Epidemiology of plant diseases. Collected by Syed Zahid

2. Epidemiology of Plant diseases
Epidemic
When a pathogen
• spreads to and affects many individual;
• Within a certain area and
• within a relatively short time, this type of
phenomenon is called an epidemic.
An epidemic has been defined as any increase of
disease in a plant population.

3. Epidemiology:
• The study of epidemics and of the factors that
influence them is called epidemiology.
Epidemiology deals with the interaction of host,
pathogen and environmental factors that leads to
an epidemic disease.
• Plant disease epidemics; sometimes called
epiphytotics, occur annually on most crops in
many parts of the world. Most epidemics are
more or less localized and cause minor to
moderate losses.

4. Examples
• Irish potato famine of 1845-1846 was caused
b the Phytophthora late blight epidemic of
potato.

5. Examples
• The Bengal famine of 1943 was caused by Bipolaris
oryzae ( Syn.Name; Cochiobolus/Helminthosporum)
brown spot epidemic of rice.

6. The Bengal Famine

7. The elements/component of an
epidemic:
• Plant disease epidemics develop as a result of the
timely combination of the elements that result in
plant disease:
• 1. Susceptible host plants
• 2. A virulent pathogen and
• 3. Favourable environmental conditions over a
fairly long period of time
• 4. Human may unwittingly help to initiate and
develop epidemic through some of their activities
(eg. By topping or pruning plants in wet weather).

8. Interaction among the component
• Interaction of the four components can be visualized as
a tetrahedron, or pyramid, in each plane represents
one of the components. This figure is referred to as the
disease tetrahedron or disease pyramid.
• So, the components/elements of epidemic:
• 1. Susceptible host
• 2. Virulent pathogen ..
• 3. Favourable environment
• 4. Time and
• 5. Human

9. Disease Triangle
• The interaction of the three components like
host, pathogen and environment may be
visualized as a triangle where each arm of the
triangle represents one of the three components.

10. Disease tetrahedron:
The interaction of the four components viz. host, pathogen,
environment and time can be visualized as a tetrahedron or
disease pyramid where each plane represents one of the
components. If the four components of the disease
tetrahedron could be quantified, the volume of the
tetrahedron would be proportional to the amount of the
disease on a host population.

12. Managed Pathosystem:
• Disease development in cultivated plants is
greatly influenced by a fifth component: human.
• Human can play a role on the other
components/factors, such as selection of the
host, planting materials, changing
planting/sowing time, irrigation, fertilization,
spraying of pesticides etc.

13. Managed Pathosystem:
• They also modify the effect of environment on
disease development by delaying or speeding up
planting or harvesting, by planting in raised beds
or in more widely spaced beds, by protecting
plant surfaces with chemicals before rains, by
regulating the humidity in produce storage areas,
etc.
• Thus proper interactions of human with the other
four factors could be the way of proper
management of plant pathogen. This
phenomenon is known as managed pathosystem.

14. Forms of epidemic:
Based on occurrence, mode of spread and severity of
infection, the epidemic may be defined in various
ways:
• Local: When an epidemic occurs in a particular area.
• Regional: When an epidemic occurs over a wide areas
or regions.
• Endemic: An epidemic which has established in a
particular area for a long time and more or less
appears constantly from year to year. Example: Gray
spot of mustard in Bangladesh caused by Alternaria
brassicicola and A. brassicae.

15. Forms of epidemic:
• Pandemic: When an epidemic occurs over most
of a continent and causes mass mortally;
Example: Late blight of potato caused by
Phytophthora infestans (lrish famine of 1845).
• Sporadic: An epidemic that occurs over a large
area with irregular intervals and location, and in
relatively fewer populations.
• Cyclic: An epidemic which appears periodically
after certain intervals of time in a particular area.