📈 Plant Disease Epidemiology

Plant disease epidemiology explains how host, pathogen, environment, and time interact to determine outbreak intensity and spread. This lesson focuses on endemic, epidemic, and pandemic patterns used in plant pathology problem solving.

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environment and time factor

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:

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Endemic or Enphytotic

When a disease is more or less constantly occurring year after year in a moderate to

severe form in a country or locality then it is called as endemic disease. eg: wart disease of

potato (Synchytrium endobioticum) is endemic in Darjeeling, citrus canker (Xanthomonas

axonopodis pv citri)in Asia and sorghum rust (Puccinia purpurea).

Epidemic or Epiphytotic

It is a sudden outbreak of a disease periodically over a widespread area in a devastatingly

severe form causing severe losses or complete destruction. This is constantly present in a locality

but it assumes severe form only on occasions. This is because of the occurrence of favorable

environment responsiblefor the rapid development of disease. eg: wheat stem rust (Puccinia

graminis tritici) and powdery mildew (Erysiphe graminis vor tritici), late blight of potato

(phytophthora infestans), red rot of sugar cane( Colletotrichum falcatum), downy mildew of

grapevine( Plasmophora viticola) and rice blast (Pyricularia oryzae).

Certain disease are endemic in one area and become epidemic in another area. Eg:

Citrus canker is endemic in Asia but epidemic in the introduced place, Florida (U.S.A).The

downy mildew of corn is a endemic disease in India but became epidemic in the Philippines.

Pandemic

When an epidemic disease spreads over continents or subcontinents and involves mass

mortality it is considered as pandemic. The outbreak of black stem rust of wheat in India during

1947 is best example for a pandemic disease.

Sporadic

Diseases which occur at irregular intervals over limited areas or locations are called

sporadic. They occur relatively in few instances. Eg: Fusarium wilt of cotton (Fusarium

oxysporum f sp. vasiinfectum) grain smut of sorghum (Sporisorium sorghi ) and loose smut of

wheat (Ustilago nuda).

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:

1. Host

2. Pathogen and

3. Environment

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Environment flow chart

PATHOGEN ENVIRONMENT

Pathogen

A course of epidemic in nature differs with the nature of the host, the pathogen and the

environment. In arecanut the Koleroga fungus, Phytophthora arecae become destructive during

monsoon period (July-Sep) and wanes away with rising temperatures and dry conditions. The

above disease once again become destructive during rainy season. This type of epidemic is

known as seasonal epidemic or annual epidemic. Outbreak of Phytophthora wilt of betelvine

occurs during rainy season in South India. In temperate zone peach leaf curl and apple scab

follow the similar course.

Epidemics caused as a result of introduction of new pathogens in the locality hither to

free from them, appear in two phases viz., destructive phase and innocent phase (due to biologic

equilibrium reached between new comer pathogen and the original inhabitant). The well known

epidemics of late blight of potato in Europe and blast disease of rice in South East Asia, powdery

mildew and downy mildew of grapevine in Europe, leaf rust of coffee in Sri Lanka and

anthracnose of grapevine in India are examples of this category. In the above diseases the

pathogens after taking heavy toll of the crops have settled down.

Factors governing epidemic or essential conditions for an epidemic

A disease is sometimes sporadic and assumes epidemic proportions under special

circumstances. The essential conditions for an epiphytotic or the factors governing epidemics can

be grouped under the three heads.

1. Nature of host

2. Nature of the pathogen and

3. Environment

An epidemic can only result from the cumulative effects of all the three factors

mentioned above, acting simultaneously. Few pathogens are capable of assuming epiphytotic

conditions while others are sporadic. The former group consists of late blight of potato, blast of

rice, downy mildew diseases and rust diseases.

Host	Pathogen	Environment
Susceptibility of the host	Introduction of a new pathogen	Temperature
Aggregation and distribution of susceptible hosts	Presence of aggressive strain of the pathogen	Moisture and humidity
Introduction of new hosts	High birth rate of the pathogen	Rainfall
Introduction of	Low death rate of the pathogen	Light and

new collateral or alternate pathogen host	Col2	shade
	Easy and rapid dispersal Wind of the pathogen Adaptability of the pathogen	Wind

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A. Host Factors

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Susceptibility of the host

Plants have ability to combat disease which manifests itself as susceptibility or resistance.

Plants are predisposed to the attack depending on their nature, environment and stage of growth.

Presence of susceptible varieties in an area may act as one of the causes of epidemic. 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. Early sown sugarcane varieties of sugarcane are more susceptible to leaf

rust in Deccan canals in Bombay area than the late sown varieties.

Wheat plant becomes susceptible to black rust (Puccinia graminis tritici) at the boot

stage but is resistant when young. Susceptibility of rice plants to blast disease (Pyricularia

oryzae) increases with application of heavy doses of nitrogenous fertilizers. Cottons plants are

susceptible to Fusarium wilt ( F.oxysporum f.sp. vasinfectum) at soil temperatures of 26 to 28°C,

brinjal to Verticillium wilt Verticillium dahliae at 20°C. But crop plants are resistant to these

soil-borne diseases at relatively lower or higher temperatures. Under the above conditions, the

pathogen multiplies faster, cause infection and effectively uses its propagules for quick

secondary spread causing epidemic.

Aggregation and distribution of susceptible hosts

Abundance of susceptible hosts in an area is one of the major causes of the spread of

epidemics. Continuous cultivation of susceptible variety or varieties in an area, that too in a large

contiguous area help in the build up of inoculum and improve the chances of epidemics. Under

the above conditions the pathogen increases the rate of multiplication of its propagules and

repeats the disease cycles in a short span. Wheat cultivation area in the U.S.A and Canada and

rice cultivation area in East Asian countries are exposed to a greater danger of epidemics by

wheat black rust and rice blast respectively.

Destructive epidemic of early and late leaf spots of groundnut in Bombay area (Gujarat

and Maharashtra States) during 1912-1913 was mainly the result of cultivation of local varieties

in a larger area. Panama wilt ( Fusarium oxysporum f.sp. v asinfectum ) susceptible table variety,

‘Son’ in banana was responsible for the destructive epidemic in parts of Bombay area (Gujarat

and Maharashtra) during 1936 – 1940 Countrywide cultivation of red rot ( Colletotrichum

falcatum ) susceptible sugarcane varieties (local varieties like Pundya, Khajuria etc.,) practically

made their cultivation impossible in Bombay area.

Introduction of new host (s)

Disease proneness in the host is induced by environment and other factors. The host is

liable to vigorous attack and successful infection by the pathogen. A resistant or moderately

resistant variety may become susceptible or highly susceptible. A susceptible variety may

become highly susceptible when conditions favouring proneness are existing and cause severe

damage. Under the above conditions the pathogen multiplies faster, cause infection and produces

more propagules for secondary spread. Introduction of an exotic cotton variety (C4 (Cambodia)

caused outbreak of bacterial blight ( Xanthomonas axonopodis pv . malvacearum) and grey

mildew (Septoacylindrium gossypii) in local variety, Deviraj, grown in Maharashtra area in

India.

Introduction of new collateral or alternate hosts

Alternate hosts are those plants on which the heteroecious pathogens pass part of their

life cycles. Similarly, collateral hosts are some wild plants in which the pathogen survives when

primary host is not available. Both alternate and collateral hosts are important in building up the

primary inoculum to the next crop. They determine the course and intensity of an epidemic.

Grass hosts (collateral hosts) of Sclerospora sacchari, S. philippinensis ( downy

mildews), Pyricularia oryzae ( rice blast), Ustilago scitaminea (sugarcane smut) may produce

abundant inoculum which aid in building up of epidemics. Outbreak of heteroecious blister rust

of pine (Cronartium ribicola ) in Europe and the U.S.A happened due to import or introduction

of Pinus strobus from the USA.

B. Pathogenic Factors

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Introduction of new pathogen

Some pathogens, epidemic in certain area, may become quite aggressive and outbreak as

epidemic when introduced to new area. For example late blight of potato caused by

Phytophthora infestans was epidemic in South America. This disease became epidemic when the

infected tubers were introduced in Europe (in 1843-45). Fire blight ( Erwinia amylovora in North

America is endemic. Fire blight spread to Pacific coast fruit-growing areas of the U.S.A in 1884

and subsequently it reached Canada. It reached New Zealand in 1919 and it appeared in England

in 1957. The mode of introduction had been through fruit boxes. Coffee rust ( Hemileia vastatrix)

is indigenous in Ethiopia, where Coffea arabica is native. The disease spread to Sri Lanka in

1869, India in 1870, Sumatra in 1876, Java in 1878 and the Philippines in 1889. It also spread

from Kenya to the Congo by 1918 and reached the Cameroons. From 1950 onwards, it spread to

the reminder of West Africa.

The mode of long distance transport of H . vastatrix is wind. Spores have been trapped at

up to 1000 m above sea level up to 150 m from infected sites. Dutch elm diseases (Ceratocystis

ulmi ) first reported in 1919 in Holland, spread throughout Europe and reached Great Britain in

1927. It was introduced to the eastern United States on elm logs imported from Europe.

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Presence of aggressive strain of the pathogen

All the strains of a pathogen are not aggressive. Only the aggressive strains are capable of

causing infectious diseases which spread as epidemic. They are characterised by rapid cycle of

infection and causing successful infection in new hosts. Rapid cycle of infection is essential for

successful infection and it happens only by aggressive strain of the pathogen. e.g., Puccinia

graminis tritici (wheat black rust) in India, 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 easily dispersed

by using water or insects and cause infections to new plants. The high degree of fecundity and

the enormous amounts of inoculum produced by some common plant pathogens are given in

table.

Fecundity rates of plant pathogens

Sl. No	Pathogen	Extent of fecundity
	Wheat stem rust_(Puccinia graminis_ tritici)	Twenty five trillion uredospores in one hectare of wheat crops.
2	Wheat stem rust_(Puccinia graminis_ tritici )	64,000 million aeciospores from aecial cups in a single barberry bush
3	Cedar rust of apple	Two billion teliospores in a single gall.
4.	A corn plant infected with downy mildew	225 million sporangia in one night
5.	A. grapevine infected by downy mildew	32,000 sporangia per sq.cm
6.	Bunt of wheat	6 to 12 million smut spores in a single kernel
7.	Smut of corn	125,000 billion smut spores in one hectare
8.	Chestnut blight	150,000,000 spores in a single spore horn
9.	Fomes applanatus	5,460 billion spores in a single fruiting body

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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.

Easy and rapid dispersal of the pathogen

The ability of the pathogen to cause epidemic depends both on the high birth rate and

dispersal. The propagules of the pathogen produced should be dispersed for development of an

epidemic. It may happen by external agencies like wind, water, insects, mites and nematodes.

Fungal spores / conidia are minute and light and resistant to adverse conditions. Fungal spores

are mostly disseminated by wind. Bacteria are mostly disseminated by water or insects. Virus

and phytoplasma diseases are mostly transmitted by insects, mites or nematodes. Epidemics are

determined by the velocity of wind, direction of wind, moisture, relative humidity, temperature,

presence and number of vectors and their rate of reproduction.

Adaptability of the pathogen

Pathogens have the capacity to adapt to adverse conditions. Fungi produce different types

of spores like oospores, ascospores and smut spores (chlamydospores) which help in tiding over

adverse conditions. Bacteria survive in diseased plant parts. Viruses and phytoplasmas live in

collateral hosts or insect vectors in the absence of the suitable crop hosts.

C. Environmental Factors

The environmental conditions such as temperature, relative humidity, rainfall, duration

and intensity of light, etc. play very important role in causing epidemics. These are actually the

deciding factors and influence almost all the stages of disease cycle. Favourable environmental

conditions are needed for sporulation, liberation of spores, dissemination of pathogen,

germination, infection and establishment of pathogen in the host.

For example, persistent optimum temperature and moisture are needed for spore

germination and entry of germ tube in the host. Similarly optimum temperature, moisture, light

and specific nutrition is required for the development of the disease and sporulation of pathogen.

Compound interest diseases and simple interest diseases The terms compound interest and

simple interest are for explaining rate of increase of pathogen. These terms were introduced by

Van der Plank in 1963 in the book ‘ Plant Diseases-Epidemics and Control’. Based on the mode

of multiplication of pathogen, the diseases are classified of two types:

1. Simple interest diseases

2. Compound interest diseases

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Simple interest diseases

In simple interest diseases the increase is mathematically analogous to simple interest in

money. There is only one generation of the pathogen in the life of the crop. The primary

inoculum is seed-borne or soil-borne. The secondary infection rarely occurs during the crop

season. That is, the pathogens do not spread from plant to plant in one growing season. Simple

interest diseases are caused by seed-or soil-borne smuts, like loose smut of wheat, covered smut

of barley and soil borne fungi which attack roots, like wilt (Fusarium oxysporum) and root rot

(Rhizoctonia spp.) diseases.

Most of the smuts infect the seedlings, grow along with the growth of the plant and

produce spores in the inflorescence on maturity of the crop. There is no secondary spread from

the smutted heads. These smut diseases are mostly systemic in nature. They do not produce

propagules external to the host during the active season of the crop. Dispersal of propagules of

these fungi is restricted by existing climatic and biotic conditions.

Compound interest diseases

In compound interest diseases the rate of increase is mathematically analogous to

compound interest in money. The pathogen produces enormous amount of spores at a very rapid

rate. These spores are disseminated rapidly by wind and infect the other plants. Both the

inoculation and sporulation period are short so that the pathogen spreads from plant to plant

during the same growing season. New crop of spores is produced, disseminated and the cycle is

repeated fast. Thus more generations of the pathogen are produced in the life of a crop. e.g., late

blight of potato, powdery mildews and rust diseases. If we consider wheat stem rust caused by

Puccinia graminis tritici as an example, the fungus produces uredospores in very large numbers

(50,000 to 4,00,000 uredospores per uredosorus).

These spores are spread by wind and infect other plants. Each of the freshly infected

wheat plant produces uredopustules within 5 to 7 days at 24°C. Thus within a week of

appearance of the first pustule in the crop several thousand new pustules are formed which could

repeat the process within a week. If the climatic conditions of about 24°C temperature and

relative humidity remain for only few weeks, the entire crop is severely affected by the disease.

Course of epidemic

The course of epidemic follows two distinct phases viz.,

i. Progressively destructive phase and

ii. the decline phase

i. Progressively destructive phase

Some epidemics develop slowly (tardive) while others develop rapidly. Slow epidemics

(or epiphytotics) usually occur among population caused by systemic pathogens. The pathogen

multiplies slowly following the characters of simple interest disease. They belong to low death

rate category and have less incubation period and sporulation period. However, the rapid

epiphytotics are greatly influenced by environmental factors.

ii. Decline phase

During early stage, an epidemic spreads vigorously causing diseases in new hosts. After

development of a saturation stage it shows a decline by itself. No epidemics may be due to non

availability of suscepts non-availability of susceptible stages of the crop, unfavourable weather

conditions and reduction in aggressiveness of the pathogens. Generally the hosts are prone to the

disease at a specific developing stage. Once this stage is crossed in a plant it’s proneness to

infections is reduced or completely lost. Under the conditions the epidemic declines. The decline

in the epidemic may also be due to unfavourable weather conditions for disease development. As

a result future spread of the disease will be checked and the epidemic will decline. Wheat crop in

Northern India usually gets the attack of rusts in January to March.

Epidemics develop during these months. Although the plant remains prone to attack

afterwards also, further development of the disease is checked because of rise in temperature

which is favourable for the pathogen. Due to the above mentioned and other causes, the

aggressiveness of the pathogen may be reduced. When all susceptible individuals are destroyed

by the pathogen, it may try to parasitize the remaining resistant individuals of the same species.

In these adverse conditions, the pathogen may lose its power of successful infection, its

reproduction may slow down and the pathogen becomes less aggressive.

Slow and rapid epiphytotics

The form of epidemic is decided by the nature of the pathogen, host and the weather.

Epidemic may develop slowly and is called ‘ tardive’. Epidemic which develops rapidly is called

‘explosive’. In between these intermediate forms of epidemic may occur.

i. Slow epiphytotics

Slow epiphytotics occur among perennial (tree) populations. Infected host survives for

several years before dying. Most of the characters of a simple interest disease are found in slow

epiphytotics. The causal agent is mostly systemic. The pathogen multiplies slowly. Their

movement from plant to plant is much slower. They are low death rate pathogen. In slow

epiphytotics, crop sanitation is the best method. e.g., Swollen shoot of cocoa.

This disease spreads very slowly from tree to tree and still less from one garden to

another garden. For instance, the incidence of 31 % swollen shoot increased to 75 % over a

period of 2.5 years. As stated by Van der Plank (1959) the rate of multiplication of a systemic

disease of trees is about ten fold a year whereas it is 10,000 fold in respect of herbaceous plants

and it is of higher

rates for local lesion pathogens e.g., late blight of potato, wheat stem rust, etc.,

ii. Rapid epiphytotics

Rapid epiphytotics occur among annual crops. It is caused by non-systemic pathogens

with high birth rate. Several generations of the pathogen is produced within a short time. Rapid

epiphytotics are largely governed by environmental factors compared to slow epiphytotics.

Disease increase is rapid and the disease rises to a beak in short time and then show sharp decline

when the weather turns unfavourable or when the host becomes resistant due to maturity or due

to restricted dispersal of propagules of pathogen. e.g.,apple scab. This type of epiphytotic is

controlled by protective spraying or dusting with chemicals.

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Summary Cheat Sheet

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References

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