Introductory Resistance Breeding PDF

Introductory Resistance Breeding PLB 304, 2(2+0) Sixth semester Krishna Hari Dhakal, PhD  INTRODUCTION Stress-free environment An optimal environment there is no interference by any environmental factor with complete expression...

Introductory Resistance Breeding PLB 304, 2(2+0) Sixth semester Krishna Hari Dhakal, PhD  INTRODUCTION Stress-free environment An optimal environment there is no interference by any environmental factor with complete expression of the genotypic potential of an individual/line Stress When some factor of the environment interferes with the complete expression of genotypic potential Considered as a significant deviation from the ideal conditions in which plants are grown, preventing them from expressing their full genetic potential for growth, development, and reproduction Biotic stress Biotic stress is stress that occurs as a result of damage done to an organism by other living organisms, such as bacteria, viruses, fungi, parasites, beneficial and harmful insects, weeds, and cultivated or native plants. Crop characters such as rapid early growth, high competitive ability and herbicide tolerance may be selected for or may be obtained through genetic modification Abiotic stress Abiotic stress is defined as the negative impact of non-living factors on living organisms in a specific environment. The stresses include drought, salinity, low or high temperatures, and other environmental extremes. Abiotic stresses, especially hyper-salinity and drought, are the primary causes of crop loss worldwide Abiotic stress An abiotic stress may vary considerably depending on the location i.e region / location specific Occurrence and degree of some of the stresses are unpredictable, e.g., drought, while some others are reliably known Degree of some stresses are likely to vary during crop season Some stresses can be relieved by appropriate management practices, e.g., drought, while some others are virtually impossible, e.g., temperature stress A given abiotic factor may increase/decrease the level of another abiotic stress, e.g., in a saline soil, moisture stress would enhance salinity stress Different crop species show marked differences in their abilities to withstand a given stress  Different varieties of a crop show large differences in their ability to tolerate/resist abiotic stresses Different growth stages of crops may show marked differences in their tolerance to an abiotic stress, e.g., differential tolerance to stress by maize Stress during reproductive phase of crops causes far more economic loss than comparable stress during earlier phases Effects generated by one abiotic stress may overlap some of those generated by another stress, e.g., overlaps of some features generated by salinity stress and drought stress Biotic stress Most important effects of almost all crop pests and diseases is reduction in yield (upto 100%). Yield reduction is caused in four main ways: ✓ Destruction of leaf tissue and hence reducing plant photosynthesis capacity or efficiency (e.g., many rusts, mildews) ✓ Stunting plants by metabolic disturbance, nutrient drain or root damage (e.g., many viruses, aphids) ✓ Reducing plant stands by killing whole plants and leaving gaps in the crops which can not be compensated with increased productivity of neighbours (e.g., vascular wilts, soil borne fungi) ✓ Killing parts of plants (e.g., boring or feeding by insects)  Spectrum of biotic stresses that may cause crop yield losses is large and diverse Number of insects that causes yield reduction is large, including those that attack the crops during the growing season, feeding on leaves, pods, fruits and roots Global warming could exacerbate incidence of insects, diseases and weeds on farms around the world Breeding efforts for developing insect resistance cultivars have not been as effective as for disease resistance Biotechnology has contribute to develop varieties resistance to insects (Bt maize) and herbicide (glyphosate resistance soybean)  MERITS AND DEMERITS OF RESISTANCE BREEDING Merits Play important role in controlling the losses caused by biotic and abiotic stresses in crop plants Lead to reduction in the cost of production resulting in increasing the cost benefit ratio Resistant varieties are non-toxic to man, farm animals, and wild life Genetic resistant is only solution of some diseases as wilts, rusts, smuts, nematodes and bacterial blights Demerits Takes longer time (10-15 years) to develop agronomically acceptable varieties when the source of resistance are readily available Breeding for resistance to one pests may lead to susceptibility to another pest, e.g., hairiness in cotton is associated with Jassid resistance but confers susceptibility to whitefly and bollworms Interspecific gene transfer posses many problems Discard of genes for undesirable traits which are linked to resistance genes takes long time Development of resistance varieties is expensive and time consuming process Some resistant varieties has lower yield and poor quality Natural Enemies Natural enemies are plant consuming organisms ranging from most simple pieces of infective DNA or RNA (viroids) to the very highly developed and complex organisms as vertebrates that kill or decrease the reproductive potential of another organism is called natural enemies According to size and ways of feeding, natural enemies are 1. Pathogens: Natural enemies belonging to the micro-organisms. Literal meaning of pathogen: generator of disease 2. Parasites: Small animals or phytophagous insects and parasitic higher plants (Striga) 3. Herbivore: Are larger, mobile animals that cause biting damage (caterpillars, locusts, rodents) Classification of natural enemies and the nature of their effects on plants Taxonomic Classification Classification of NE Effect on plants Viroid Pathogens Disease, Infection Virus Pathogens Disease, Infection Phytoplasm Pathogens Disease, Infection Bacterium Pathogens Disease, Infection Fungus Pathogens , Parasites Disease, Infection; Infestation Higher Plant Parasites Infestation Nematode Parasite Infestation Mite Parasites Infestation Insect Parasites, Herbivores Infestation, Biting damage Snail and Slug Herbivores Infestation, Biting damage Vertebrate Herbivores Biting damage  Some Terminologies Disease: Physiological disturbance of a plant /part of a plant caused by a stress factor or a combination of stress factors resulting in symptoms. Host: Susceptible to a natural enemies and serves both as source of nutrition and as living substrate. Parasite: Organism that more or less permanently and sometimes for part of its life cycle lives in close connection with a living organism and withdraws its nutrients completely or partially at the expense of that organism. Pathogen: Having the capacity to infect plants; exploits the plants as a source of nutrition. Infection: The use of a plant as nutrient source by a pathogen, usually resulting in reproduction of that enemy.  Some Terminologies Infestation: The use of a plant as nutrient source by natural enemies and lives on that plant. Sign: Visible parts of pathogen by which it can be recognized. Symptom: Deviation from normal growth and development of a plant by stress factor(s). Stress factor: Identifiable factor that potentially results in damage. Damage: Reduction in physical or economical yield of a crop due to stress factors.  Classification of Pathogens According to their characteristics of the infection process Biotrops: Withdrawing nutrients from living host tissue. (viruses, PM, rust, fungi, loose smuts) Necrotrops: Withdrawing nutrients from tissues killed by natural enemy. (Septoria, Helminthosporium) Hemi-biotrophs: Withdrawing nutrients from living host tissue that soon after this will die. (Phytophthora infestans, DM) Weakness pathogen: That only can infect weakened plant parts with reduced fitness. (Botrytis, Pythium) Vascular wilts: Pathogen that causes wilting of the host plant by blocking of the xylem and lives in and spreads through the vascular tissue. (Fusarium, Verticillium) Principles underlying selection for disease resistance i) Sources of resistance must be identified from existing materials: cultivar itself, commercial cultivars, other varieties, land races, weedy relatives, related species or genera. ii) Screening technique for resistance: by exposure to the disease pathogen under natural or artificial induced epiphytotic is necessary to distinguish between resistance and susceptible plants. iii) Mode and inheritance of resistance must be understood. iv) The resistance gene must be transferred to an adapted cultivar. v) Progeny testing of resistant plants to verify the inherent nature of resistance  Losses due to diseases Diseases reduce total biomass production by the crop in one or more of the following ways: 1. Killing of plants 2. Killing of branches 3. General stunting 4. Damage to the leaf tissues 5. Damage to the reproductive organs including fruits and seeds Disease development The development of any disease depends on a close interaction among three diverse factors. They are I) The host (sum total of resistance genes and other host features affecting disease reaction): It should be suitable II) The pathogen (sum total of virulence, inoculum abundance): It should be aggressive in nature and III) The environment (sum total of the conditions influencing disease development): It should be favourable. Defence mechanisms of host against pathogen/parasite Avoidance It reduces the chance of contact between prospective host tissue or food plant and a potential natural enemy usually as a result of particular morphology, phenology or smell of the potential host plant. Thorns, hairs, spines, smell, colour, taste, repellent odours are some of the examples. Resistance It is the ability of the host to reduce the growth and development of the parasite or pathogen. It is of two types: ✓ Race specific/vertical resistance/gene for gene resistance: A resistance that is effective to specific (avirulent) genotypes of a pathogen species. ✓ Race non specific/horizontal resistance/no gene for gene resistance: A resistance that is equally effective to all the genotypes of a pathogen species. Tolerance It neither restricts parasitic contact nor the growth and development of the parasite after establishment. But, it reduces the amount of damage/symptoms per unit quantity of parasite or pathogen present. Susceptibility Incapacity of a plant to reduce the growth, development and reproduction of the natural enemy. Sensitivity Character of the host plant to develop relatively severe symptoms or severe damage per unit quantity of the natural enemy. Sensitivity = { (Yield without pathogen -Yield with pathogen)/ Yield without pathogen}/ Con. of the pathogen Gene for gene relationship Flor 1956 based on his work on linseed rust and postulated the gene for gene relationship between a host and pathogen. This relationship states that "A resistance gene R, is only effective if the infecting pathogen carries the corresponding avirulence gene, A” Host resistance is conditioned by dominant allele, R. In the pathogen, virulence (ability to damage/infect host) is conditioned by recessive allele, a. Resistance reaction occurs when complementary genes in both host and pathogen are dominant. A host genotype that caries no dominant alleles at any of the loci is susceptible for all the races of pathogen (even if avirunlent) 'A' avirunlent allele is dominant over 'a' virulent allele and resistant allele 'R' is dominant over susceptible allele 'r'. Compatibility depends on the genotype of the host and the genotype of the pathogen.  Host genotype Pathogen genotype A a R - + r + + Where, + indicates susceptible and – indicates resistant reaction  Host Pathogen genotypes genotype A1A2a3 a1A2a3 A1a2A3 a1a2A3 A1A2A3 a1A2A3 a1a2a3 r1r2r3 + + + + + + + R1r2r3 - + - + - + + r1R2R3 - - - - - + + R1R2r3 - - - + - - + R1r2R3 - + - - - - + r1r2R3 + + - - - - + R1R2R3 - - - - - - +  Variety Virus conc. Yellowing Yield with virus Yield without (kg) virus (kg) A 100 8 80 90 B 60 5 100 110 C 50 4 75 90 D 70 6 50 100 a) Which variety is the most susceptible and why? A b) Which variety is the most resistant and why? C (Low damage and low yield difference c) Which variety is the most tolerant and why? B between with and without virus) d) Which variety is the most sensitive and why? D ❑ With the help of following given table, write the genotypes of the host (A, B and F1) and pathotype and inheritance of resistance. Pathotype Variety Progeny ratio A B F1 F2 P1 R S All R 3R 1S P2 S R All R 3R 1S P3 R R All R 15 R 1S P1+ P2 S S All R 9R 7S Genotypes: Inheritance of resistance Var A: AAbb Inoculum P1 or P2 : F2 ratio, 3 : 1; single gene with Var B: aaBB two alleles F1: AaBb P1: A1A1a2a2 Inoculum P3 : F2 ratio, 15 : 1; duplicate gene action P2: a1a1A2A2 P3: A1a1A2a2 Inoculum P1 + P2 : F2 ratio, 9 : 7; complementary P1+P2: A1A1a2a2+a1a1A2A2 gene action.  Types of resistance 1. Broad Resistance ❖ Resistance that results from an identifiable mechanism that is effective to several species of potential natural enemies. Eg. Toxic alkaloids in potato, glucosinolates in Brassica, latex production in Asclepias. ▪Active Resistance ❖ Becomes operative in reaction to an attempt of infection by natural enemies. ❖ Eg, production of host specific but pathogen non specific phytoalexins (pisatin by pea, hydrophaseolin by soybean, Phaseolin by bean) etc. take places in cells around the place of penetration or wounding ❖ Pathogenesis Related (PR) protins - plant produces such proteins as a response to infection, biting damage or other types of stress and that occurs systematically through whole plant; the effect is called induced or systemically acquired resistance. ❖ Formation of papillae- cell-wall appositions formed and save from penetration by fungus. Eg. Rye leaf rust in barley. ▪ Passive Resistance ❖ Toxic compounds or mechanical barriers are present in the plant, irrespective the presence or absence of the pathogen. Eg, alkaloids in potato. 2. Non-host Resistance: ❖ Nonhost resistance is typically considered the ability of a plant species to repel all attempts of a pathogen species to colonize it and reproduce on it. ❖ The complex of characters of a plant species that are responsible for making it a non-host to a certain potential natural enemies is non-host resistance. ❖ Plant species of which no single individual is susceptible to a natural enemies is called as non-host. ❖ Based on this common definition, nonhost resistance is presumed to be very durable and, thus, of great interest for its potential use in agriculture. Host range: List of plant species that can be exploited by a natural enemies as source of nutrients. Generalists/polyphagous: Natural enemies with a wide host range. Specialists/oligophagous/monophagous: Natural enemies with a narrow host range or even a single plant species. Race: A group of genotypes within a pathogen species that is distinguished by its virulence spectrum. Biotype: synonym of race but used for phytophagous insects and nematodes and for parasitic higher plants. (Virus and Bacteria: biotype; Nematode: pathotype term is used in stead of the term race) Forma specialis: Unit within the natural enemy species that is distinguished by its host range; differ in pathogenicity (capacity of a microbial organism to infect a plant species) but belong to the same pathogen species. Mechanism of non-host resistance May not be stimulated to germinate: soil borne pathogen, nematode Spore germinate but fail to penetrate: leaf pathogens Penetrate the epidermis cell and abort just before or after the plant cell penetration Blocking of plant cell wall penetration by formation of papillae Cell wall penetration and necrosis of plant cell Breeding for non-host resistance: Crossing between non-host and cultivated species and selection of target plant generation after generation Protoplast fusion of non-host and host plants and grow regenerants then repeatedly backcrossing to the host species  Types of host resistance Hypersensitivity resistance: Capacity of a plant to defend itself against a pathogen/parasite by reacting with necrosis of plant cells at the infection sites (infected side die rapidly and prevented to spread further). Mechanism Occurs as reaction to infection by biotroph and hemibiotroph viruses, fungus, bacteria, nematodes and even insects It is an active defense mechanism and associated with other active defense mechanisms like production of phytoalexins and pathogen related proteins Immune plant: showing no macroscopic visible sign of infection whatsoever after exposure to a pathogen Race specificity Is very high specific: pathogen-species specific and specific to certain genotypes of pathogen Virulent: pathogen genotypes to which the resistance is not effective Avirulent: pathogen genotypes to which the resistance is effective Type of resistance: vertical Virulence spectrum is the range of resistance lines or specified genes for resistance to which an isolate shows virulence Gene-for-gene relationship Resistance of hypersensitivity type very frequently is due to a dominant gene “Avirulence is dominant over virulence” The outcome of infection of plant tissue depends on genes in the plant as well as on genes in the pathogen (genetic interaction between plant and pathogen) Do you find race specific and race non specific resistances? If so, which lines are race specific and which are race non specific to which isolate(s)? Breeding line Isolate of Bremia lactucae 1 2 3 4 5 A 0 0 0 0 0 B 20 0 22 19 19 C 10 9 13 10 9 D 51 53 55 54 52 E 0 0 35 23 23 A - race non specific and resistant for all isolates B, C, D, E – race specific, B is resistant for isolate 2 and cultivar E is resistant for isolates 1 and 2, all others are susceptible. Among B,C,D and E, D is comparatively more susceptible.  Expression of resistance gene Resistance allele (incomplete, complete) Genetic background of cultivars Genotype of the pathogen like homozygous, heterozygous state Developmental stage of the plant Environmental factors like temperature Suppressors Suppressors are those that prevents the expression of defense genes and stop the resistance reaction. They are either dominant or recessive. ❑ Two homozygous host plants are intercrossed. Both have the same dominant major gene for hypersensitivity resistance. One of the two plants, however also has a dominant gene for suppression of resistance. The other plant has a recessive allele on the suppressor locus that does not interfere with the expression of resistance. In what frequencies do you expect resistant and susceptible phenotypes in the two parents, F1 and F2 after inoculation with a pathogen isolate carrying the corresponding allele for avirulence? R1R1, SS X R1R1, ss Sus. Res. F1 R1R1, Ss (Sus.) F2 1R1R1, SS : 2R1R1, Ss : 1R1R1, ss Sus. Sus. Res  Durability Breaking down of resistance: Phenomenon that the effectiveness of resistance against a natural enemy decreases as a result of changes in the population of that enemy. Resistance that remain effective for a long period in which it is applied at large scale in an environment conducive to the natural enemy is durable resistance. Within the same plant-patho system, the degree of durability of the resistance varies. Hypersensitivity resistance is infamous for its very limited durability. Hypersensitivity against PM, DM, rust fungi and loose smut are usually very ephemeral (lasting within very short time) These pathogens are all biotroph or hemibiotroph with a narrow host range and a large number of races known.  Causes of Breaking Down of Resistance Natural selection One clubroot gall produces millions to billions of resting spores and that population of spores can contain multiple Plasmodiophora brassicae pathotypes. However, one or two pathotypes tend to be dominant across a field, with other pathotypes present only at low levels. First generation CR varieties are resistant to the most common pathotypes. Natural selection can occur with repeated use of the same type of resistance, which will keep the common pathotypes from increasing but opens the door for rapid increases of some less common pathotypes that were present in the field at low levels. Over time, these less common pathotypes could become the new dominant pathotypes in the population. This is the “selection pressure” that makes natural selection work on clubroot. Mutation This refers to a permanent change in the DNA (nucleotide sequence) of an organism, which can be passed on to its progeny. Primarily the consequence of a loss mutation in the avirulence locus in the pathogen i.e. AVR1 to avr1. For example, a mutation in a P. brassicae could result in a brand new pathotype developing which can cause infection in a resistant canola variety. Mutations are one way that diversity can be introduced into a pathogen population, but mutations like this are rare and slow. Recombination This refers to the exchange of genes during sexual reproduction (chromosomal crossover) that leads to the offspring having different gene combinations from the parents. This can also add diversity to the pathogen population and result in the emergence of new strains. AVR1avr2 x avr1AVR2 gives 15 avirulent : 1 virulent (i.e., avr1avr2) genotypes of pathogen. Monocyclic : Pathogens that produce only one cycle of development (one infection cycle) per crop cycle are called monocyclic. In one growing season only one sexual/asexual reproduction cycle Test (evaluation) being carried out during one reproduction cycle Polycyclic: Pathogens that produce more than one infection cycle per crop cycle are called polycyclic. Having several sexual/asexual reproduction cycles per crop growing season. Test (evaluation) being carried out after several reproduction cycles of the natural enemy. Residual resistance ❖ The resistance that may be observed after a virulent race of the pathogen appears and infects a formerly completely resistant cultivar is a residual resistance. With the help of following given results, which cultivar has the highest level of partial resistance? Give your logics. Components of Cultivars Partial Resistance A B C D Infection frequency Low (%) 100 75 40 60 Latency period High (days) 8 10 15 17 Spore production/ Low Uredospore (No.) 100 65 50 90 Sources of resistance  Diploid and perennial Why landraces, wild progenitor, related species and genera? ❖ Adaptation ❖ Quality ❖ Disease ❖ Insects ❖ Yield Difficulties/ Drawbacks of wild progenitor, related species and genera Limited crossability Hybrid sterility Poor chromosome pairing and poor exchange of chromosome fragments Lower fertility Frequent backcrossing and selection to get rid of the numerous undesirable traits of the donor Other source of resistance 7. Non host 8. Mutations 9. Somaclonal variation 10. Unrelated organisms 11. Germplasm collection Alternative methods of creating resistance Induction of mutation Genetic modification Pathogen specific genes from plants Disadvantages of genetic modification of crop Genes for broad resistance plants Non vegetable genes (Bt gene) Genes of viral origin (coat protein) Recalcitrant to genetic transformation (monocotyledons, leguminosae) Public acceptance is low Considerations of mutagenic treatment Genes are not always expressed Breaking down of resistance Not necessarily durable Low frequency of the mutation for resistance Huge number of plants should be tested Efficient methods of screening Undesirable side effects Test of Resistance How to apply natural enemy? Dipping: soil borne pathogen; seeds or roots of the seedlings Spraying: leaf pathogen (PM, P. infestans, rust fungi, Septoria tritici) Dusting: leaf pathogen (PM, P. infestans, rust fungi, Septoria tritici) Injection: Xanthomonas campestris pv oryzae Using spreader plants: A susceptible cultivar is planted in a row behind the test plots and perpendicular to the plant rows in the plots. The spreader rows are inoculated early in the season. How to compose the inoculum? Aspects to be considered Phytosanitary aspects: no exotic races can be used Screen and characterize the pathogen by using differential set of cultivars or by molecular marker fingerprints Known variation in the pathogen Most naturally occurring complex race Composition of inoculum a) A single isolate and b) Mixture of isolates Test will be carried out with the most complex race (race with a wide virulence spectrum) available Mixture of isolates belonging to various races that have as many as virulence factors represented  Tested Infected lesions Result I Result II cultivars caused by mixtures Isolate 1 to 4 Isolate Isolate 1 2 3 4 1 2 3 4 A 160 40 40 40 40 40 40 40 40 B 60 15 15 15 15 20 0 20 20 C 40 10 10 10 10 20 20 0 0 D 20 5 5 5 5 0 0 0 20 In Result I could be race non specific. Resistance is equally effective to all isolates. Cultivar A and D are relatively susceptible and resistant respectively. Resistance is equally effective to all isolates. In Result II, cultivar A is race non specific but cultivar B, C and D are race specific. Cultivar B is resistant to isolate 2, C is resistant to isolate 3 and 4 and D is resistant to isolate 1, 2 and 3. Resistance is not equally effective to all isolates except cultivar A. Partial, race non specific resistance should be looked for with genetically pure isolates rather than with mixtures  Incubation (period) ❑ The time elapsed between exposure to the pathogenic organism or a chemical or radiation and when symptoms and signs are first apparent is called incubation. Factors ✓ Relative air humidity ✓ Temperature ✓ Light intensity Such conditions can be controlled much better in the labs and greenhouse than in the field. Aspects to be evaluate: After incubation the effects of the infection may become visible Show symptoms or biting damage Differ in degree or nature of infection or severity of symptoms expression Strong indications of differences in resistance Relatively mild symptoms may also indicate tolerance Quantitative aspects: Incidence: degree of infection in a population of the host species that is expressed as the percentage infected or symptoms showing individuals Polycyclic leaf pathogens: percentage of leaf tissue covered with lesions, mycelium or postules AUDPC: amount of infection Measurement: Visual, image analysis, biochemical or serological tests Lower the AUDPC value, considered as resistant cultivar  Disease incidence (%) = No of dead plants/total no of plants X 100 in the given area AUDPC = Sum of (X1 + X2)/2 X (T2-T1) + (X2 + X3)/2 X (T3 –T2) +……………..+(Xm + Xn)/2 X (Tn -Tm). Whereas X1, X2 ,.....Xm, Xn are the disease severity on the respective date and T1 ,T2 ,.....Tm, Tn are the dates on which the disease is scored. A novel form of indirect enzyme-linked immunosorbent assay (ELISA) has been devised for the detection of viruses in plants. Calculate AUDPC based on following information and determine which genotype is most resistant.  Assessment of damage Reduction of physical or economic yield damage Higher the amount of infection, the higher damage should be expected Quantity of the pathogen relative to that on a susceptible cultivar is determined by the level of resistance or avoidance Amount of damage is determined by both the amount of pathogen and the level of tolerance Heterogeneity of soil fertility cause a large experimental error Perform in replicated trials Cultivars with the least yield reduction are of course the most desirable Can either be resistant or tolerant or combination of both  How to determine? ❑ Resistance: ✓ Amount of pathogen in or on cultivar ❑ Tolerance: ✓ Yield/value of cultivars in the absence of pathogen ✓ Yield/value of cultivars in exposed conditions of pathogen ✓ Per unit of pathogen in order to calculate the damage ❑ Sensitivity = { (Yield without pathogen -Yield with pathogen)/ Yield without pathogen}/ Con. of the pathogen ✓ Higher the sensitivity, the higher the damage Effect of powdery mildew (PM) on yield reduction of 6 barley cultivars with different levels of tolerance and or resistance is given below. With the help of that given information, interpret it. Cultivar Infection % yield reduction (%leaf area) Observed Due to Lack of Res Lack of Tol A 80 21 21 0 B 60 18.5 15 3.5 C 50 12 12 0 D 50 7 12 -5 E 40 12.5 9 3.5 F 20 3 3 0  Interpretation ❖ Higher the figure the more susceptible or sensitive the cultivar ❖ Negative values indicate tolerance or resistance ❖ The most tolerant cultivar is D, Why? ❖ The most resistance cultivar is F, Why? ❖ The most sensitive cultivar is B and E, Why? ❖ The most susceptible cultivar is A, Why? Breeding material of a cereal crop and infection by PM/rust fungi is given below. What is the meaning of letters and figures and aspects of the infection? Line Infection Line Infection Which line(s) do you select for type type breeding program? And why? Which line is the most resistant, 1 45S 6 5MR tolerant and susceptible? 2 20S 7 50R Which of the 10 lines are especially interesting to include in a breeding 3 0R 8 5S program for resistance ? Line 8: Partial resistance, durable 4 20MS 9 10MR type of resistance Line 3: Resistant, but non durable! 5 80S 10 5MR  Early plants will escape from heavy infection by NE that appear late in the season, late cultivars may escape or recover from infection by NE that thrive earlier in the season Early lines should be compared with early checks and late lines with late checks  Phenomenon that in an experimental field with small adjacent plots plant genotypes with quantitative resistance are infected more severely, and susceptible plants less severely than when they would be grown in isolated field. The lines that are to be tested is grown in experimental plots along with susceptible line. These lines are to be grown in monoculture situation. Record the level of infection in both situations and estimate the difference  Summary Hypersensitivity resistance is the best option in a number of plant patho-systems Partial resistance is best option if breaking down of resistance occurs Avoidance, non preference and non acceptance mechanisms are best options for insects Broad resistance is applicable to generalists (broad range of host) Race specific hypersensitivity resistance may be applied in well conceived strategies by gene pyramiding, diversification and cultivar mixtures Breeding for tolerance is too labourious Non durable resistance Hypersensitivity resistance is non durable in general Pathogen population becomes predominantly virulent, when one gene after the other is introduced All genes for resistance available in the crop will be finished Strategies for controlling plant pathogens and pests Exclusion of pathogen from the host: (legislation, plant quarantine, crop inspection or crop isolation) Reduction or elimination of the pathogen’s inoculum (crop rotation, sanitation, soil drainage, weeding, soil sterilization, seed treatment Improvement of host resistance: introduction of genetic resistance into adapted cultivars Protection of the host by using chemicals (pesticides) Strategies to enhance the durability of ❖ Pyramiding of Resistance Gene resistance ❖ Diversification ❖ Multiline Variety and Cultivar mixture ❖ Integrated Control  Why is not it feasible to realize a multiline cultivar of potato with resistance to Phytophthora infestans? What about a cultivar mixture? Is it feasible to realize a multiline cultivar of wheat with resistance to rust? What about a cultivar mixture? 4. Integrated control (a) Exploitation of the period without crop Many natural enemies pass a bottle neck at least once a year Population increases in favourable periods and decreases in unfavourable periods (host crop unavailable) Eg., Powdery mildew in barley. Virulent mutant formed in summer will relatively survive less in winter if winter barley is prohibited to grow. (b) Crop rotation and phytosanitation Cultivation of different non host crop leads to a decrease of the population of the pathogen by a factor 2/3rd per year as compared to cultivation of resistant cultivar by a factor 1/5th per year. Effective to soil borne pathogen and nematodes. Volunteer plants should be removed during the season where other crop plants are grown. C.  Erosion of crops horizontal resistance to disease during a breeding cycle due to the presence of strong vertical resistance characterized by presence of vertical resistance gene (oligogenes). Van der Plank introduced the term Vertifolia effect derived from the name of a German potato variety “Vertifoia” having the late blight resistant gene R3 and R4. The variety become susceptible when the appropriate pathotype P (3,4) evolved leading to a near complete failure of the crop, such a total failure of vertical resistance leading to disease epidemic. b.  Factors affecting expression of disease and insect resistance ❑ Certain specific factors may complicate breeding for resistance that may be environmental or biological in nature. 1. Environmental factors ❖ Temperature: Low or high temperature over a period of time may cause loss of resistance. ❖ Light: Light intensity affects the chemical composition of plants that is relategd to pest resistance (e.g., glycoside in potato increased by light intensity). ❖ Soil fertility: High soil fertility makes plants more succulent and more susceptible to disease development. ❖ Biological factors ❖ Age: The response of a plant to a pathogen or insect pest may vary with age. Some diseases are more intense at the early stage in plant growth than others. ❖ New pathotypes or biotypes: New variants of the parasite that overcome the current resistance in the host may exhibit a different kind of disease expression.  Breeding for resistance to insects Mechanisms Non preference: Character of a phytophagous species to refuse (or accept less) a plant as a source of nutrients, when in the environment more attractive alternatives are available. Non acceptance: Character of a phytophagous species to refuse (or accept less) a plant as a source of nutrients, irrespective whether or not in the environment attractive alternatives are available. Antibiosis (Resistance): Refers to an adverse effect of feeding on a host plant on the development and or reproduction of the insect pest. In several cases, it may even lead to the death of the insect pest. Tolerance (similar to disease tolerance) Avoidance: same as disease escape. It is not a case of true insect resistance. Sources of insect resistance Screening techniques ❖The cultivated cultivar ❖ Field screening ❖Commercial cultivars ❖ Glass house screening ❖Other varieties ❖Landraces ❖Related species Breeding methods of insect ❖Related genera resistance ❖Unrelated organisms ❖Germplasm collection ❖ Same as breeding methods of disease resistance  Breeding for Abiotic Resistance Phenotypic performance of a population is determined by genotype, environment and G x E interaction. In an optimal environment, there is no interference by any environmental factors (stress free environment). Any factor of the environment interferes with the complete expression of genotypic potential, it is called stress. Yield loss can be minimized by crop management and development of resistant/tolerant varieties. Stress during reproductive phase leads greater yield loss that of during earlier growth phase. Suitable soil amendments should be used to improve the soil status of the field. Different varieties of a crop also show large differences in their ability to withstand a given stress. Different growth stages of crop may show large differences in their tolerance to concerned stress (seedling...........heading). Common abiotic stresses Drought Heat Cold Mineral stress (deficiency, toxicity) Oxidation Salinity Water logging Screening Techniques Field Screening Green House screening Which genotype is the most drought tolerant and which one is the most sensitive? Genotype Yield W/o Drought Yield in drought Wt. after Loss Q/ha (Turgid) Q/ha (Fresh) Q/ha (Dry) A 45 30 15 B 50 45 05 C 60 50 10 D 70 30 25 E 30 20 10 RWC = Relative Water Content Estimate RWC Genotypes Turgid SN Weight (g) Fresh weight (g) Dry weight (g) 1 WK1701 30 20 15 2 WK1544 25 20 15 3 WK1204 30 25 20 4 WK1720 20 15 14 5 WK1777 15 12 10  Breeding Methods ❖Selection ❖Introduction ❖Germplasm Collection ❖Hybridization followed by Back crossing ❖Mutation ❖Somaclonal Selection ❖Genetic Engineering Estimate STI and SSI SN Non-stressed condition Stressed condition Genotypes Grain yield (kg/plot) Grain yield (kg/plot) 1 Pasang Lhamu 25 20 2 Godawari 25 15 3 WK1701 32 25 4 WK1544 15 8 5 WK1204 30 25 6 WK1720 29 19 7 WK1777 28 18  Breeding for Heat stress Temperature is basic to life processes, which increase temperature within a limited range. Effect is expressed as Q10, which is the ratio of the rate of biochemical process at one temperature to that at a temperature 10 0C lower. Normal growth and development of each genotype is in optimal range of temperature. When temperature moves beyond optimal range, it generates temperature stress. Temperature stress may be grouped as heat, chilling and freezing stresses  Maize crop is grown at a constant temperature of 25 OC and characterised by a temperature sum of 750 d OC until anthesis and a temperature threshold value of 10 OC. Find days to flowering of this crop under such condition. If, it is grown at a constant 35 OC, how many days does it take to start flowering? Present of logics on the basis of these results. The threshold temperature for the development of wheat is 2 OC. If a wheat variety is grown at a constant temperature of 15 OC and starts flowering at 70 days after sowing, find the temperature sum until anthesis.