Host genotype diversity and its effects on pathogen populations

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Host genotype diversity and its effects on
pathogen populations
Mixtures of red-grained and white-grained sorghum
(courtesy of Dr. Henry Ngugi, Pennsylvania State
University)
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Host genotype diversity host heterogeneity

• What is it, why the interest in it
• Mechanisms
• why does it work
• factors that make it more / less effective
• Empirical experience
• Effects on pathogen populations
• does it select for complexity?

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• Technically
• Mixture different species
• fescue bluegrass
• Blend different varieties, same species
• wheat varieties NC-Neuse USG3592
• Multiline mixtures of near-isogenic lines
  (NILs)
• 1950s Browning developed oat multilines for
  managing oat crown rust in Iowa

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Basic idea if monoculture maximizes selection
for new virulences, take a step back
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Host genotype diversity

• Mechanisms
• Dilution of inoculum
• Barrier effect
• Induced resistance

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• Mid-component the mean of the mixtures
  component cultivars when they are grown as pure
  stands
• Mixture performance is typically evaluated by
  comparison to the mid-component
• Disease severity
• Yield
• Yield stability
• Quality

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Mechanisms of disease reduction in small-grain
blends

• Induced resistance -- accounted for 27 of total
  stripe rust reduction in wheat cultivar mixtures
  (Calonnec et al, 1996, Eur. J. Plant Pathol.
  5733-741)
• Differences in genetic background among cultivars
  (partial resistance) -- led to additional 33
  reduction of powdery mildew beyond that accounted
  for by R-gene differences (Wolfe et al., 1981, in
  Jenkyn and Plumb, eds, Strategies for the Control
  of Cereal Disease, Oxford Blackwell)
• Compensation of resistant cultivars through
  increased tillering -- accounted for 6 of total
  disease reduction by club wheat mixtures
  inoculated with stripe rust (Finckh Mundt,
  1992, Phytopathology 82905-913)
• Disease reductions below mid-component were
  statistically significant in 20 of 58 two-,
  three-, and four-component mixtures

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Club wheat mixture components can be
distinguished by head color
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From data in Finckh Mundt, 1992, Stripe rust,
yield, and plant competition in wheat cultivar
mixtures, Phytopathology 82905-913
of total reduction in disease severity due to of total reduction in disease severity due to of total reduction in disease severity due to of total reduction in disease severity due to
1987 1987 1988 1988
Two-cultivar club wheat mixtures Compensation (tillering by R) Epidemic reduction Compensation (tillering by R) Epidemic reduction
F-R 2 54 -4 39
F-Y 5 49 7 41
J-R 20 58 17 33
J-Y -3 42 0 35
M-R 19 49 -
M-Y 1 38 -
Mean 7 48 5 37
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Characteristics that predict whether blends will
reduce disease (Garrett Mundt, 1999)

• Things that maximize allo- vs. autodeposition
• Genotype unit area
• Shallow dispersal gradient of pathogen spores
• Small lesion size
• Short pathogen generation time
• Strong host specialization

• Small GUA

Large GUA
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Different plant sizes relative to pathogen
dispersal gradient (Garrett Mundt, 1999.
Epidemiology in mixed populations.
Phytopathology 89984-990.)
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Garrett Mundt, 1999. Epidemiology in mixed
populations. Phytopathology 89984-990.
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Spatial scale is predicted by theory to be
important to blend effectiveness

• Research plots may underestimate blend effects in
  controlling spread from disease foci that would
  be seen on a commercial scale
• Host-diversity effects of reducing disease should
  be larger for larger fields of mixtures than for
  smaller fields, because the difference in
  epidemic velocity between pure and mixed stands
  will increase with distance from the focus
• Large-plot wheat / stripe rust experiments in
  eastern Oregon Cowger et al, 2005. Velocity of
  spread of wheat stripe rust epidemics.
  Phytopathology 95972-982

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Cowger et al, 2005. Velocity of spread of wheat
stripe rust epidemics. Phytopathology 95972-982.