Q. What is E? Describe various factors involved in making
a disease epidemic.
EPIDEMIOLOGY:-
Edpidemiology or epiphytology is the study of the outbreak
of disease, its course, intensity, cause and effects and the various factors
governing it. Based on the occurrence and geographical distribution they are
classified as follows:
For more Notes
·
Endemic or Enphytotic
(A disease is more or less constantly occurring year after year in a
locality or country crops.
·
Epidemic or
Epiphytotic (is constantly present in a locality but it assumes severe
form only on occasions due to favorable environmental conditions.).
·
Pandemic (When an
epidemic disease spreads over continents or subcontinents and involves mass
mortality it is considered as pandemic).
·
Sporadic (Diseases
which occur at irregular intervals over limited areas or locations)
Factors involved in making a disease epidemic:-
An epidemic may cause widespread and mass destruction of crop in a short
time or may persist for long periods depending upon the three following factors
responsible for the disease:
(i) HOST
FACTORS
(ii) PATHOGEN FACTORS
(iii) ENVIRONMENTAL FACTORS
·
HOST FACTORS THAT AFFECT THE DEVELOPMENTOF
EPIDEMICS:-
Several internal and external factors of particular host
plants play an important role in the development of epidemics.
·
Susceptibility of the host/ Levels
of Genetic Resistance:-
Obviously, host
plants carrying race-specific (vertical) resistance do not allow a pathogen to
become established in them, and thus no epidemic can develop. Host plants carrying partial
(horizontal) resistance will probably become infected, but the rate at which
the disease and the epidemic will develop depends on the level of resistance
and the environmental conditions. Susceptible host plants lacking genes for
resistance against the pathogen provide the ideal substrate for establishment
and development of new infections.
Therefore, in the presence of a virulent pathogen and a
favorable environment, susceptible hosts favor the development of disease
epidemics.
For example,
late maturing varieties of groundnut are more susceptible to early leaf spot (Cercospora arachidicola ) and late leaf spot (Phaeoisariopsis) than the
early maturing varieties. Similarly late maturing varieties of wheat are
susceptible to loose smut (Ustilago nuda tritici) than the early maturing
varieties.
·
Degree of Genetic Uniformity of Host Plants:-
When genetically uniform host plants,
particularly with regard to the genes associated with disease resistance, are
grown over large areas, a greater likelihood exists that a new pathogen race
will appear that can attack their genome and result in an epidemic. This
phenomenon has been observed repeatedly, for example, in the Cochliobolus
(Helminthosporium) blight on Victoria oats and in southern corn leaf blight on
corn carrying Texas male-sterile cytoplasm. For similar reasons of genetic
uniformity, the highest rates of epidemic development generally occur in
vegetatively propagated crops, intermediate rates in self-pollinated crops, and
the lowest rates in cross-pollinated crops.
This
explains why most epidemics develop rather slowly in natural populations, where
plants of varying genetic makeup are intermingled.
·
Type of Crop:-
In
diseases of annual crops, such as corn, vegetables, rice, and cotton, and in
foliar, blossom, or fruit diseases of trees and vines, epidemics generally
develop much more rapidly (usually in a few weeks) than they do in diseases of
branches and stems of perennial woody crops such as fruit and forest trees.
Some epidemics of fruit and forest trees, e.g., tristeza in citrus, pear
decline, Dutch elm disease, and chestnut blight, take years to develop.
·
Age of Host Plants:-
Plants
change in their reaction (susceptibility or resistance) to disease with age.
The change of resistance with age is known as ontogenic resistance.
In some plant–pathogen combinations, e.g.,
Pythium damping off and root rots, downy mildews, peach leaf curl, systemic
smuts, rusts, bacterial blights, and viral infections, the hosts (or their
parts) are susceptible only during the growth period and become resistant
during the adult period (adult resistance). With several diseases, such as
rusts and viral infections, plant parts are actually quite resistant to
infection while still very young, become more susceptible later in their growth,
and then become resistant again before they are fully expanded.
In other diseases,
such as infections of blossoms or fruit by Botrytis, Penicillium, Monilinia,
and Glomerella, and inall postharvest infections, plant parts are resistant during
growth and the early adult period but become susceptible near ripening.
Apparently then, depending on the particular plant–pathogen
combination, the age of the host plant at the time of arrival of the pathogen
may affect considerably the development of infection and of an epidemic.
2. PATHOGEN FACTORS THAT AFFECT DEVELOPMENT OF EPIDEMICS:-
·
Levels of Virulence:-
Virulent pathogens capable of infecting the
host rapidly ensure a faster production of larger amounts of inoculum, and,
thereby, disease, than pathogens of lesser virulence.e.g., Puccinia graminis
tritici (wheat black rust) in Pakistan, stripe rust, bunt and loose smut of
wheat in the U.S.A. and Europe. The possibility of outbreak of epidemics
increases with the number of physiologic forms or pathogenic strains of the
pathogen present in a locality.
·
High birth rate of the pathogen :
Pathogen
with high reproductive capacity and capable of rapid dissemination over wide areas
mostly cause epidemics. The fungal members causing powdery mildews, downy
mildews, rusts, blasts, blights etc., produce enormous amount of spores. These
spores are ea Low death rate
Epiphytotics may also be caused by
low death rate diseases. These diseases are caused by agents of systemic nature
which are protected by plant tissues. As they are protected by plant tissues
the chances of high mortality is reduced to the minimum. In these diseases the
chief source for accumulation of inoculum for epiphytotics is the diseased
plant organ used for vegetative propagation (corms, setts, tubers, etc,). Here
the buildup of epidemics is comparatively low compared to high birth rate
diseases. When a particular area is planted and covered with diseased planting
material the chances of occurrence of epiphytotics are very high. e.g., virus
and phytoplasma diseases in crops propagated through vegetative plant parts.
·
Quantity of Inoculum near Hosts:-
The greater the number of pathogen propagules (bacteria, fungal spores
and sclerotia, nematode eggs, virus-infected plants, etc.) within or near
fields of host plants,the more inoculum reaches the hosts and at an
earliertime, thereby increasing the chances of an epidemic greatly.
·
Type of Reproduction of the Pathogen
All pathogens produce many offspring. Some of them,such as most fungi,
bacteria, and viruses, produce agreat many offspring, while a few fungi, all nematodes,and,all
parasitic plants produce relatively small numbersof offspring. Some plant
pathogenic fungi, bacteria, and viruses have short reproduction cycles and
therefore are polycyclic, i.e., they can produce many generations in asingle
growing season. Polycyclic pathogens includemfungi that cause rusts, mildews,
and leaf spots and are,responsible for most of the sudden, catastrophic plant,disease
epidemics in the world.
·
Ecology of the Pathogen:-
Some
pathogens, such as most fungi and all parasitic higher plants, produce their
inoculum (spores and seeds,respectively) on the surface of the aerial parts of
the host From there, spores and seeds can be dispersed with ease over range of distances and can
cause widespread epidemics. Other pathogens, such as vascular fungi and bacteria,
mollicutes, viruses, and protozoa, reproduce inside the plant. In this case,
spread of the pathogen is rare or impossible without the help of vectors.
·
Mode of Spread of the Pathogen:-
The spores
of many plant pathogenic fungi, such asthose causing rusts, mildews, and leaf
spots, are released into the air and can be dispersed by air breezes or
strongwinds over distances varying from a few centimeters upto several
kilometers. These kinds of fungi are respon-sible for the most frequent and
most widespread epi-demics. In terms of their ability to cause sudden and widespread
epidemics, the next most important groupof pathogens includes those whose
inoculum is carriedby airborne vectors. Many of the viruses are transmittedby
aphids, whiteflies, and some other insects. Mollicutesand fastidious bacteria
are transmitted by leafhoppers,plant hoppers, or psyllids. Some fungi (such as
the cause
of Dutch elm disease), bacteria (such as the cause of bacterial
wilt of cucurbits), and even nematodes (such asthe cause of pine wilt disease)
are disseminated primarily by beetles. Pathogens that are transmitted by
wind-blown rain (primarily fungi causing diseases such as anthracnoses and
apple scab, and most bacteria) are almost annually responsible for severe but
somewhat localized epidemics within a field, a country, or a valley
3. ENVIRONMENTAL FACTORS THAT AFFECT
The environment may affect the availability, growth stage,
succulence, and genetic susceptibility of the host plants. It may also affect the
survival, vigor, rate of multiplication, sporulation,and ease, direction, and
distance of dispersal of the pathogen, as well as the rate of spore germination
and penetration. In addition, the environment may affect the number and
activity of the vectors of the pathogen. The most important environmental
factors that affect the development of plant disease epidemics are moisture,
temperature, and the activities of humans in terms of cultural practices and
control measures. Favorable environmental conditions are needed for sporulation,
liberation of spores, dissemination of pathogen, germination, infection and
establishment of pathogen in the host.
·
Moisture:-
Abundant,
prolonged, or repeated high moisture, whether in the form of rain, dew,or high
humidity, is the dominant factor in the development of most epidemics of
diseases caused by oomycetes and fungi (blights, downy mildews, leaf spots,
rusts, and anthracnoses), bacteria (leaf spots, blights, soft rots), and nematodes.
Moisture not only promotes new succulent and susceptible growth in the host,
but, more importantly, it increases sporulation of fungi and multiplication of
bacteria. Moisture facilitates spore release by many fungi and the oozing of
bacteria to the host surface, and it enables spores to germinate and zoospores,
bacteria, and nematodes to move. The presence of high levels of moisture allows
all these events to take place constantly and repeatedly and leads to
epidemics. In contrast, the absence of moisture for even a few days prevents
all of these events from taking place so that epidemics are interrupted or
stopped completely. Some diseases caused by soilborne pathogens, such as
Fusarium and Streptomyces, are more severe in dry than in wet weather, but such
diseases seldom develop into important epidemics. Epidemics caused by viruses
and mollicutes are affected only indirectly by moisture, primarily by the
effect that higher moisture has on the activity of the vector. The
activity ofthese vectors are reduced drastically in rainy weather.
·
Temperature
Epidemics are sometimes
favored by temperatures higher or lower than the optimum for the plant because they
reduce the plant’s level of partial resistance.
Low
temperature reduces the amount of inoculum of oomycete fungi, bacteria, and nematodes
that survives cold winters. High temperature reduces the inoculum of viruses
and mollicutes that survives hot summer temperatures. In addition, low
temperatures reduce the number of vectors that survive the winter. Low temperatures
occurring during the growing season can reduce the activity of vectors.
The
most common effect of temperature on epidemics, however, is its effect on the
pathogen during the different stages of pathogenesis, i.e., spore germination
or egg hatching, host penetration, pathogen growth or egg hatching, host
penetration,
Pathogen
growth. When temperature stays within a favorable range for each of these
stages, a polycyclic pathogen can complete its infection cycle within a very
short time (usually in a few days). As a result, polycyclic pathogens can
produce many infection cycles within a growing season.
In
reality, moisture and temperature must be favorable and act together in the
initiation and development of the vast majority of plant diseases and plant
disease epidemics.
Source of Information and study/ References
·
PLANT
PATHOLOGY book by. GEORGE N. AGRIOS
Department
of Plant Pathology
University
of Florida
Plant Pathogens & Principles of
Plant Pathology book
Principles of Plant Pathology – Das
Gupta M. K. 1999. Allied Publishers,
Pvt.
Ltd. New Delhi.
Introduction to Principles of Plant
Pathology – Singh R. S. 1984. Oxford &
IBH
Publishing Co., New Delhi.
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