How does host genotype diversity affect the pathogen population?

1
How does host genotype diversity affect the
pathogen population?

• Mixtures (blends) and multilines (mixtures of
  NILs)
• Mechanisms
• Dilution of inoculum
• Barrier effect
• Induced resistance

2

• Technically
• Mixture different species
• fescue bluegrass
• Blend different varieties, same species
• E.g., wheat varieties NC Neuse USG 3592

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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
• Quality

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More on disease reduction mechanisms in
small-grain blends

• Induced resistance accounted for about 30 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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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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Club wheat mixture components can be
distinguished by head color
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Good reviews of host genotype diversity literature

• Smithson Lenné, 1996, Varietal mixtures a
  viable strategy for sustainable productivity in
  subsistence agriculture, Ann. Appl. Biol,
  128127-158.
• Garrett Mundt, 1999, Epidemiology in mixed
  host populations, Phytopathology 89984-990.
• Mundt, 2002, Use of multiline cultivars and
  cultivar mixtures for disease management, Annu.
  Rev. Phytopathol. 40381-410.

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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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Wheat blends stabilize yields, The Furrow,
John Deere, January 1997.

• Reduce year-to-year yield fluctuation
• Insure against future stresses
• Planting blends is one way to reduce the
  chances of picking the wrong variety for the
  conditions that develop. Jim Shroyer, Kansas
  State University agronomist

Ted Wolf grows equal amounts of three different
varieties, then plants them as a blend the
following year.
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Wheat blends in the U.S.(NASS, planted acres)
J
of wheat acres in blends of wheat acres in blends of wheat acres in blends of wheat acres in blends of wheat acres in blends
State Ttl wheat ac. 2003 2004 2005 2006 2007
KS 10.0 million 15 15 11 10 10
NE 1.9 million 10
WA 2.3 million 13 14 15 14 10
OR 0.9 million 13 6 7 4 6
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Cereal blends internationally

• German Democratic Republic (DDR former East
  Germany) largest-ever blend experiment
• From 1984 to 1990, cultivar mixtures comprised up
  to 92 of the barley acreage.

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• Barley mixtures planted until political
  re-unification of East and West Germany when
  government support for the project stopped.
• Barley monoculture has been re-established in
  Germany and fungicide use has increased.
• Wolfe M.S., 1997. Variety mixtures concept and
  value. In Variety Mixtures in theory and
  practice, Wolfe, M. S. (ed.). European Union
  Variety and Species Mixtures working group of
  COST Action 817. Online at http//www.scri.sari.a
  c.uk/TiPP/Mix/Booklet/default.htm

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Other blend experiences with disease reduction

• Small grains
• 10 of bread wheat acreage is currently in blends
  in Switzerland, Poland
• Rice mixtures (planted by rows) in 10 Chinese
  townships (6,000-acre experiment!) reduced rice
  blast on S varieties by 94 (Zhu et al, 2000,
  Nature 406718-722)

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• Potato late blight mixed results
• blends reduced late blight severity for focal and
  general patterns of primary inoculum in Oregon,
  but had little effect in Ecuador (Garrett
  Mundt,1999, Phytopathology, 89984-990 2000,
  Phytopathology, 901307-1312)
• alternating S-R rows reduced late blight severity
  on S cultivar (Andrivon et al., 2003, Plant
  Pathol., 52586-596)
• no late-blight reduction by mixtures in UK
  (Phillips et al. 2005, Ann. Appl. Bio.,
  147245-252)

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Hemileia vastatrix coffee rust

• Coffee rust is the most economically important
  coffee disease in the world, and in monetary
  value, coffee is the most important agricultural
  product in international trade.
  (http//www.apsnet.org/Education/LessonsPlantPath/
  Coffeerust/default.htm)

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• So far, nine rust R-genes identified, mostly from
  C. canephora and C. liberica
• The challenge is to deploy R genes in such a way
  they are not immediately overcome by new races of
  H. vastatrix.

• Figure 12. Susceptible Coffea arabica on the
  left a resistant hybrid with C. canephora on the
  right.

• So far, gt 40 races of H. vastatrix identified.
  New ones continue to appear some are able to
  attack previously resistant hybrids.
• To reduce the rate of selection of virulent
  races, Cenicafé, a national coffee research
  center in Colombia, created a composite cultivar
  with uniform agronomic characteristics and coffee
  quality, but a mixture of genes for rust
  resistance.

17
Willows (and poplars) grown as short-rotation
coppice major energy crop in N. Ireland, UK,
Sweden

• A plantation yields 8 -18 tonnes of dry woodchip
  per hectare per year, and can be harvested on a
  2-5 yr cycle up to 30 years before replanting.
• Melampsora epitea causes rust on Salix (willow)
• M. larici-populina causes rust on Populus
  (poplar)
• Cultivars range from MS to immune resistance can
  break down after 8-12 yr of heavy epidemics
  (e.g., N. Ireland)

• Concerns that large-scale willow monocultures
  place strong selection pressure on Melampsora
  population
• For pests such as brassy and blue willow beetles,
  as well as rust, planting a carefully selected
  mix of varieties is recommended. Fungicides not
  economically practical.
• Will super-races evolve in mixtures?

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Rust caused by the fungus Melampsora is the most
serious disease in short rotation coppice (SRC)
willow plantations for renewable energy. Willows
(Salix) are grown as a major SRC crop  for energy
because of their yield potential and coppicing
ability. In most willows, rust only infects
expanded leaves (left picture). However, in some
willows, such as Salix viminalis and S. caprea,
rust also attacks stems and young leaves. When
severe, rust defoliates susceptible plantings
prematurely (right picture) and reduces yields by
as much as 40. Severe rust also predisposes
plants to infections by secondary pathogens which
may lead to death of the plants.
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• Huge willow mixture experiments in Northern
  Ireland (McCracken Dawson, Eur. J. For. Path.,
  1998, 28241-250).
• 20 Salix genotypes, 96x14 m. plots 5-, 10-, 15-,
  and 20-way mixtures

• Melampsora rust reduced by 50 or more on S
  genotypes in mixtures, but impact still so great
  they contributed little to yield
• However, yield from 10-, 15- and 20-way mixtures
  not adversely affected despite loss of S trees
• Epidemic initiation slowed by up to 3 wks in
  mixtures vs. pure stands
• Pathotype diversity greater in mixtures than in
  pure stands possible dilution of aggressive
  pathotypes

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• (McCracken Dawson, Eur. J. For. Path., 1998,
  28241-250).

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Didelot et al, 2007, Effects of cultivar mixtures
on scab control in apple orchards, Plant Pathol.
561014-1022
Venturia inequalis causes apple scab
Within-row mixtures 7-21 reduction in
incidence over 2 yrs, 35 reduction in severity
in second yr, compared to monoculture of S
cultivar
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Will host diversity select for super-races?

• super-races pathogens with complex virulence
  (i.e., virulence to multiple R genes) and high
  fitness
• Experimental data are scarce
• A multiline favored wheat rust strains with
  intermediate numbers of virulences (Kolmer, 1995,
  Can. J. Bot. 7310811088)

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Chin and Wolfe (1984, Plant Pathol. 33535-546)
planted pure stands and mixtures of 2 or 3
barley cultivars (H, W, M), each with a different
gene for mildew resistance

• In pure stands, pathogen genotypes with simple
  virulences were more fit than those with more
  complex virulence.
• In mixtures, the mean fitnesses of simple
  genotypes over both cultivars were reduced
  relative to the complex genotype.

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Pure
Mixture

• Chin and Wolfe, Fig. 1. , 1978 trial, isolates
  collected from H, W, or M 31 and 53 d from
  sowing.
• Changes in the frequencies of different virulence
  combinations on Hassan, Wing, and Midas in pure
  stands (HH, WW, MM) and in mixed stands
  (HWM
• On H selection for simple races, even in
  mixtures
• On W complex races increased in frequency,
  though simple race predominated
• On M mixed results, but complex races dominated

H
W
M
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Summary Chin and Wolfe

• Contrary to Vanderplank Rate of decline of an
  unnecessary virulence gene depends on host (e.g.,
  W virulence on H and M)
• Simple races usually fitter in pure stands, but
  relative fitness of complex races increased in
  mixtures because of their greater flexibility
• Although mixtures encouraged gradual selection
  for complex virulence, absolute numbers of
  complex genotypes were reduced due to reduction
  in population size.
• However, if same host mixture is grown
  continuously several seasons, there may be
  selection for modifiers that improve performance
  of complex pathogen types.
• Evolution of super-races?

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Pathogen evolution in mixtures

• Other mechanisms besides a cost of virulence may
  counter super race emergence in mixtures
  (Mundt, 2002 Lannou and Mundt, 1997, TAG
  94991-999)

• Quantitative adaptation to host genetic
  background disruptive selection
• Effects on relative fitness of
• Pathogen genotype frequencies
• Pathogen density
• Immigration

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Mixtures of cultivars with different levels of
partial resistance

• Models predict these mixtures can increase,
  decrease, or have no effect on disease severity
  relative to component pure stands (Jeger et al,
  1981, Ann. Appl. Biol. 98187-198)
• Benefit if cultivars differ in infection
  frequency (IF) or sporulation rate (SR)
• If differ in both IF and SR, and are ranked same
  by both, benefit is greater
• If differ in both IF and SR, and are ranked
  differently, disadvantage can occur

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• Most empirical studies of mixtures of susceptible
  and partially resistant cultivars are of
  splash-dispersed necrotrophs on cereals
• Rynchosporium secalis on barley
• Stagonospora nodorum on wheat
• Bipolaris sorokiniana on wheat
• Mycosphaerella graminicola on wheat
• Results on average, relatively low levels of
  disease control. A few larger reductions similar
  to those expected for race-specific, obligate
  pathogens.

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Effects of wheat cultivar mixtures on epidemic
progression of Septoria tritici blotch and
pathogenicity of Mycosphaerella graminicola,
2002, Cowger and Mundt, Phytopathology 92617-623.
Percent disease
1998
1999
Inconsistent performance of a mixture confronted
with a splash-dispersed epidemic depending on
environment
2000
Percent disease
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Pathogen evolution in mixtures the bottom line

• A given R gene should be more durable deployed in
  a mixture than a monoculture
• Complex pathotypes may be favored in mixtures
  (especially wind-dispersed biotrophs with
  major-gene resistance?)
• This selection of complex types can be slowed by
  rotation of components in space and/or time
• Evolution to complexity should be more than
  offset by reduction in inoculum loads