Epidemiology or epiphytology is the study of the disease outbreak, its course, intensity, cause, and effects, and the various factors governing it. Based on the occurrence and geographical distribution, they are classified as follows:
- Endemic or Enphytotic (A disease 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 occasionally due to favorable environmental conditions.).
- Pandemic (When an epidemic disease spreads over continents or subcontinents and involves mass mortality, it is considered a pandemic).
- Sporadic (Diseases that 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 crops 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 DEVELOPMENT OF EPIDEMICS:-
Several internal and external factors of a particular host
plants play an essential role in the development of epidemics.
Susceptibility of the host/ Levels of Genetic Resistance:-
Host plants carrying race-specific (vertical) resistance do not allow a pathogen to become established in them; thus,; thus, no epidemic can develop. Host plants carrying partial (horizontal) resistance will probably become infected.
Still, 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 establishing and developing new infections.
Therefore, susceptible hosts favor the development of disease epidemics in the presence of a virulent pathogen and a favorable environment.
For example, late-maturing groundnut varieties are more susceptible to early leaf spot (Cercospora arachidicola ) and late leaf spot (Phaeoisariopsis) than the early-maturing varieties. Similarly, late maturing wheat varieties are more 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 concerning 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 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 mildew, 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 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 in all postharvest infections, plant parts are resistant during growth and the early adult period but become susceptible to ripening.
Then, depending on the particular plant–pathogen combination, the age of the host plant at the time of arrival of the pathogen may considerably affect the development of infection and of an epidemic.
2. PATHOGEN FACTORS THAT AFFECT THE 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 (black wheat rust) in Pakistan, stripe rust, bunt, and loose smut of wheat in the U.S.A. and Europe.
The possibility of an outbreak of epidemics increases with the number of physiologic forms or pathogenic strains of the pathogen present in a locality.
The high birth rate of the pathogen :
Pathogens with high reproductive capacity and capable of rapid dissemination over wide areas mainly cause epidemics. The fungal members causing powdery mildews, downy mildews, rusts, blasts, blights, etc., produce enormous spores. These spores are a Low death rate.
Low death rate diseases may also cause epiphytotic. These diseases are caused by agents of systemic nature, which are protected by plant tissues.
As plant tissues protect them, the chances of high mortality are reduced to the minimum. In these diseases, the chief source for the accumulation of inoculum for epiphytotic 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 the occurrence of epiphytotic 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, sclerotia, nematode eggs, virus-infected plants, etc.) within or near fields of host plants, the more inoculum reaches the hosts and at an earlier time, thereby increasing the chances of an epidemic significantly.
Type of Reproduction of the Pathogen
All pathogens produce many offspring. Some of them, such as most fungi, bacteria, and viruses, produce many offspring, while a few fungi, all nematodes and all parasitic plants, produce relatively small numbers of offspring.
Some plant pathogenic fungi, bacteria, and viruses have short reproduction cycles and therefore are polycyclic, i.e., they can produce many generations in a single growing season.
Polycyclic pathogens include fungi that cause rusts, mildews, and leaf spots and are responsible for most of the sudden, catastrophic plant disease epidemics worldwide.
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 easily dispersed over a range of distances and cause widespread epidemics.
Other pathogens, such as vascular fungi and bacteria, mollicutes, viruses, and protozoa, reproduce inside the plant. In this case, the 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 as those causing rusts, mildews, and leaf spots, are released into the air and can be dispersed by air breezes or strong winds over distances varying from a few centimeters up to several kilometers.
These kinds of fungi are responsible for the most frequent and widespread epidemics.
Regarding their ability to cause sudden and widespread epidemics, the next most important group of pathogens includes those whose inoculum is carried by airborne vectors.
Aphids, whiteflies, and other insects transmit many viruses. Leafhoppers, plant hoppers, or psyllids transmit mollicutes and fastidious bacteria. Some fungi (such as the cause
of Dutch elm disease), bacteria (such as the cause of bacterial wilt of cucurbits), and even nematodes (such as the 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
DEVELOPMENT OF EPIDEMICS
The environment may affect the host plants’ availability, growth stage, succulence, and genetic susceptibility.
It may also affect the survival, vigor, rate of multiplication, sporulation, ease, direction, and distance of dispersal of the pathogen, as well as the spore germination and penetration rate. In addition, the environment may affect the number and activity of the pathogen vectors.
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, the liberation of spores, dissemination of the pathogen, germination, infection, and establishment of the 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 mildew, leaf spots, rusts, and anthracnoses), bacteria (leaf spots, blights, soft rots), and nematodes.
Moisture promotes new succulent and susceptible growth in the host and, more importantly, increases the sporulation of fungi and the 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.
High levels of moisture allow all these events to occur constantly and repeatedly, leading to epidemics. In contrast, the absence of moisture for even a few days prevents all these events, so epidemics are interrupted or stopped altogether.
Some diseases caused by soilborne pathogens, such as Fusarium and Streptomyces, are more severe in dry than wet weather, but such diseases seldom develop into essential 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 of these vectors is 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 inoculum of oomycete fungi, bacteria, and nematodes that survive 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 the temperature stays within a favorable range for each stage, a polycyclic pathogen can complete its infection cycle quickly (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 initiating and developing the vast majority of plant diseases and disease epidemics.
Source of Information and Study/References
- PLANT PATHOLOGY book by. GEORGE N. AGRIOS Department of Plant PathologyUniversity 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.