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Genetics of Plant Disease • Pathogen Has 1 or more Genes for Pathogenicity, for Specificity & for Virulence against particular Host • The Gene Combination that Makes a Plant Susceptible Is Present only in that 1 Kind of Plant or a few Related Kinds • Nonhost Resistance – Inability of a Pathogen to Infect a Plant because the Plant Lacks something the Pathogen Needs or Contains Substances Incompatible with the Pathogen Variability in Resistance & Susceptibility in Both Pathogens & Plants – Causes of Variability • Mutations – Abrupt Change in the Genetic Material – Changes in Sequence of Bases in DNA • Recombination of Genes Genes & Disease • Pathogens Are often Host-Specific – Fusarium oxysporum f. sp. lycopersici causes tomato wilt but attacks only tomato – F. oxysporum f. dianthii – Puccinia graminis f. sp. tritici attacks only wheat http://en.wikipedia.org/wiki/Stem _rust • A Few Pathogens Attack many Kinds of Plants – Many Diverse Genes for Virulence – The Genes less Plant Specific than more Specialized Pathogens • Each Plant Species Susceptible to fairly Small Number of different Pathogens • Plants Have Existing & Induced Defenses to Resist most Pathogens – Resistant Plants probably Have Numerous Genes for Resistance Mutation Mechanism Often Favors Pathogens • Probably not more Frequently • Pathogens Produce a Great Number of Progeny • Monoculture Characteristic of Agriculture – Segregation & Recombination during Meiotic Division of the Zygote in Sexual Reproduction 1 Pathogen Designations • Genus & Species— – Similar Morphology & Phenotype • Race or Strain— – Same Variety but Attack different Varieties of Plants within a Species • Variant— – Offspring of a Race of Pathogens that can Suddenly Attack New Variety • Varieties— – Same Species but Attack different Plant Species – Special Designations for Varieties • Biotype— – The ‗Clones‘ or Offspring of Variants that Reproduce Asexually • ―Special Form‖ (Fungi) – formae specialist = f. sp. • ―Pathovar‖ (Bacteria) • ―Type‖ (Viruses) • ―Race, Biotype, Pathotype, Strain‖ (Nematodes) Example of Designations 1. Genus • Genus & Species = Puccinia – Daylily Rust Is the Species Puccinia hemerocallidis – The Pathogen that can Cause Stem Rust in Small Cereal Grains Is Grouped into the Species Puccinia graminis • Variety – Stem Rust in Wheat: the Variety Puccinia graminis tritici – Stem Rust in Oats: the Variety Puccinia graminis avenae • Specific Races/Strains of Pathogen can Attack only Specific Varieties of a Plant Species Puccinia Puccinia graminis 2. Species Puccinia hemerocallidis Puccinia graminis tritici 3. Variety 4. Races/Strains P. graminis tritici-compacti Puccinia graminis avenae P. graminis tritici-inficiens – (There Are over 200 Races of P. graminis tritici) Another Example • Fusarium oxysporum – Fusarium Wilt Disease • Fusarium oxysporum f. sp. niveum – Fusarium Wilt on Watermelon • F. oxysporum f. sp. niveum race 0, race 1, & race 2 – Wilt Disease of Specific Varieties of Watermelon Types of Plant Resistance to Pathogens • Non-Host Resistance – Plants Completely Resistant to Certain Pathogens • True Resistance – Host, Pathogen Incompatible • Horizontal Resistance – Partial Resistance against all Races of a Pathogen – Controlled by many Genes • Vertical Resistance – Complete Resistance to some Races of a Pathogen but not to other Races – Controlled by a few Genes 2 Resistance Plant Variety B Resistance Plant Variety A 1 2 3 4 5 6 7 1 Races of a Pathogen • Escape – All 3 Disease Factors not Optimum • Tolerance 3 4 5 6 7 Races of a Pathogen • Apparent Resistance – Plants usually Susceptible to a Disease Remain Free of Infection or Symptoms – They Escape or Tolerate Disease 2 Gene-for-Gene Concept • Plants & Pathogens Evolved Together • Genetic Changes in One Is Balanced by Changes in other – Ability to Produce Crop even when Infected Breeding Resistant Varieties Hypersensitive Response • Natural Variability Exists • Plant apparently Recognizes Pathogen Elicitors • Activates Cascade of Genes • Recognition Response also Leads to Activation of other Defense-Related Genes that Result in other Types of Resistance, including Horizontal Resistance – Breeders Combine most Useful Genes for Yield, Quality, Uniformity, Hardiness, Resistance • Sources of Resistance Genes – – – – Native or Foreign Varieties Older, Heirloom Varieties Wild Plant Relatives Induced Mutations • Treating Plant Material with Mutagenic Agents 3 • Recent Work includes Genetic Engineering Techniques – Breeders can Introduce Genetic Material (DNA) into Plant Cells Directly • Ballistic Devices • Vectors • Protoplast Fusion – Tissue Culture to Obtain Plants with different Characteristics • A Few ‗Improved‘ Varieties Replace Most or all Others – The most Successful Improved Varieties Adopted Abroad & Eventually Replace Local Varieties – Genetic Base of the Varieties often Narrow – Vulnerable to Sudden Outbreaks of Catastrophic Plant Disease Epidemics • Initial Steps in Breeding generally Increase Genetic Variability – Combines Genes from Plants Widely Separated by Distance • One Most Useful Genes Are Identified, further Breeding Tends to Eliminate Variability • Classical Breeding Techniques for Disease Resistance – Depends on Plant‘s Mating System – Historically Complicated – Resistance Could Be Determined Only by Making Plants Diseased – Recent Molecular Techniques Allow Breeders to Select for Resistance instead of Inoculating Plants • Molecular Markers • Breeding for Vertical Resistance – – Aimed against Specific Pathogens or Races Most Effective • • – Annual Crops that Are Easy to Breed Directed against Pathogens that Do not Reproduce & Spread Rapidly or Mutate Frequently Easier to Manipulate in Breeding Program than Horizontal Resistance • Horizontal Resistance Requires Insertion of many Minor-Effect Genes – Can Be Complete at First but may Be ShortLived • Strategies to Improve Vertical Resistance – Multilines • – Plant Mix of Varieties that Differ in Resistance Genes Pyramiding • Cross Several Varieties that Have different Resistance Genes & Select Those that Contain Mixtures • Breeding for Horizontal Resistance – Confers Incomplete but more Durable Protection 4