of Insecticides: There are four major forms or groups of insecticides used in the control of insect pests of crops. These forms or groups and their mode of action are: Groups and Mode of action powder – contact. liquid – systemic. granules – stomach. Gas (eous) – fumigation. These groups of insecticides can be used either directly on insects, seeds, plants and plant parts to check the activities of insects. Examples of chemicals used to control pests are: pesticides – chemicals to control pests. insecticides – chemicals to control insects, e.g Grasshoppers. rodenticides – chemicals to control rodents, e.g rats. avicides – chemicals to control birds, e.g weaver birds. nematicides – chemicals to control nematodes, e.g. eel Worms. SIDE EFFECTS OF THE VARIOUS PREVENTIVE AND CONTROL METHODS OF DISEASES AND PESTS OF CROPS The use of the various control methods of diseases and pests has some side effects .. These side effects include the following: a) Chemical Method of Control Some beneficial insects and soil organisms may be destroyed. The chemical used may be toxic to man and domestic animals. It may leave undesirable residue in the environment. Pests and diseases may develop resistance to chemicals. Some are washed out of soil to rivers and streams where they can endanger aquatic life and cause pollution. Empty containers could be a source of poisoning when used as containers for consumables. b) Biological Method of Control The new organism introduced may start attacking crops which were originally free from attack. The predators expected to control others may rather feed on other beneficial insects. The activities of the new organism introduced may cause serious imbalance in the ecosystem. c) Cultural Method of Control The use of fire to kill harmful pests may also result in the destruction of other beneficial organisms. Resistant varieties may become adapted to the environment so that the resistance is short-lived where fire is used. If care is not taken, fire may spread to other farms. The use of fire may cause the destruction and loss of organic matter from the soil. It may also lead to the destruction of soil structure and cause soil erosion.
A plant disease may be defined as a departure or deviation of the plant from the normal state of health, presenting marked symptoms or outward visible signs. In other words, disease is an unfavourable condition caused either by the pathogens present within a living organism or by nutritional deficiency. It usually results in physiological and anatomical abnormalities expressed in characteristic symptoms. The harmful effects of a disease to the plant or to any of its parts and products generally result in the reduction of the economic value of the plant. Causes of Diseases The primary causes of disease can be classified into two groups – pathogens and physiological factors. a) Pathogens: Pathogens are disease-inducing living organisms or agents which pass through a regular cycle of development and reproduction. Examples are: Viruses. Bacteria. Fungi. Nematodes. b) Physiological Factors: The physiological factors may be physical, chemical or environmental. Many of them are essential for normal growth of plants, but when they are deficient or present in excess, they cause diseases.Examples of physiological factors are: Nutrient deficiency. Heat. Inorganic salts. Water. GENERAL EFFECTS OF DISEASES ON CROP PRODUCTION Diseases cause lots of damage to crop and their effects include the following: Diseases generally reduce the yield or productivity of crops. They also reduce the quality of crops. They cause the malformation of parts of plants or the whole plants. They can kill or cause the death of a whole plant. They cause reduction in the income of the farmer. They increase the cost of production through the expenses incurred in the course of controlling them. They render vegetables and fruits unattractive and unmarketable. Their activities cause retarded growth in crop plants. It leads to delayed maturity. Damage to crop parts leads to reduction in photosynthesis. Ways by which Diseases Spread on a Crop Farm Diseases can spread on a crop farm through any of the following ways: By rain splash. Through the use of contaminated tools and equipment. The use of infected planting materials. Wind blowing pathogens to other crops. Through animals, especially predators during feeding. Through insect vectors. Through visitors to the farm. Through weeds which may harbour pathogens. Through irrigation water. GENERAL CONTROL OF CROP PLANT DISEASES Diseases of crop plants can be controlled by the following methods: cultural control; biological control; chemical control. Cultural Control: This involves the use of crop rotation, resistant varieties, tillage practices, regular weeding, fallowing, timeliness of planting, pruning, uprooting and burning of infected crops, soil treatment or sterilization, roguing or uprooting of infected crops, maintenance of farm hygiene, timeliness of harvesting, etc to control or prevent diseases. Biological Control: This involves the use of natural enemies of the disease to reduce or totally eliminate the disease. Chemical Control: This involves the use of chemicals such as fungicides, nematicides, and insecticides to dust or spray plants and plant materials in order to prevent or control plant diseases. Summary of General Control Measures of Diseases of Crops Weed the farm regularly to prevent the disease pathogens from being haboured by weeds. Practise crop rotation. Remove and burn infected plants. Plant disease-resistant varieties of crops. Use healthy seeds or stems for propagation. Practise seed dressing with plant protection chemicals such as Fernasan D before planting. Spray plant protection chemicals such as fungicides and nematicides. Early planting helps the crops to escape the period of disease occurrence. Spray insecticides to control the insect vectors. Destroy crop residues after harvesting to prevent the build up of disease pathogens or practise good farm sanitation or hygiene. Sterilise soil to control soil-borne diseases. Avoid close planting to reduce the rate of spread of diseases. Imported seeds and plants should be quarantined before their introduction into the country. Timely harvesting reduces the period of exposure.
The processes of crop improvement include: Introduction. Selection. Breeding or Hybridization. a) Introduction: This involves the importation of introduction or some varieties of crops with desirable characteristics into areas where they have not existed before. In other words, it involves the movement of crop from its origin or supposed centre of evolution to another region or area where it can adapt. Crops with desired qualities are identified in their centres of origin and then procured. Such crops are then introduced into the breeder’s country for testing and multiplication. Such imported crops are usually quarantined by the nation’s plant quarantine service before introduction into the country importing it. Introduction is usually carried out to upgrade the qualities of the local varieties of crops. Advantages of Introduction It helps in bringing new varieties of crops to a new area. It may enhance greater productivity. It may perform better if there is better climatic condition in the new location. It may also perform better if there is better soil condition in the new area. Absence of pests and diseases. It helps to upgrade the quality of the local varieties of crops. Disadvantages of Introduction There is the possibility of introducing new crop diseases. The new crop may not be able to adapt to climatic conditions of the new environment. It may also introduce new pests to the new environment. The introduced crop may not be able to adapt to soil conditions of the new environment. b) Selection: This involves the artificial picking of crops with desirable characteristics which are most favoured by the environment. In other words, selection is the ability to choose certain crops which possess specific artificial or external characteristics which are easily recognised by merely seeing them. Methods of selection include Mass selection: Crop plants are selected or rejected on the basis of their own performance or merit. Crop plants with the desired characteristics are chosen in preference to those not possessing them from a large group of crops. Pure line selection: This is the type of selection in which only one crop plant with good characteristics is selected. It leads to the concentration of that character which is uniform and pure. Pedigree selection: Crop plants are selected on the basis of the performance of their ancestors. It is based on the belief that the crop plant selected is likely to perform equally or even better than the ancestors. Progeny selection: Crop plants are selected on the basis of the performance of their offspring or progeny. Advantages of Selection It ensures that only the best naturally available crop is grown. Crops with desired qualities are selected. Seeds from best stands are multiplied for distribution. Crops with undesirable characters are detected and rejected. It reduces the spread of diseases and pests. Disadvantages of Selection Selection is tedious and time consuming. It is very expensive in terms of time and money. It requires expertise which may not be available. It brings about the elimination of some desirable traits of the parent stock. c) Breeding or Hybridization: Hybridization is a method by which an offspring is produced through the crossing of two different plant varieties of the same species. In other words, it refers to the development of plants by combining inherited qualities of one plant with that of another through the fertilization of female with male gametes. Types of Breeding In-breeding: This is the pollination and fertilization of closely related crop plants in order to retain certain desirable characteristics. This can lead to pure breed or pure line. Pure line: A plant which has been self fertilised or crossed continuously with closely related species (inbred) for many generations, so that the desirable qualities it possesses do not change from generation to generation, is called a pure line. Pure line will always reproduce itself with great precision. Cross breeding: This is the pollination and fertilization of unrelated crop plants belonging to different breeds. This results in the production of an offspring which is superior to the average performance of the parents. This is called hybrid vigour “heterosis”. Advantages of Breeding It can produce a superior offspring resulting in hybrid vigour or heterosis (cross breeding). Progeny grows more rapidly (cross breeding). Production of pure line (inbreeding). Offsprings can withstand variations of environment (cross breeding). Disadvantages of Breeding It could lead to “inbreeding depression”, which is the depression or loss in vigour and performance of offspring (in-breeding). There is a drop in production or yield of crops in terms of quantity and quality (in-breeding). It may lead to poor or low resistance to disease attack (in-breeding). METHODS OF IMPROVING CROP PRODUCTIVITY Productivity of crops can be achieved through a combination of methods which include: Crop Improvement Methods: As discussed earlier, crops can be improved through introduction, selection and hybridization. Proper Timing of Planting: Crops should be grown at the right time to avoid high temperature, inadequate rainfall or abundance of pests and diseases during growth. Adoption of Better Cultivation Methods: The adoption of better methods of cultivation like crop rotation which adds nutrients to soil, prevents erosion, pests and diseases outbreak and helps to increase yield. Use of Manures and Fertilizers: The use of manure like farm yard compost and green manure in combination with the use of fertilizers helps to add nutrients to soil and promotes good growth of crops. Control of Pests of Crops: The control of pests of crops which cause reduction in yield and growth can help in the improvement of crops. Control of Diseases of Crops: Diseases also cause a reduction in growth and productivity of crops. Where they are adequately prevented, it will lead to improvement of such crops. Use of Resistant Varieties: Some varieties of crops are capable of resisting disease’s attack and can mature early thereby increasing the yield of crops. Use of good crop varieties: There are some varieties of crops which naturally will grow well in different environmental conditions. Such crops do help to increase the overall production of such crops.
Gregor Mendel (1822-1884) was a monk in an Augustinian monastery in Brunn, Austria. He is often regarded as the father of genetics because his work formed the foundation for scientific study of heredity and variation. Mendel’s Experiments Gregor Mendel carried out several experiments on how hereditary characters are transmitted from generation to generation. He worked with garden pea (Pisum sativum). His major aim was to find out the pattern of inheritance of different characteristics on the pea plant. Methods used by Gregor Mendel in his experiment Gregor Mendel used two major methods in conducting his experiments. These methods were grouped into monohybrid inheritance and dihybrid inheritance. Reasons for Mendel’s Choice of Pea Plant Gregor Mendel decided to use the pea plant for his experiment because of the following reasons: Peas are usually self-pollinating and he could pollinate them by himself. They have a very short life span because they are annual plants. The pea plant was known to have several unique characteristics which exist in contrasting pairs such as: some seeds were round while others were wrinkled; some plants were tall while others were short; some seeds were yellow while others were green; some flowers were axial while others were terminal; some pods were green while some were yellow; some flowers were white while some were red; some pods were smooth while some were constricted. Methods Used by Mendel in His Experiment Gregor Mendel used two major methods in conducting his experiments. These methods were grouped into monohybrid inheritance and dihybrid inheritance. Monohybrid Inheritance Mendel used artificial method to cross two different plants at a time, which differed in one pair of contrasting characters, e.g. tall and short plants. This procedure was called a monohybrid inheritance and it was an example of complete dominance. He carried out the experiment in the following order: He planted tall plants for several generations and discovered that the plants produced were all tall plants. In the same way, he planted short plants for several generations and discovered that the plants produced were all short. He proceeded to plant tall plants and short plants. By the time the flowers were produced, he collected the pollen grains of the tall plants tagged the male and pollinated the stigma of the short plant tagged the female. He also collected the pollen grains of the short plant and place them on the stigma of the tall plant. Mendel then covered the artificially pollinated flowers with small paper bags to prevent the chance of natural pollination by insects. Mendel once again picked the seeds formed after the cross. When he planted the seeds, the plants obtained were all tall plants. These he referred to as the first filial generation or F1. Mendel then crossed the F1 plants, collected their seeds and sowed them. The plants he got from these were tall and short plants in a ratio of 3:1 respectively. He then called this stage the second filial generation or F2. The outcome of this experiment led to Mendel’s first law of inheritance. Mendel’s First Law of Inheritance This first law is also called the law of segregation of genes. The law states that genes are responsible for the development of the individual and that they are independently transmitted from one generation to another without undergoing any alteration. Explanation All the offspring in the F1 generation are all tall. It shows that the genes for tallness (TT) is dominant over the recessive genes (tt). In the F2 generation, three of the offspring are tall while only one is short (tt). From Mendel’s first law of segregation of genes, the actual segregation occurs in the F2, generation. The phenotypic and genotypic ratios in F, generation can be summarised as follows: Phenotypic ratio = 3:1 (i.e ., 3 tall and 1 short) Genotypic ratio = 1:2:1 (i.e ., 1TT, 2Tt, 1tt) Note: Letters are used to represent the genotypes of the traits. In the case of complete dominance, the capital letter form of the first letter of the dominant trait is used to denote the dominant gene. The small letter form of it is used to represent the recessive gene. Since tallness in the plant is dominant over shortness, T represents gene for tallness. TT represents genotype of the pure breeding tall plants. Such a plant is described as homozygous for tallness. t represents gene for shortness. tt represents genotype of the pure breeding short plant, homozygous for shortness. A cross between two organisms is shown by a multiplication sign x. Each gamete is represented by only one encircled letter, i.e. (T) or (t) depending on the trait being discussed. This is in compliance with Mendel’s law of segregation of germinal units. A heterozygous individual is represented by one dominant gene and one recessive gene, i.e; Tt. Such individuals are called carriers of a trait. Dihybrid Inheritance Gregor Mendel also carried out several experiments in which he crossed plants which differed in two pairs of contrasting characteristics such as seed shape (round and wrinkled seeds) and seed colour (yellow and green seeds). Mendel therefore called the whole set up as dihybrid inheritance because two pairs of contrasting characters are involved. When Mendel crossed plants which had round and yellow seeds with those which had wrinkled and green seeds, all the F1 plants produced round and yellow seeds. However, when the F, plants were self pollinated, the F, plants were of four types: plant that produced round and yellow seeds, wrinkled and yellow seeds, round and green seeds, wrinkled and green seeds. All these were in the ratio of approximately 9:3:3:1. Mendel then concluded that this could result if the contrasting characteristics of round and wrinkled seeds and the contrasting characteristics of yellow and green seeds were inherited independent of each other. The outcome of this experiment led to Mendel’s second law of inheritance. Mendel’s Second Law of Inheritance This second law is also called the law of independent assortment of genes. Mendel’s second law of independent assortment of genes states that each character behaves as a separate unit and is inherited independently of any other character. Mendel’s work can be represented by letters and their explanations as below: Parents Round yellow x wrinkled green. The four phenotypes which appear in the ratio 9:3:3:1 are as follows: 9 round yellow r1, 2,3,4,5,7,9,10,13 3 round green r 6,8,14 3 wrinkled yellow r 11, 12, 15 1 wrinkled green r 16 The 9 genotypes which include 4 homozygous and 5 heterozygous conditions are: 1 is homozygous for both round and yellow (1). 1 is homozygous for both round and green (6). 1 is homozygous for both wrinkled and green (16). 1 is homozygous for both wrinkled and yellow (11). 2 are homozygous for round and heterozygous for yellow (2, 5). 2 are heterozygous for round and homozygous for yellow (3,9). 2 are heterozygous for round and homozygous for green (8, 14). 2 are homozygous for wrinkled and heterozygous for yellow (12, 15).
In order to understand the Mendelian Laws and its usefulness in plant breeding, it is important to know some technical words which we shall come across in crop improvement. Genes: Genes are hereditary units or basic units of inheritance. They are located in chromosomes and are responsible for the transmission of characters from parents to offspring. Chromosomes: Chromosomes are rod or thread-like bodies found in the nucleus of a cell. The chromosomes house or contain the genes. Characters or traits: These are the inheritable attributes or features possessed by an organism, e.g. seed colour, seed size, plant height, etc in plants. Gamete: Gamete is a matured sex cell which takes part in sexual reproduction. There are two types: male gamete or spermatozoon (in animals) and pollen grains (in plants) and female gamete, egg or ovum (in animals) and ovules (in plants). Gametes are usually haploid. Zygote: Zygote is a single cell formed as a result of the union of a male gamete with a female gamete. Gamete is usually diploid. Allelomorphs: Allelomorphs are pairs of genes on the position of a chromosome (i.e. locus) that control contrasting characters. A pair of allelomorphs are called allelic pair while each member of the pair is the allele of the other. Phenotype: Phenotype is the sum total of all observable features of an organism, i.e. the physical, physiological and behavioural traits, e.g. height, weight and skin colour. Genotype: This term is used to describe those traits or sum total of the genes inherited from both parents. In other words, the genotype of an individual is his genetic make-up or constitution. Genotype includes both the dominant and the recessive traits that form the genetic make-up of an individual. Dominant character: This is a trait or character that is expressed in an offspring when two individuals with contrasting characters or traits are crossed. Dominant genes on the other hand are genes which control dominant characters. For instance, in a very tall plant, there may be the gene for shortness but the gene has no influence on the gene for tallness. Recessive character: This is the character or trait from one parent which is masked or does not produce the effect in the presence of dominant character. From the illustration above, shortness is the recessive character while tallness is the dominant character. Recessive genes on the other hand are genes which control recessive characters. Homozygous: An individual is said to be homozygous if it has two similar genes for the same character, i.e ., it has two identical alleles at the same position on a pair of chromosomes; the pair of genes controlling a given pair of contrasting characteristics are identical, e.g. (TT) for tallness or (tt) for shortness. Heterozygous: An individual is said to be heterozygous if the two members of a pair of genes controlling a pair of contrasting characters are different, i.e ., it has two different or contrasting alleles located on the same position on a pair of chromosomes, e.g. (Tt) for tallness or a plant with Rr genetic composition has a heterozygous red flower plant. Filial generation: The offspring of parents make up the filial generation. The first, second and third generations of offspring are known as the first, second and third filial generation respectively, and are denoted by the symbols F1, F2, and F3, respectively, too. F1, generation givesrise to F2, generation. Hybrid: Hybrid is an offspring from a cross between parents that are genetically different but of the same species. Hybridization: Hybridization is the crossing of plants with contrasting characters. Monohybridization involves the crossing of two pure traits while Dihybridization involves the crossing of plants with two pairs of contrasting characters. Locus: Locus is the site for location of a gene in a chromosome. Haploid: Haploid is when an organism has one set of chromosomes in the gamete. Gametes at certain stages in the life cycle of plants are haploid. It is represented by small letter “n”. Diploid: Diploid is when an organism has two sets of chromosomes in the body cell. The bodies of animals and plants are diploids. Diploid number is double the haploid number of chromosomes and is represented by “2n”. Mutation: Mutation is a change in the genetic make-up of an organism resulting in a new characteristic that is inheritable. Back cross: Back cross is the crossing of an organism with the homozygous recessive organisms from the original parental generation. Test cross: Test cross is the crossing of an organism with the homozygous recessive organism. Back cross and test cross are used to determine the genotype of organisms showing dominant phenotype.