Pathogen Terminology (for now):

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• Pathogen Terminology (for now)
• Pathogenic The ability of a pathogen to cause
  disease (host range).
• Virulence The degree of pathogenicity of a
  pathogen.
• Aggressiveness The relative disease-producing
  ability of a pathogen on a susceptible host.
• Important to understand the different use of
  these terms based on the type of pathogen in
  question.

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Concepts Terminology
Host Plant Pathogen
Immunity Pathogenicity
Race-specific Virulent
Race-nonspecific Aggressiveness
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• Expression or Description of Resistance
• Resistance can be functionally described using 3
  criteria
• - Magnitude or Effect of the resistance (high or
  low).
• - Whether or not the resistance is differential
  (effective against one race/strain/biotype/pathoty
  pe or against many races).
• - Its inheritance or genetic basis (single genes
  or multiple genes).

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Race-specific resistance effective against some
races ineffective against other races (can be
moderate or partial). aka Vertical Qualitative
Inoculum-reducing Major-effect Hypersensitive
Monogenic Oligogenic Seedling R gene.
100 severity

Amount of disease
0 severity
Race 1 Race 2 Race 3 Race 4
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Race-nonspecific resistance equally (more or
less) effective against most or all races. aka
Horizontal Quantitative Rate-reducing
Minor-effect General Polygenic Additive
Adult-Plant Influenced by environment.
100 severity

Amount of disease
0 severity
Race 1 Race 2 Race 3 Race 4
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Theoretical types of resistance
Major effect Differential Monogenic Minor effect Differential Monogenic
Major effect Nondifferential Monogenic Minor effect Nondifferential Monogenic
Major effect Differential Multigenic Minor effect Differential Multigenic
Major effect Nondifferential Multigenic Minor effect Nondifferential Multigenic
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Potato late blight (Phytophthora infestans)
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Wheat Stripe rust resistance - susceptibility
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Effects of barley yellow dwarf virus on wheat
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Eyespot (Helminthosporium spp.) on rice
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Cabbage Yellows Fusarium oxysporum
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Cucumber scab Cladosporium cucumerinum
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Oat Victoria Blight Cochliobolus victoriae
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• Infection type The phenotypic expression of a
  host-pathogen interaction. (Fleck, Resistant,
  Moderately Resistant, Moderately Susceptible,
  Susceptible)
• - wheat stem rust 0, , 1, 2, 3, 4, X ( or -)
• - wheat stripe rust 0-3(R), 4-6(Intermediate),
  7-9(S)
• - rice blast 1-2(R), 3(Intermediate),
  4(Susceptible)

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0 1 2 3 4 X

• Infection types for wheat leaf rust and stem rust

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Wheat stripe rust infection types
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0
1
2
3
4
5
6
7
8
9
Wheat powdery mildew Infection types
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Jin et al. Pl Dis 2007 V97 P1096
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Spatial Context of Resistance Breeding
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• Importance of Pathogen Variability
• Major-effect type resistances (hypersensitive
  reaction) exert tremendous selection pressure on
  the pathogen population. The dangers associated
  with pathogen variation are greatest when the
  expression of resistance is very high.
• Responsible for the breakdown of resistance.
• Typically, new resistances select for
  pre-existing virulent pathotypes in the pathogen
  population (directional selection).
• Variability for virulence is directly responsible
  for the durability of host resistance.
• More cultivars of crop plants are removed from
  commercial production due to their disease
  resistance becoming ineffective, than due to any
  other biologic or agronomic factor.

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Theoretical Boom Bust (1) Resistance allele
selected when virulence allele is low. (2)
Virulence allele selected as resistance allele
becomes more frequent. (3) Resistance allele
decreases as virulence allele becomes more
prevalent. (4) Virulence allele decreases due to
decreased fitness with new plant resistance
allele (stabilizing selection).
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Real Boom Bust Cycle (5) Resistance allele
rises if in a popular variety. (6) Even if
virulence is low, a resistance allele is not used
if variety unpopular. (7) Resistance is durable
variety is popular. (8) Virulence selected as
acreage increases. (9) Variety remains popular
despite high virulence due to other factors. (10)
Acreage decreases if new resistant variety
introduced. (11) Virulence keeps rising if
associated with new virulence or decreases if not.