Host Plant Resistance and Conservation of Genetic Resources

1
Host Plant Resistance and Conservation of Genetic
Resources
2
Crop Cultivars

• Improved agronomic characters (yield, durability,
  timing and uniformity of harvest)
• Resistance to pests and diseases (resistant
  cultivars available in many plant species vs many
  different kinds of plant pests)

3
Adaptations of Pests Overcoming Resistance

• To pesticides (many examples with insects)
• To resistant cultivars
• To cultural/agronomic practices

4
Resistance Management

• Important so pests dont overcome resistance
• Important to minimize selection pressure to slow
  development of resistant pests
• Use IPM, reduced dosages, rotation of methods,
  different cultivars, etc.
• Avoid overusing same method or cultivar

5
Soybean Cultivars and Resistance to Nematodes
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Soybean Cyst Nematode and Resistant Soybean
Cultivars

• Soybean cyst major pest of soybeans in midwest
  US
• Many isolates
• Managed by soybean cultivars resistant to a
  specific isolate
• Use of same resistant cultivar for 2-3 yrs in a
  row selects for new isolates and breaking of
  resistance

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Soybean Cyst Nematode and Resistant Soybean
Cultivars

• Soybean cyst major pest of soybeans in midwest
  US
• Many isolates
• Managed by soybean cultivars resistant to a
  specific isolate
• Use of same resistant cultivar for 2-3 yrs in a
  row selects for new isolates and breaking of
  resistance
• Solution rotate soybean cultivars, using
  resistant cv only one year at a time. e.g.,
  nonhost --- resistant cv --- susceptible cv

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Costs of obtaining/utilizing resistant or
high-yielding cultivars
Wild relatives Crop species Selection Plant
breeding Cultivar (variety)
Germplasm
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Costs of obtaining/utilizing resistant or
high-yielding cultivars
Wild relatives Crop species Selection Plant
breeding Cultivar (variety)
Biotechnology accelerates development of new
cultivars
Germplasm
10
Costs of obtaining/utilizing resistant or
high-yielding cultivars
Wild relatives Crop species Selection Plant
breeding Cultivar (variety)
Crop genetic diversity decreases
Agronomic performance increases
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Costs of obtaining/utilizing resistant or
high-yielding cultivars

• Cultivar has desirable attributes (high yield,
  resistance to some pest)
• Potential cost is that because of loss of genetic
  diversity, it may become susceptible to other
  problems
• Most breeding programs now recognize this, and
  include broader genetic base in new cultivars

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Unimproved germplasm can have high genetic
diversity (high variability)
Wild relatives Crop species Selection Plant
breeding Cultivar (variety)
Germplasm
13
Least genetic diversity in vegetatively
propagated plants (clonal propagation)
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Genetic Diversity (not same as HPR)

• Advantages
• Mixed genotypes can provide future source for
  developing new resistant cultivars
• Mixed crop genotypes can slow epidemics,
  invasion of new pests, withstand biological and
  environmental events

15
Genetic Diversity (not same as HPR)

• Disadvantages
• - - Lower yields than genotypes selected for high
  yield
• - - Less effective than resistant cultivars
  against single pests

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Maintaining Genetic Diversity

• On farm, if mixed crop genotypes are planted
  genetic diversity maintained within plant
  population (e.g., local crop varieties in many
  tropical areas)
• Within region, if different cultivars are planted
  genetic diversity among plant populations
  (e.g., some US crops like corn, that have many
  cultivars)

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Classical examples of lack of genetic diversity

• Potato produced from tuber seed pieces clonal
  propagation low genetic diversity
• Potatoes in Ireland consisted of two genotypes,
  introduced on two clones of potatoes in 1500s
• Became highly successful and productive crop,
  supported human population growth

18
Classical examples of lack of genetic diversity

• Potato produced from tuber seed pieces clonal
  propagation low genetic diversity
• Potatoes in Ireland consisted of two genotypes,
  introduced on two clones of potatoes in 1500s
• Became highly successful and productive crop,
  supported human population growth
• Potato late blight fungus (Phytophthora
  infestans) appears in 1840s
• Both clones highly susceptible to late blight
• Irish potato famine results

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Classical examples of lack of genetic diversity

• Potato produced from tuber seed pieces clonal
  propagation low genetic diversity
• Potatoes in Ireland consisted of two genotypes,
  introduced on two clones of potatoes in 1500s
• Became highly successful and productive crop,
  supported human population growth
• Potato late blight fungus (Phytophthora
  infestans) appears in 1840s
• Both clones highly susceptible to late blight
• Irish potato famine results

South America great diversity of potato clones,
epidemics dont establish
20
Classical examples of lack of genetic diversity
Green Revolution, 1960s

• High yielding wheat and rice cultivars (high
  genetic uniformity)
• Widely planted in Tropics
• Uniform genotypes highly susceptible to pests and
  environmental conditions

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Conservation of Genetic Resources

• Gene banks International Research Centers,
  research stations, etc., maintain plant germplasm
  (seeds, other plant materials, etc.)
• Habitat and community-based conservation wild
  crop relatives maintained in natural habitats,
  borders of fields, etc.

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Conservation of Genetic Resources

• Gene banks International Research Centers,
  research stations, etc., maintain plant germplasm
  (seeds, other plant materials, etc.)
• Habitat and community-based conservation wild
  crop relatives maintained in natural habitats,
  borders of fields, etc.

Political controversy Who owns/controls genetic
resources?
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Future Host Plant Resistance

• Development of resistant plant cultivars is an
  ongoing process
• Need to be prepared to develop new cultivars
  before pests overcome resistance in the old ones
• Always looking for new sources for resistance and
  for cultivar development
• Need to insure that these genetic sources will be
  available in the future

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References

• Text, Ch. 9, esp. pp. 148-150, 155-162.
• Carroll et al., 1990. Ch. 6 (pp.173-177), Chs.
  16,19.
• Tivy, 1992. Ch. 6 (pp. 106-107).