Plant Disease Epidemiology

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Plant Disease Epidemiology

• --03-3 Disease Epidemic

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Epidemic disease increase in a population
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Epidemic development is influenced by

• Environmental factors
• Rate of pathogen reproduction
• Mode of pathogen dispersal
• Efficiency of pathogen survival
• Level of aggressiveness of the pathogen
• Level of host plant resistance

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What is epidemiology used for?

• 1 Model disease progress

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A monocyclic pathogen completes just one disease
cycle per season. There are no secondary disease
cycles.Can you think of some examples of
monocyclic pathogens?
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Soilborne pathogens are usually monocyclic due to
physical constraints--inoculum is not dispersed
within the growing season.
Verticillium wilt of strawberry
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Some rust and smut fungi are monocyclic because
their life cycles take a full season to complete.
Cedar-apple rust
Oat smut
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Polycyclic pathogens have several secondary
disease cycles each season.
Oat stem rust
Halo blight
Soybean mosaic
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Disease increase in plant populations is
sometimes compared to the increase of invested
capital over time.

• Diseases caused by monocyclic pathogens are
  analogous to investment with simple interest
  diseases caused by polycyclic pathogens are
  analogous to investment with compound interest.

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With simple interest, capital grows at a constant
rate (the interest bearing capital remains
unchanged)
MONEY
YEARS
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With compound interest, invested capital grows at
an increasing rate over time as the earned
interest is reinvested.
MONEY
YEARS
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The mathematical formula describing increase of
capital at continuous compound interest is the
same formula used in biology to describe
exponential growth. The formula can also be
applied to epidemics caused by polycyclic
pathogens.
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Exponential Disease IncreaseCompound-interest
DiseasePolycyclic Pathogen
X amount of disease at time t X0 initial
disease/primary inoculum (at time t 0) e
2.73 (base of natural log) r rate of disease
increase t time period
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EXPONENTIAL GROWTH
LOGISTIC GROWTH
Disease
Time
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In reality, polycyclic diseases increase
exponentially only during the very early stages
of an epidemic (lt 5 disease).Why do you think
this is so?
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Reasons why disease does not increase
exponentially for long

• As the amount of healthy tissue becomes limiting,
  the rate of disease increase slows down.
• Infection occurs intermittently, not
  continuously.
• Newly infected tissue is not immediately
  infectious.
• Disease tends to occur in foci, not evenly
  throughout the field.

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Disease progress curve for a typical polycyclic
pathogen is an S-shaped curve.
LOGISTIC GROWTH
Disease
Time
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Logistic GrowthCompound-interest
DiseasePolycyclic Pathogen
r apparent infection rate t time x2
proportion disease at t2 x1 proportion disease
at t1
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Disease progress curve for a typical monocyclic
pathogen
DISEASE SEVERITY
TIME
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Mathematical model for monocyclic diseases
rm infection rate (monocyclic) t time x2
proportion disease at t2 x1 proportion disease
at t1
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What is epidemiology used for?

• 1 Model disease progress
• 2 Assess effectiveness of alternative control
  measures

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Progress of late blight in plots of susceptible
(Hudson) and resistant (Sebago) potatoes.
Hudson r 0.43
PROPORTION DISEASE
Sebago r 0.21
10
20
30
50
40
DAYS
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Effect of different fungicide dosages on
development of potato late blight. (Cultivar
Russet Rural weekly applications in kg/ha.)
0.00
0.22
PROPORTION DISEASE
0.67
1.79
10
20
30
50
40
DAYS
(Fry, W. E. Phytopathology 65908)
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Effect of different levels of weekly fungicide
applications on Late Blight epidemics in plots of
Russet Rural and Sebago potatoes.
(Fry, W. E. Phytopathology 65908)
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Effects of fungicide dose on yield of Sebago and
Russet Rural potatoes under conditions favorable
for Late Blight.
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Sebago
Difference is approximately 0.25 kg/ha/week
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Yield of max.
Russet Rural
50
25
0
0.5
1.0
Fungicide (kg/ha)
(Fry, W. E. Phytopathology 65908)
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The cultivar with a higher level of general
resistance requires less fungicide to produce an
acceptable yield.If weekly applications of 1.75
kg/ha gave adequate control with Russet Rural, we
could expect that 1.50 kg/ha would be sufficient
for Sebago.
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Example How much lettuce mosaic virus infection
can be tolerated in lettuce seed lots?

• r 0.12 per day
• Season is 70 days long
• 1 infection tolerated in crop at harvest

We will use this equation to determine tolerance
level in a 30,000 seed assay.
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Lettuce mosaic virus seed assay (cont.)
1.00 X0(4447)
0.0002 x0
1 in 30,000 0.003, so seed lots must have 0
in 30,000 to pass certification.
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Any disease management practice is aimed at doing
one of three things

• Reducing the amount of initial inoculum (X0)
• Reducing the infection rate (r)
• Reducing the time disease is allowed to develop
  (t)

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Choose a typical polycyclic disease

• Describe a disease management strategy which will
  reduce the amount of initial inoculum.
• Describe a disease management strategy which will
  reduce the rate at which disease develops.
• Determine how each of these management strategies
  will affect the disease progress curve.

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Reduce X0
No control
Disease
Reduce r
Time
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What is epidemiology used for?

• 1 Model disease progress
• 2 Assess effectiveness of alternative control
  measures
• 3 Predict disease spread

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Disease Gradient change of disease with distance
from a source
Pathogens A- splash dispersed B- large spores,
wind dispersed C- small spores, wind
dispersed
Disease
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Spatial patterns of disease
Random
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Clustered (point source)
Clustered (line source)
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x x x x x x x x x x x x x x x x
x x x x x x x x x x x x x
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What is epidemiology used for?

• 1 Model disease progress
• 2 Assess effectiveness of alternative control
  measures
• 3 Predict disease spread
• 4 Predict yield loss

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Yield loss models
Stem rust
Powdery mildew
Yield loss (ton/ha)
Southern leaf blight
Disease severity
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What is epidemiology used for?

• 1 Model disease progress
• 2 Assess effectiveness of alternative control
  measures
• 3 Predict disease spread
• 4 Predict yield loss
• 5 Disease forecasting

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Relationship of temperature and duration of leaf
wetness to infection by ascospores of Venturia
inequalis
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Hours of continuous leaf wetness
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20
10
10
15
20
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Temperature (C)