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Website to brush up on bacterial
diseases
Bacterial spot of pepper and tomato
http://www.apsnet.org/edcenter/intropp/lessons/prokaryotes/Pages/Bacterialspot.aspx
Potato blackleg and soft rot
http://www.apsnet.org/edcenter/intropp/lessons/prokaryotes/Pages/Blacklegpotato.aspx
Crown gall
http://www.apsnet.org/edcenter/intropp/lessons/prokaryotes/Pages/CrownGall.aspx
Fire blight of apple and pear
http://www.apsnet.org/edcenter/intropp/lessons/prokaryotes/Pages/FireBlight.aspx
Ice nucleation phenomenon
Catalysis of ice crystal
formation in supercooled
water has been
Droplet
demonstrated for some
plant pathogenic bacteria
e.g. Pseudomonas
syringae
freezing assay:
In this sample, icenucleating bacteria
were not present. The
droplets supercool but
do not freeze.
In this
sample,
icenucleating
bacteria
were
abundant,
and ice
was
formed
These epiphytic bacteria
initiate ice formation at
temperatures higher than
normally required.
A gene in the bacterium
codes for protein in cell
wall that confers ice
nucleating ability.
Ice damage creates wounds
Types of Symptoms
The temperature of the wax
covered foil is -3°Celcius
Cankers
Pseudomonas
syringae
Necrosis of woody stem
tissue, usually involves
phloem tissue
Bark of stem appears
sunken with sharp
borders
Often initiated in early
spring with epiphytic P.
syringae populations
promoting freezing/
thawing injury
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Field application of INA-minus
Pseudomonas syringae for frost
protection. It was one of the first
field releases of a geneticallyengineered microorganism
Biological
control
of frost
injury
From G.L. Schumann 1998
Ice nucleation protein
of Pseudomonas syringae
is sold to ski areas to improve
efficiency of snow making
http://www.johnsoncontrols.com/publish/us/en/products/building_efficiency/market/snowmaking/snomax.html
Overwintering of bacterial pathogens
There are no specialized survival structures
Methods of overwintering:
In host or in seed or in clonal planting stock
Most common
On host as epiphyte
Pseudomonas spp. (bacterial canker of woody plants)
Free living in soil, water or debris (generally rare)
Crown gall pathogen and bacterial scab pathogens of
root & tuber crops (soil), soft rotters (ponds, oceans)
How bacteria cause disease
The typical life strategy of a bacterial plant pathogen is either
facultative parasite or facultative saprophyte
Many survive grow and reproduce
saprophytically (no disease) on plant
surfaces. This growth is called
‘epiphytic’ (upon the plant).
Entry into plants is through
wounds or natural openings:
Bacterial colony growing
‘epiphytically’ on a leaf surface
Epiphytic growth is favored
by a humid environment
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What does it mean to be an epiphyte?
• It is the ability of bacteria (and some fungi) to grow and
reproduce saprophytically on plant surfaces
jargon: to live in the ‘rhizosphere’ (on roots)
or in the ‘phyllosphere’ (on a foliar surface)
or within a ‘biofilm’
Bacterial colony growing
‘epiphytically’ on a leaf surface
• Typically, with bacterial pathogens,
an epiphytic phase occurs
prior to infection
• Epiphytic growth is favored by a humid environment
• A bacterial epiphytic phase has been shown to influence a
plant host’s sensitivity to frost injury –
ice nucleation phenomenon
• Epiphytic (preinfection) phases of bacterial (and fungal)
pathogens are frequently susceptible to biological control
Bacterial pathogens possess the ability to grow and
reproduce on or in a plant without necessarily
inciting disease
Bacterial colony growing
‘epiphytically’ on a leaf surface
This growth is called:
‘epiphytic’ growth (upon the plant)
or ‘endophytic’ (within the plant).
Fungal
Pathogens
Non-pathogenic
bacterial epiphytes
associate superficially
TTS
TTS
TTS
TTS
Pathogenic
bacterial epiphytes
associate more intimately
Plant tissue
Effect of an H202 treatment on
survival of epiphytic bacteria:
Pathogen strains
Wilson, Hirano, & Lindow 1999
AEM 65:1435-1443
Non-pathogen strains
Bottom line: Pathogens are specialized to associate intimately with their host
-- this association make biocontrol hard to achieve
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Biological control
Definition:
- reduction of the amount of inoculum or disease producingactivity of a pathogen by or through one or more organisms
other than man
Cook & Baker 1983
How is it accomplished?:
- Introduce saprophytic microorganism to plant environment
- Modify environment or cultural methods to favor specific
saprophytic microorganisms that can accomplish biocontrol
Example : Biocontrol
experiment from lab
Inoculated with
Agrobacterium tumefacians
strains 529
Pretreated with
Agrobacterium radiobacter strain K84
followed by A. tumefacians
Thus, in this example, the biocontrol agent (strain K84) protects the
plant from infection by the pathogen.
In the Oregon nursery industry, it is very common practice to dip the
roots of plants in a suspension of K84 prior to planting.
Question: What is the mechanism of protection?
Important: The mechanism of
suppression influences the quality of
the biocontrol interaction
Mechanisms of biocontrol:
• Competition
• Antibiosis
• Parasitism
Pathogen
Biocontrol
strain
K84
Zone of inhibition is due to diffusion of antibiotic
produced by biocontrol strain K84
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The importance of antibiosis to the
effectiveness of K84
Tomato
Disease incidence
Antibiosis ineffective
Antibiosis effective
Antibiosis ineffective
Antibiosis effective
Cherry
Antibiosis is ineffective
either when K84 doesn’t
make its antibiotic or
when the pathogen has
become resistant to the
antibiotic
Dose of biocontrol agent
Biological control
Works best when:
• The site of infection on plant supports growth of competing
saprophytes (wounds, flowers, seed and root surfaces)
• when the biocontrol agent possesses
multiple tools to suppress pathogen
activity (superior competitor, antibiosis,
ability to induce SAR)
Antibiosis
zone of
inhibition
• when disease is monocyclic and the
A
pathogen is restricted to a short window of opportunity
(wound healing, seed emergence, bloom period)n
t
i
b sclerotia,
• when pathogen exposed is outside the plant (e.g.,
i
hyphae)
o
Course Content Since Midterm I
Theme 1: Disease dynamics
How does disease develop in populations: Polycycle? Monocycle?
What is ‘r’? What is initial disease?
What are the axis labels for a graph relating to speed of increase, how far it moves, or
how many propagules are required to give a certain amount of disease?
What does each type of curve look like?
How does sanitation and host resistance influence disease progress?
When is disease increase fastest: Relatively? Absolutely?
Theme 2: Host resistance
Two major types
How do we know which type we have?
How do we breed and select for resistance?
What is the gene-for-gene concept?
What are pathogen races? How do define and determine number of races?
How do we manage R-gene resistance?
How do we select for non-race-specific resistance?
Theme 3:Basidiomycetes
Describe the disease (life) cycle of a heteroecious, macrocyclic rust.
How do we manage rust diseases with host plant resistance?
Theme 4: Bacteria
What are they? How do we identify them?
How do they cause disease?
What is an epiphytic phase?
What is the significance of different types of secretion systems?
How do bacteria biocontrol agent control bacterial (and fungal) pathogens?
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