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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Host Reactions



Resistance (HR)
Susceptible
Resistance
Huckelhoven, R., et al. Plant Physiol.
19991191251-1260
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NematodesPathogen Reproduction adults or eggs
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Host resistance interferes with the soybean cyst
nematodes life cycle
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Disease EscapePlant canopy not conducive for
infectionPathogen misses infection court
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Ergot of Small Grains Claviceps purpurea Example
of disease escape
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Ascospores to grain florets
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Reaction related to length of time florets are
open during pollination
Rye longest Barley Wheat Oat very short
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Qualitative differences in interaction phenotype
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Chrysanthemum Fusarium Wilt
R
S
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Aphanomyces euteiches and PeaDisease severity
classes used to rate pea lines for partial
resistance
0
1
2
3
4
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Disease Severity Classes
DSI 60
Sum rating for plot 238 DSI
---------------------------------------- x 100 4
x number of plants rated (100)
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Interaction Phenotypes of plant tissue covered
by symptoms

• Chlorosis
• Necrosis

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Interaction phenotypes typical of rust pathogens

• Interaction Phenotypes
• R necrotic flecks
• MR sparse sporulation in flecks
• MS small pustules chlorosis
• S large and sporulating pustules

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Resistance to Viruses

• Complete lack of symptoms
• Latent infection?
• Virus reproduction
• Bioassys
• ELISA

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HR Recognition of pathogen by host
Avr (avirulence gene)
R gene
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LE 39-30a
Signal molecule (ligand) from Avr gene product
Receptor coded by R allele
R
Avr allele
Avirulent pathogen
Plant cell is resistant
If an Avr allele in the pathogen corresponds to
an R allele in the host plant, the host plant
will have resistance, making the pathogen
avirulent.
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LE 39-30b
R
No Avr allele virulent pathogen
R allele plant cell becomes diseased
Avr allele
No R allele plant cell becomes diseased
Avr allele virulent pathogen
No Avr allele virulent pathogen
No R allele plant cell becomes diseased
If there is no gene-for-gene recognition because
of one of the above three conditions, the
pathogen will be virulent, causing disease to
develop.
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LE 39-30
Signal molecule (ligand) from Avr gene product
Receptor coded by R allele
R
Avr allele
Plant cell is resistant
Avirulent pathogen
If an Avr allele in the pathogen corresponds to
an R allele in the host plant, the host plant
will have resistance, making the pathogen
avirulent.
R
No Avr allele virulent pathogen
R allele plant cell becomes diseased
Avr allele
Avr allele virulent pathogen
No R allele plant cell becomes diseased
No Avr allele virulent pathogen
No R allele plant cell becomes diseased
If there is no gene-for-gene recognition because
of one of the above three conditions, the
pathogen will be virulent, causing disease to
develop.
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avirulence
virulence
Gene-for-Gene theory
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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
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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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Reaction of Cultivar Corsoy 79 to Genotypes A
and B of Phialophora gregata
B
Non-inoc.
A
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Reaction of Cultivar Jack to Genotypes A and B
of Pg
Non-inoc.
A
B
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Race specific resistance Complete resistance
Non-race specific resistance Partial resistance
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Puccinia coronata Crown rust
Puccinia graminis Stem rust
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Molecular Plant Breeding Partial
resistance Multiple R genes Strong
environmental effect on IP
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Multiline of 6 resistance phenotypes
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Monocyclic vs. Polycyclic Diseases
Polycyclic Disease
Monocyclic Disease
Host Variety Can Influence Disease Progress
Curves for Monocyclic and Polycyclic Diseases