plant disease epidemiology

1. WELCOME
PPATH 503: Epidemiology and
Forecasting of plant disease
K. M. GOLAM DASTOGEER
LECTURER
DEPARTMENT OF PLANT PATHOLOGY
BANGLADESH AGRICULTURAL UNIVERSITY

2. Epidemic
Gr. Epi=upon, among and Demons=people
Epidemic What is among people
"Change in disease intensity in a host population over time and
space."
Change: often increase -- a dynamic process
Disease: dealing with diseases, not just the pathogen (or plant/crop)
Host: Organism infected (or potentially infected) by another organism
Population: a population phenomenon
Time and space: two physical dimensions of interest.

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Epiphytotic Unger (1833), Whetzel (1920's)
However, equally valid meaning from Greek:
"what is in (or among) a population" ("demio")
"Epidemic" used for plants for a long time…..
•1728: Duhamel 1691,1842: book titles
•1858: Kuhn 1901: Ward
Thus, no valid reason to use "epiphytotic“
Therefore the issue has been resolved!!!
NOTE:
If one used epiphytotic
(instead of
epidemic), then one
should use
epiphytotiology instead
of epidemiology!
-(Epiphytology is the
study of epiphytes).

4. Epidemiology
• Study of epidemics.
• Science of disease in populations. Vanderplank (1963)
• Ecology of disease.
• Study of the spread of diseases, in space and time, with the objective to trace
factors that are responsible for, or contribute to, epidemic occurrence.
• The science of populations of pathogens in populations of host plants, and the
diseases resulting therefrom under the influence of the environment and human
interferences.
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5. History (ancient to modern times)
Disease
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Hippocrates (~400 BC): First use of "epidemic", widespread disease (human diseases
Theophrastus (~340 BC): Plant diseases in fields, Environmental influences
Pliny (~50 AD): Plant diseases; soil; climate
Duhamel de Monceau (1728 AD): Disease progress curves, Comparison of plant and
animal epidemics
Late 19th Century and forward…
Kuhn (1858) - 1st textbook of plant pathology
Ward (1901): book "Diseases in Plants" emphasized ecology (populations) of disease
Jones (1913) - role of the environment
Gaumann (1946): "Principles of Plant Infection” -Disease spread, -Conditions leadin
to an epidemic, -'Infection Chain' (= disease cycle), -compare with medicine (diseases
of humans)

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Large (1952, and others)
-Disease progress curves
-Crop losses
-Disease assessment (measurement)
Horsfall & Dimond (1960)- "Plant Pathology, Volume 3"
-Populations
-Inoculum density:disease relations
-Spore dispersal
-Analysis (mathematics)
-Forecasting, prediction
-Traditional definition ---> Modern definition
Gregory (1963, 1973)
"The Microbiology of the Atmosphere"
-spore dispersal, disease spread
Aerobiology
Vanderplank (1963) (used to be van der
Plank)
"Plant Diseases: Epidemics and Control"
-Populations
-Rates (dynamic processes)
-Analysis, mathematics
-Models, theory
-Link epidemiology and control
-Established the science of plant disease
epidemiology
Other pioneers:
Zadoks (1960-1995), The Netherlands
Kranz (1968-1995), Germany
Waggoner (1960-mid --1980s), USA
S. Nagaranjan 1983-India
Note: many developments in other fields…
Ecology, medical epidemiology,
Biomathematics, etc.

7. Elements of an Epidemic
1.Host
2. Pathogen
3. Environment
Interactions of the 3 main components
are described by the disease triangle.
The Disease Triangle
Disease development is also affected by
4. Time
5. Humans
Disease Tetrahedron
Interactions of the 5
components are
described by the disease
pyramid.

8. Elements of an Epidemic (cont’)
i. Genetic resistance or susceptibility of Host
–Vertical Resistance
–Horizontal Resistance
ii. Degree of genetic uniformity of host in a particular field
–Monoculture, especially Clones
–Natural, Intermingled Populations
iii. Type of crops
- Annual crops & foliar or fruit diseases develop much
more rapidly (in weeks)
- Perennial woody diseases take longer time to develop
(in years)
iv. Age of host plants
- Some plants are susceptible only during growth period
& become resistant during mature period
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How the Plant Affects Development of Epidemics

9. How Pathogens Affect Development of Epidemics
i. Levels of virulence
–Faster Production of Larger # Inoculum
ii. Quantity of inoculum near hosts
iii. Type of reproduction of the pathogen
–Monocyclic
–Polycyclic
•Responsible for most Sudden, Catastrophic Epidemics
–Polyetic
iv. Ecology of the pathogen
–Reproduce on Surface of Aerial Parts of Plant
–Reproduce inside Plant
–Reproduce on Infected Plant Parts in Soil
v. Mode of spread of the pathogen
–Breezes or Strong Winds
•Most Sudden & Widespread Epidemics
–Inoculum Carried by Airborne Vectors
–Wind-Blown Rain
–Carried on Seed, Tubers, Bulbs
–Beetles
–Pathogens Spreading through Soil
•Usually Local, Slow-Spreading Diseases of Considerable Severity
Elements of an Epidemic (cont’)

10. Elements of an Epidemic (cont’)
3. Environmental factors
i. Moisture
- Rain, dew, high humidity
- Dominant factor in diseases caused by
oomycetes, fungi, bacteria & nematodes
ii. Temperature
- Affects disease cycles of pathogens
Disease development is also
affected by
4. Time
Time factors
Season of the year
Duration & frequency of favorable temp. &
rains
Appearance of vectors, etc.
5. Humans
 Site Selection & Preparation
 Selection of Propagative Material
 Introduction of Exotic Pathogens
 Cultural Practices
 Disease control measures
I ntroduction of new pathogens or disease
How Humans Affect Development
of Epidemics

11. Monocyclic pathogen
A monocyclic pathogen completes
just one disease cycle per season.
primary inoculum is the only
inoculum available for the entire
season, and there is no secondary
inoculum and no secondary
infection.
 Examples of Monocyclic
Diseases: Blackleg of potato
(Erwinia caratovora),
Verticilliumwilt, Cereal Cyst
Nematode.
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Can you think of some examples
of monocyclic pathogens?

12. Monocyclic Disease
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• Examples: smuts, rusts, which require two alternate hosts,
many soil-borne diseases, root rots and vascular wilts
• In general, there are three types of plant diseases that tend
to produce only one infection cycle per host cycle (1)
postharvest diseases, (2) diseases caused by soil-borne
plant pathogens, and (3) rusts without a urediniospore
stage.

13. Polycyclic pathogens/Disease
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Pathogens that produce more than one (2 to 30) infection cycle per
crop cycle
Disseminate primarily by air or airborne vectors (insects)
Responsible for epidemics on most crops
downy mildews, late blight of potato, powdery mildews, leaf
spots and blights, grain rusts, and insectborne viruses.

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15. Polyetic(multiyear) pathogens
In some diseases of trees,fungal vascular
wilts,phytoplasmal declines, and viral
infections, pathogen may not complete
a disease cycle, it may not produce
inoculum that can be disseminated and
initiate new infections, until at least the
following year and some may take
longer.
• Several rusts of trees and the
mistletoes,they attak several years to go
through all the stage sof their life cycle
and to initiate new infections. Dutch elm
disease, cedar apple rust, white pine
blister rust, and citrus tristeza
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16. Disease Progress Curve
• The representation of plant disease over time is
referred to as a “Disease Progress Curve”.
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WHY?
 Compare control measures
‐ Compare effect of environment on disease development
‐ Predict future disease development
‐ Disease forecasting for improved control

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Disease progress curve for a typical monocyclic pathogen

18. Progress curve for polyclic epidemic
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Measuring Disease in a Population
Disease incidence
Actual number or proportion of plants diseased
Number diseased out of total number of plants
observed
Disease severity
•Area of plant tissue affected by disease
•For many diseases, severity is the area of plant surface covered by lesions
•Measured using assessment scales or by determining the area under a disease
progress curve (AUDPC)
3. Yield loss
•The proportion of yield that the grower will not be able to harvest due to disease
•Results in economic loss

22. Disease gradients:
• Change in disease
severity along a straight
line away from the
source of inoculum.
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23. Epidemic Manganement
• Ways to reduce disease
(inoculum) at beginning (xo)
• Affects monocyclic and polycylic
diseases
• Fumigation Certified seed
• Sanitation Seed treatments
• Quarantine Host plant
resistance
• Ways to decrease the rate of disease
development (infection rate) (r)
• Change the environment
• Fertilizer application
• Pesticide Application
• Host plant resistance
• Ways to change t
• Harvest early before disease becomes
severe.
• Plant early (cereal cyst nematode)
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24. Importance of Epidemiology
• The epidemiological study is essential for the formulation of control
measures for which no resistant variety is available and the farmers have to
rely of the chemicals and other sanitary measures.
• It undergirds the technology to disease management through understanding
disease behaviour over time and space.
• It accomplishes perspective governing disease management scheme.
• It protects bases for evaluating for an general disease management
scheme.
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1. Molecular tools
Polymerase Chain Reaction (PCR), Enzyme
Linked Immunosorbant Assay (ELISA), DNA
Fingerprinting, etc.
For rapid & accurate detection & identification
of pathogens
2. Data management
Geographic Information System (GIS), Global
Positioning System (GPS), Remote Sensing, etc.
To assist in disease control strategies
3. Disease modeling & forecasting
To predict the probability of outbreaks
New Tools in Epidemiology

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Books Recommended

27. Two more books
• Plant disease Epidemiology–S. Nagaranjan-India
• A text book of plant pathology- H.
Ashrafuzzaman-Bangladesh
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28. THANKS
For Your
Kind Attention
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