Master Gardener General Plant Pathology Talk

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Use of Plant Varieties to Manage Plant Diseases
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Plant Reactions to Pathogens
Interaction Phenotype appearance of host
infected by pathogen at a given point of time and
growth stage of host
Highly susceptible
Complete resistance
Partial resistance
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avirulence
virulence
Gene-for-Gene theory
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The arms race explained
An avirulence gene mutates so that its product
is no longer recognised by the host resistance
gene.
It therefore becomes a virulence gene relative to
the host, and the pathogen can infect.
The host resistance gene mutates to a version
which can detect the elicitor produced by the new
virulence gene.
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Multiline of 6 resistance phenotypes
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In some cases, more than one host gene
contributes to resistance

• Referred to as polygenic or quantitative
  resistance
• Many genes contribute to resistance
• If function of just one gene is impaired,
  resistance is not lost completely
• The genes might encode information for making
  defense structures or toxic substances

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Race specific resistance Complete resistance
Non-race specific resistance Partial resistance
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Monocyclic vs. Polycyclic Diseases
Polycyclic Disease
Monocyclic Disease
Host Variety Can Influence Disease Progress
Curves for Monocyclic and Polycyclic Diseases
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LE 38-12
Just a few parenchyma cells from a carrot gave
rise to this callus, a mass of undifferentiated
cells.
The callus differentiates into an entire plant,
with leaves, stems, and roots.
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LE 38-13
50 µm
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Figure 38-14
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LE 38-16
Genetically modified rice
Ordinary rice
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Figure 38-15
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Plum pox virus
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Plum pox virus resistance due to insertion of
PPV coat protein gene
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Courtesy of Marc Fuchs, Cornell University, NYSAES
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2005 Field Trials Damage is from natural
infections
Transgenic CZW-3
Control
Courtesy of Marc Fuchs, Cornell University, NYSAES
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Zucchini yellow mosaic virus
Cucumber mosaic virus
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Yellow Squash and Zucchini

• Asgrow Seed Co. develops variety resistant to
  zucchini yellow mosaic virus
  and watermelon mottle virus 2, 1995
• Asgrow transferred the GE virus-resistance to
  zucchini
• by conventional breeding, 1996
• Asgrow achieves resistance to cucumber mosaic
  virus,
• 1997
• First triple virus resistant variety planted by
  farmers, 1998

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What Happened and Why

• Yellow Squash and Zucchini
• Adoption of Asgows GE variety very limited
• Multiple virus problem virus infection usually
  several viruses
• at once
• If protection against some, but not all, viruses
  same control
• measures are needed so little cost savings
  to offset high priced seed

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What Happened and Why Successful Crops
of Acreage Planted to GE Varieties In U.S. in
2003
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Lessons Learned From Successful Crops

• Success of
  Field Crops
• Limited Species Diversity
• Dozens of different lettuce varieties each
  transformation
• requires separate regulatory approval
• Size of Market Matters
• Benefits Benefit per acre Number of acres
• Cost of RD from Discovery to Release gt 100
  million
• Feed grain buyers are farmers comfortable with
  technology
• Fiber crops (cotton) food safety concerns do not
  apply
• Large savings economically and environmentally
• Large potential profits, increasingly
  concentrated industry

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Molecular Plant Breeding Partial
resistance Multiple R genes Strong
environmental effect on IP
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Puccinia coronata Crown rust
Puccinia graminis Stem rust
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LE 39-31
Signal
Signal transduction pathway
Hypersensitive response
Signal transduction pathway
Acquired resistance
Avirulent pathogen
R-Avr recognition and hypersensitive response
Systemic acquired resistance