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Breeding for Resistance. Control important crop diseases and pests Must have genetic variability Consider both genetic variability in the plant and in the pest Sources: Fehr, ch. 21 Slusarenko, Fraser, and van Loon, 2000. Types of Genetic Resistance. Qualitative Resistance

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breeding for resistance
Breeding for Resistance

Control important crop diseases and pests

Must have genetic variability

Consider both genetic variability in the plant and in the pest


  • Fehr, ch. 21
  • Slusarenko, Fraser, and van Loon, 2000
types of genetic resistance
Types of Genetic Resistance

Qualitative Resistance

Distinct classes of resistance and susceptible plants

Controlled by one or a few genes

Also called “Vertical” resistance

Quantitative Resistance

Continuous variation among genotypes

Many loci

Also called “Horizontal” resistance

qualitative resistance
Qualitative Resistance
  • Controlled by major genes (one or two)
  • Genes are readily transferred from one genotype to another
  • Presence of genes can be determined by exposing plants to particular races
qualitative resistance1
Qualitative Resistance


  • Easier screenings
  • Easier transfer of genes “all or nothing”


  • Vulnerability to new races
  • Continuous use of particular cultivar may lead to development of new race or shift in pest population (shift from race 1 to race 2)
qualitative resistance2
Qualitative Resistance


Leaf Rust Resistance in Wheat

quantitative resistance
Quantitative Resistance

Controlled by many genes each with minor effects


  • Can control a broad range of races due to the fact that many loci are involved


  • Difficult to transfer resistance from one genotype to another
  • Individual genes harder to identify
quantitative resistance1
Quantitative Resistance


Fusarium Head Blight in Wheat

sources of resistance
Sources of Resistance

Primary Gene Pool

Other breeding programs


Germplasm collections (GRIN)

Wild Relatives

Tomato: Lycopersicon genus

Wheat: Agropyron genus

Aegilops tauschii, Triticum monococum

Mutant Transformations: powdery mildew in Barley

vertical vs horizontal resistance
Vertical vs Horizontal Resistance



Vertical Resistance to Races 2, 5, and 6

vertical vs horizontal resistance1
Vertical vs Horizontal Resistance



Horizontal Resistance to all Races

hypersensitive response
Hypersensitive Response
  • Stem Rust of Wheat
  • Programmed cell death
  • A lot of black boxes
  • Pathogen recognition, signaling proteins, transcription factors,
gene for gene hypothesis
Gene-for-Gene Hypothesis

Proposed by Flor (1956)-study on flax rust

“For each resistance gene in the plant there is a gene in the pathogen that determines if the pathogen will be able to injure the plant.”


Gene combinations and Disease Reaction

Types in the Gene for Gene concept


Complementary Interaction of Two Host Genes

For Resistance and the Corresponding Two

Pathogen Genes for Virulence


a or


A genes

In the


races of pests
Races of Pests
  • Genetic variability within the pathogen population
  • Different isolates cause different responses among host genotypes
  • Example: Soybean Cyst Nematode Races
minimizing changes in races
Minimizing Changes in Races
  • Alternation of Resistant and Susceptible Cultivars (soybean cyst nematode)
  • Mixtures of Resistant and Susceptible Genotypes
  • Prevention of New Races
resistance vs tolerance
Resistance vs Tolerance

Resistance: no establishment of the pathogen in/on the host, or a limited establishment

Tolerance: the host develops, continues to grow, and produces well despite the pathogen’s presence

breeding for specific resistance
Breeding for Specific Resistance

Individual Major Genes



breeding for specific resistance1
Breeding for Specific Resistance

Individual Major Genes

  • Breed cultivars with major genes that control the prevalent pest races
  • Select progeny from a segregating population or …
  • Transfer major genes from other sources
  • Example: Black Shank Tobacco Varieties
  • Advantages and Disadvantages
breeding for specific resistance2
Breeding for Specific Resistance


  • Develop many individual lines each with individual major resistance genes and then …
  • Mix the seed of these lines together to get …
  • Protection against a broad spectrum of races
  • Example: wheat cultivar Miramer 63
  • Advantages and Disadvantages
breeding for specific resistance3
Breeding for Specific Resistance


  • Put all known major genes into one line
  • Advantages and Disadvantages
  • Example: Leaf Rust in Wheat

Comparative structural analysis of the Lr10 homologous regions from diploid wheat, rice 5 (japonica), indica rice and rice 1 (japonica).


qtl and resistance
QTL and Resistance
  • Quantitative Trait Loci
  • Identify QTL that control variation in resistance observed in genotypes
  • Use markers to screen genotypes (throw out the susceptibles)
  • Applications in Marker Assisted Selection (MAS)
qtl and resistance1
QTL and Resistance

Wheat chromosome 1A

Leaf rust gene ‘Lr10’


J.C. Nelson et al. 1997 Crop Science 37:1928-1935

qtl for fusarium head blight resistance in wheat
QTL for Fusarium Head Blight Resistance in Wheat

Ning 7840/Clark population

11 AFLP markers associated with resistance

1 marker explained 53% of the variation

Bai, Kolb, Shaner, and Domier. Phytopathology 89:343-348.

Sumai 3/Stoa population -- used RFLP markers

5 genomic regions were associated with resistance (3 from Sumai 3, 2 from Stoa)

Waldron, Moreno-Sevilla, Anderson, Stack, Frohberg. Crop Sci 39:805-811.

example of quantitative genetics
Example of Quantitative Genetics
  • 3 F2:3 populations, 40 families, 3 reps, 13 individual observations per rep
  • Separated variance into two parts
  • Among families
  • Within families
  • Narrow sense heritabilities were estimated
example from population 2
Example from Population 2

Estimate from 2555