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1. Epidemiology of crop diseases,
Weather factors and their role in
disease development

2. What is Epidemiology?
Epidemiology or epiphytology is the study of
outbreak of disease, its course, intensity, cause
and effects and the various factors governing it.

3. Environment
Pathogen Host
Disease Triangle
A plant disease is the outcome of interaction between the plant,
pathogen and the environment. The interaction of host, pathogen
and environment constitutes the traditional disease triangle.
Host influences the environment and pathogen where as the
pathogen affects the host alone. If the environment and the host are
not favourable the pathogen is ineffective.

4. All the components of the triangle must be balanced.
Epidemic is the outcome of breakdown of ecological
equilibrium, the pathogen becoming more virulent and the host
is being susceptible to the attack due to the favourable
environment.
Adding time to the disease
triangle as a fourth dimension
gives a disease pyramid.
Time is important in many
ways viz., time of year when
host and pathogen coexist,
time and duration of leaf
wetness, etc.
Disease Tetrahedron

5. Man
Pathogen
Environment
Host
In the fig. host, pathogen and
environment are represented by
one of the sides of triangle.
Time is represented as the
perpendicular line arising from the
centre of the triangle and man as
the peak of the pyramid whose
base is the triangle and the height
is the length of time.
Often, human activities interfere
with epidemics through
management practices and
influence each of the other four
components of disease

6. Disease measurement
Disease incidence: It is the measurement of prevalence of the
disease in the population and it reflects the proportion of plant units
diseased.
Out of 100 plants if 10 plants show disease the incidence is 10%. It
has direct relationship with severity and loss in the case of vascular
wilts, root rot diseases but not in foliar pathogens.
In foliar diseases, percentage or proportion of plant area destroyed
by the pathogen is expressed as disease severity (Per cent Disease
Index), which is closer to the yield loss caused by the disease.
The level of disease (amount of plant damage) at which the
incremental control costs just equal incremental crop return is called
economic threshold.

7. Disease severity is expressed in terms of balance between qualities
and weaknesses of the host and the pathogen
Disease severity = Inoculum potential x Disease potential
= (Inoculum density x capacity) x (Proneness x susceptibility)
Inoculum potential of the pathogen is a function of the number of
infective propagules (inoculum density) and their pathogenic
capacity (inoculum capacity)
Disease potential is concerned with the condition of the host. Due to
unfavourable environments, poor or unbalanced nutrition,
susceptible stage of growth, etc. the host may be predisposed to
attack by the pathogen. This proneness is different from inherent
susceptibility, which is genetically determined.

8. Disease progress curve
Disease incidence and severity as influenced by time and
human interferences can be expressed in progress curves or
mathematical models of the disease development.
When pattern of epidemic in terms of number of lesions, the
amount of diseased tissues or the number of diseased plants is
plotted against time it gives a disease-progress curve.
The amount of disease is maximum near the source of primary
inoculum appearing in the field as infection foci. It decreases
as the distance from the infection foci increases. This is shown
by disease gradient curve.

9. Sigmoid disease progress curve vary according to nature of the
host plant, pathogen and the environment.
A B C
Level of
infection
(%)
Time

10. Course of epidemic / Stages of epidemic cycle
1. Lag phase
The rate of spread of disease is slow
2. Logarithmic / Exponential / Progressively destructive phase
Rate of spread is rapid – availability of susceptible host,
favourable environment leads to vigorous spread of disease.
3. Post-logarithmic phase / Decline phase
Non availability of susceptible stage of the host, unfavourable
environment leads to declining phase.
E.g. North India – Wheat crop attacked by rust during Jan – March.
Epidemics develop during these months.
After March – disease decline due to unfavourable
temperature (rise in temperature), non availability of
susceptible host – aggressiveness of pathogen is reduced.

11. Types of epidemics / epiphytotics
When epidemic develops slowly it is called tardive or slow epiphytotics
When Eidemic develops rapidly it is called explosive or rapid epiphytotics
Slow epiphytotics
Associated with perennial tree populations and the
pathogens are systemic. Pathogen multiplies less rapidly and
spread is also slower. The disease is simple interest disease and the
pathogens are low death rate pathogens. Disease is controlled by
crop sanitation. E.g. Swollen shoot of cocoa.
Rapid epiphytotics
Associated with annual crops. Caused by non-systemic
pathogens with high birth rate. Rate of spread is very fast and it
belongs to compound interest disease. Diseases are controlled by
protective spraying with chemicals.
E.g. Late blight of potato, wheat stem rust.

12. Factors governing epidemics
1. Nature of host
2. Nature of the pathogen
3. Environment
Host Pathogen Environment
Susceptibility of the host
Aggregation and
distribution of
susceptible hosts
Presence of collateral or
alternate host
Introduction of new hosts
Distance from the source
of inoculum
Type of crops
Introduction of a new pathogen
Presence of aggressive strain
of the pathogen
High birth rate of the pathogen
Low death rate of the pathogen
Easy and rapid dispersal of the
pathogen
Adaptability of the pathogen
Temperature
Moisture and
relative humidity
Rainfall
Dew
Light and shade
Wind
Soil factors (pH)

13. 1. Host factors
a. Susceptibility of the host
Late maturing varieties are more susceptible to groundnut leaf
spots when compared to early types
Nursery stage – susceptible to Pythium
Rice crop – heavy N – become susceptible
Cotton – soil temp.26 - 28º C – susceptible to Fusarium wilt
Brinjal – soil temp at 20º C susceptible to Verticillium wilt
b. Aggregation and distribution of susceptible host
Abundance of susceptible host or continuous cultivation or
monocropping helps in the build up of inoculum and cause
epidemic. Eg. Rice – South East Asia – Blast
Sugarcane – red rot epidemic in North India.

14. c. Presence of collateral / alternate host
Grass hosts (Collateral host) of sugarcane downy mildew,
sugarcane smut, rice blast produce abundant inoculum, which aid
in building up of epidemics.
d. Introduction of new host
Introduction of exotic variety caused epidemic
e.g., Cotton (C4 Cambodia) exotic variety caused out break of
bacterial blight (Xanthomonas axonopodis. pv. malvacearum) in
local variety grown in Maharastra.
e. Distance of susceptible plants from the source of inoculum
f. Type of crops: Epidemics develop more rapidly in annual crops
than in perennial woody crops

15. 2. Pathogen factors
a. Introduction of new pathogen
Fire blight of apple (Erwinia amylovora)
Endemic in N. America. It spread USA, Canada (1884) – New
zealand (1919) England (1957)
Mode of transmission through fruit boxes.
Coffee rust – Spread to Sri Lanka (1869) – (Native of Ethiopia
endemic) India (1870) – Sumatra (1876) – Java (1878) –
Phillippines (1889) – Kenya, Congo (1918) – West Africa (1950)
Transmission – wind (spores trapped up to 1000 m MSL)

16. b. Presence of aggressive strain of the pathogen
Aggressiveness – capacity of a parasite to invade and grow in its
host plant and to reproduce on or in it.
Rapid cycle of infection is essential to become aggressive.
Virulence is a measure or degree of pathogenicity in a qualitative
sense
e.g. Wheat black / stem rust of India
More the physiological races/ pathogenic strains in a locality,
more the possibility of occurrence of epidemic

17. c. High birth rate of pathogen
High reproductive capacity - rapid dissemination and epidemic
E.g., Powdery mildew, downy mildew, rust, blast, blight, etc.
Wheat stem rust - 25 trillion uredospores/ ha of wheat crop
64,000 million aeciospores/ a single barberry bush
Grapevine downy mildew –32,000 sporangia / sq. cm
d. Low death rate
Pathogen is protected by plant tissues - low death rate of diseased
planting materials for vegetative propagation.
Chances for occurrence of epiphytotics are very high
Virus and phytoplasma diseases in crops propagated through
vegetative plant parts

18. e. Easy and rapid dispersal of the pathogen
Occurrence of disease epidemics depends on quick dispersal
External agencies – wind, water, insects, mites, nematodes.
Fungal spores – wind
Bacteria – water, insects
Viruses and Phytoplasma – insects, mites, nematodes.
f. Adaptability of the pathogen
Adaptability of pathogen during adverse environmental conditions
Oospores, ascospores, chlamydospores (smut), Sclerotia, etc.,
Bacteria on dead plant tissues
Viruses and Phytoplasma – collateral hosts or insect vectors.

19. 3. Environmental factors
a. Temperature
Temperature plays an important role for development and spread of
pathogen.
High temp (30ºC - 40ºC) – favourable for dry root rot pathogen
Low night temp (below 20ºC) favours rice blast
Less than 20º C favours Verticillium wilt disease also
Loose smut of wheat is severe at 19-20ºC & no disease at 29-30ºC
BLB of rice – optimum temp 25 - 30º C

20. b. Relative humidity and moisture
Free water or RH exceeding 90 % - induce germination of most
fungal spores.
Exception: Conidia of powdery mildew germinate even at 0% RH.
(Because conidium itself contains 70% moisture as against 10% in
most fungal spores). But germination is best at higher RH.
Potato Late blight – More sporulation above 90% RH and not below
80%RH
Wet soil favours Pythium, Phytophthora, Plasmodiophora
(Production and movement of zoospores are favoured by water).
Dry soil favours Fusarium diseases and dry root rots.
Indirect effect
Presence of water – opening of lenticels, cuticle becomes thin –
favourable for the entry of pathogens.
Water stress – increased transpiration, reduced water uptake –
susceptible to facultative pathogens (root rot).

21. c. Rain
Frequent drizzling favours ergot of bajra and sorghum
BLB of rice – Disease develops rapidly when there is a combination
of rainy weather, strong wind and temperature of 22-26oC
Heavy rainfall during September accompanied by favourable
temperature (25-30oC) followed by continuous cloudy weather and
low solar radiation caused epidemic of Brown spot in rice
d. Dew
It favours many fungal pathogens (Late blight of potato, rice blast,
onion purple blotch)
Rice blast – dew period of 6-8 hours highly favours the disease
Onion Purple blotch highly favoured by dew formation

22. e. Light and shade
Light is known to influence the infection of fungal pathogens
Increasing light duration increased sporangial production of bajra
downy mildew. But sporangial liberation occurs under darkness
Tobacco necrosis virus infection is severe under low light intensity.
Under high light intensity the disease is negligible.
Heavy shade increases brown spot of rice and blister blight of tea
f. Wind
Heavy wind causes severe injury to rice leaves which leads to
infection by BLB pathogen.
After heavy wind the disease become serious.

23. g. Soil factors
Soil moisture
Low soil moisture favours potato scab (Streptomyces scabies)
Moderate / low soil moisture favours dry root rot
High soil moisture favours damping off disease
Soil temperature
Verticillium wilt favoured by less than 20oC
High soil temperature (35ºC-40ºC) favours Macrophomina root
rot (dry root rot)
Fusarium wilt favoured at 25-30oC
iii. Soil reaction (pH)
Alkaline soil: Potato scab (Streptomyces scabies) is favoured by
alkaline soil with pH above 8.0.
Gypsum application is recommended for the control
Acid soil
Acid soil favours club root of cabbage, Plasmodiophora brassicae
Lime application is recommended for the control

24. Boom and burst cycle
In varietal improvement, incorporation of monogenic vertical
resistance genes does not last long. When a single gene-based
resistant variety is widely adopted, the impact would be the arrival
of new matching pathotypes.
These pathotypes soon build up in population to create epidemics
and eventually the variety is withdrawn. This phenomenon is
generally called “ boom and burst”.
To avoid boom and burst phenomenon, use of durable host
resistance is advocated in several crops. Durable resistance
remains effective even though it may be widely grown over a long
period of time, in an environment that favours the disease.

25. Epidemic
Frequency
increases
New race
detected
Introduction of
variety with
new
resistance
Frequency
decreases
Frequency
increases
Frequency
decreases
HOST PATHOGEN
BOOM & BUST CYCLE

26. BOOM & BUST CYCLE
Eg. Introduction of barley cultivar, Triumph in the UK in 1980.
With high resistance to powdery mildew
(2 genes – ml Ab and ml a7 conferred resistance)
Area increased and new race detected
Frequency of new race increased and epidemic occurred
cultivar, Triumph become susceptible to powdery mildew by 1987
Variety withdrawn

27. Disease tolerance is also known as Disease Endurance
Host after being infected by the pathogen tolerates the
infection and suffers less damage. There is no substantial
decrease in yield and it is influenced by external factors.
e.g. application of potash and phosphatic fertilizers
Klendusity is a type of disease escape mechanism of a
plant. Ability of a susceptible variety of plant to escape
infection because of the way it grows. e.g. early maturing
varieties, change in planting season, etc.
Pseudo-resistance is conferred by disease escape

28. Thank you