BSc 2nd Year Effect of Environment on Disease Development Notes Study Material

BSc 2nd Year Effect of Environment on Disease Development Notes Study Material

BSc 2nd Year Effect of Environment on Disease Development Notes Study Material BSc is a three-year program in most universities. Some of the universities also offer BSc Honours. Out of those, there are BSc 2nd Year Study Material, BSc Sample Model Practice Mock Question Answer Papers & BSc Previous Year Papers. At you can easily get all these study materials and notes for free. Here in this post, we are happy to provide you with BSc 2nd Year Plant Pathology Effect of Environment on Disease Development Notes Study Material.

BSc 2nd Year Effect of Environment on Disease Development Notes Study Material
BSc 2nd Year Effect of Environment on Disease Development Notes Study Material

BSc 2nd Year Effect of Environment on Disease Development Notes Study Material

The mere presence of the pathogen and its susceptible host in the field does not necessarily mean that the disease will develop. For instance, keeping in view the environmental conditions that prevail in our country, most diseases develop well during winter rather than summer months of the year. Although all pathogens, all perennials, and many annual plants are present in the field throughout the year, diseases would occur only or develop best during the colder part of the year.

Also, it is common to see that most diseases appear and develop best during wet, warm days or that plants heavily fertilized with nitrogen usually are more severely attacked by some pathogens than are less fertilized plants. These examples indicate that environmental conditions in both air and soil, after contact of a pathogen with its host, may greatly affect the development of disease and frequently these conditions determine whether a disease will occur or not. The environmental factors, which affect the initiation and development of infectious plant diseases, are (i) temperature, (ii) moisture, (iii) light, (iv) soil nutrients, and (v) soil pH.

Their effects on disease may be through their influence on the growth and susceptibility of the host, on the multiplication and activity of the pathogen, or on the interaction of host and pathogen and its effect on the severity of symptom development. For a disease to occur and to develop optimally, a combination of three factors must be present: a susceptible plant, an infective (virulent) pathogen, and a favorable environment. (BSc 2nd Year Effect of Environment on Disease Development Notes Study Material)

Although plant susceptibility and pathogen virulence remain essentially unchanged in the same plant for at least several days, weeks or months, the environmental conditions may change more or less suddenly and to various degrees. Such changes influence the development of disease in progress, or the initiation of new diseases, more or less drastically. Of course, a change in any environmental factor may favor the host or the pathogen or both, or it may be more favorable to one than is to the other, and the expression of the disease will be affected accordingly. (BSc 2nd Year Effect of Environment on Disease Development Notes Study Material)

[I] Temperature

The too-low temperatures of winter and very high temperatures of summer are, below the minimum and above the maximum respectively required by most pathogens. Therefore, diseases may not be initiated and those in progress may come to a halt during these periods of the year in our country. With the advent of favorable temperatures, pathogens become active and if other conditions are favourable they can cause the disease. Pathogens differ in their choice of low or high temperatures.

Thus some species of Fusarium and Typhula thrive only in cool seasons or cold areas. The late blight pathogen, Phytophthora infestans is most destructive only during winter in our country and other sub-tropical areas. Some diseases on the other hand are favoured by high temperatures. Thus fusarial wilts, the Phymatotrichum root rots, the brown rot of stone fruits (Monilinia fructicola), and the bacterial wilt of solanaceous plants (Pseudomonas solanacearum) are more prevalent in warmer areas.

The effect of temperature on the development of a particular disease after infection depends on the particular host-pathogen combination. For stem rust of wheat (Puccinia graminis tritici), the time required for a disease cycle (from inoculation with uredospores to new uredospore formation) is 22 days at 5°C, 15 days at 10°C and 5-6 days at 23°C. Similar time periods for the completion of a disease cycle are required in many other diseases caused by fungi, bacteria, and nematodes.

If the minimum, optimum, and maximum temperatures for the pathogen, the host, and the disease are about the same, the effect of temperature on disease development is apparently through its influence on the pathogen. In many diseases, the optimum for disease development seems to be different from those of both the pathogen and the host. Thus, in black root rot of tobacco (by a fungus, Thielaviopsis basicola), the optimum for the disease is at 17 to 23°C, while that for tobacco is 28 to 29°C and for the pathogen is 22 to 28°C.

Here host grows so poorly and is so much weaker, than the pathogen that even a weakened pathogen can cause maximum disease development. In root rots of wheat and corn Gibberella zeae) the maximum disease development on wheat occurs at a temperature above the optimum for the development of both the pathogen and the wheat, but on corn, it occurs at a temperature below the optimum for the pathogen and for com. This is because wheat grows best at low temperatures whereas corn grows best at high temperatures.

Temperature, in combination with sunlight, may determine the seasonal appearance of symptoms in various viral diseases. Viruses causing yellows or leaf rolls are most severe in summer whereas those causing mosaics or ring spots during spring or winter.

[II] Moisture

Moisture also affects the initiation and development of diseases in many interrelated ways. The most important influence is on the germination of fungal spores and on the penetration of the host by the germ tube. Moisture also activates bacterial, fungal, and nematodal pathogens. As splashing rain and running water, moisture also plays important role in the distribution and spread of many pathogens on the same plant or from one plant to another. Moisture also increases the succulence of host tissues.

The occurrence of many diseases in a particular region is closely correlated with the amount and distribution of rainfall within the year. Thus late blight of potato, apple scab, downy mildews, fire blight, etc. are found or are severe only in areas with high rainfall or high relative humidity during the growing season. Most fungal pathogens depend on the presence of free moisture on the host or of high relative humidity in the atmosphere only during the spore germination and become independent once they can obtain nutrients and water from the host.

Some, such as late blight of potato and downy, mildews, however, require high relative humidity in the air throughout their development. These diseases come to a halt as soon as dry hot weather sets in and resume only after rain or after the return of humid weather.

Contrary to most fungal and bacterial pathogens of above-ground plant parts (that require water film), the spores of powdery mildew can germinate, penetrate and cause infection when there is the high relative humidity in the atmosphere surrounding the plant. In some powdery mildews, most severe infections occur when relative humidity is rather low (50-70%).

Thus powdery mildews are more common and more severe in drier areas of the world. In the root, tuber, and young seedlings diseases, caused by fungi (Pythium, Phytophthora, Rhizoctonia, Sclerotium), bacteria (Erwinia, Pseudomonas), and most nematodes, most severe symptoms develop when the soil is wet. The severity of disease in some cases is proportional to the soil moisture. However, the common scab of potatoes (Streptomyces scabies) is most severe in dry soils. Most bacterial diseases and many fungal diseases of young tender tissues are favoured by high moisture or high relative humidity.

[III] Wind

Wind affects disease development mainly through its effect on the spread of pathogens, and to some extent through its speeding up of drying of wet plant surfaces. Most diseases spreading rapidly and likely to assume an epiphytotic form are caused by pathogens (fungi, bacteria, viruses) that are either spread directly by the wind or are spread by insect vectors (which themselves are carried over long distances by wind). Uredospores of rusts and other conidia are carried over long distances by wind.

Wind-blown rains help release spores and bacteria from infected tissues and then carry them through the air and deposit them on wet surfaces. Wind also injures plant surfaces, which become susceptible to viruses, fungi, and bacteria.

[IV] Light

Light is relatively less important under natural conditions. It affects hosts where reduced light causes etiolation of plant parts, which increases their susceptibility to some unspecialized pathogens – Fusarium, and Botrytis. Reduced light however decreases the susceptibility of wheat to stem rust fungus. Reduced light generally increases the susceptibility of plants to viral infections.

[V] Soil pH

This is important in the occurrence and severity of diseases caused by soil-borne pathogens. Club-root of crucifers (Plasmodiophora brassicae) is most prevalent and severe at pH 7, whereas its development drops sharply between pH 7 and 2, and is completely absent at pH 7.8. But, the common scab of potato (Streptomyces scabies) is severe at a pH range from 5.2 to 8.0 or more and its development drops sharply at pH less than 5.2. (BSc 2nd Year Effect of Environment on Disease Development Notes Study Material)

[VI] Host-plant nutrition

Nutrition affects the rate of growth and the state of readiness of plants to defend themselves against diseases. Excess of nutrients such as nitrogen results in the production of young, succulent growth and may also prolong the vegetative growth period and delay maturity, which makes plants more susceptible to pathogens that prefer to attack such tissues. Lack of nitrogen makes plants weaker, slow growing, and faster aging and may make them susceptible to pathogens that attack weak tissues. Thus high nitrogen fertilization increases the susceptibility of pear to fire blight (Erwinia amylovora), of wheat to rust (Puccinia), and to powdery mildew (Erysiphe), etc.

Reduced nitrogen increases the susceptibility of tomato to Fusarium wilt, of many solanaceous plants to Pseudomonas solanacearum wilt, of sugar beets to Sclerotium rolfsii, and of most seedlings to Pythium damping off. Perhaps the form of nitrogen (ammonium or nitrate) rather than the amount is also important in decreasing or increase of host susceptibility to diseases. For instance, in many root rots, wilts, foliage diseases, etc. each of the two forms of nitrogen had exactly the opposite effect on a disease than did the other form of nitrogen.

Like nitrogen, other elements such as phosphorus, potassium, calcium, and also micronutrients also have similar effects on disease development. In general, plants receiving balanced nutrition are more capable of protecting themselves from new infections and of limiting existing ones than when one or more nutrients are supplied in excessive or deficient amounts.

Environment and plant disease epidemics (Epiphytotics)

Under favourable conditions, pathogens multiply and spread through populations of susceptible plants, sometimes causing extensive outbreaks of disease. On the basis of the mode of multiplication of pathogens, the diseases may be of two types: simple interest disease and compound interest disease.

1. Simple interest disease. Many pathogens do not spread from plant to plant during the growing season of the crop. There is only one generation of the pathogen in the life of the crop. The number of infected plants may increase as the season progresses but these represent new infections from the primary source of inocula, rather than spread from one plant to another. Such pathogens include soil-borne fungi attacking roots and seeds, soil borne smuts infecting seedlings.

2. Compound interest disease. Some pathogens, however, spread from plant to plant during the growing season. This denotes a disease whose increase is mathematically analogous to compound interest in money. The propagules or spores of the pathogen are produced which are disseminated and infect other plants which, in turn, produces spores that are disseminated and infect further plants, and so on. There are several generations of pathogens in the life of a crop. Many destructive diseases as potato late blight and black stem rust of wheat belong to this category. (BSc 2nd Year Effect of Environment on Disease Development Notes Study Material)


Epiphytotics are epidemics of plant diseases. Pandemics are destructive epiphytotic developing on a continental scale.

It is difficult to say at what point an outbreak of disease becomes an epiphytotic or even to define it. There may be slow epiphytotic or rapid epiphytotic.

1. Slow epiphytotic. These are often associated with perennial, long-lived plants such as trees, and the pathogens are systemic to varying extents. Systemic pathogens spread less rapidly than others. Systemic pathogens within perennial plants tend to be quite long-lived and have been described as low-death rate pathogens as distinct from high-birth rate pathogens which produce numerous spores rapidly. The latter are often controlled by fungicides or resistant varieties, the former by crop sanitation.

2. Rapid epiphytotic. These are chiefly caused by non-systemic pathogens with high rates of multiplication (high birth rate pathogens) and fairly short generations. Annual crops or perennial plants are grown as annuals are often attacked.

Elements of an Epidemic

When a pathogen spreads to and affects many individuals within a population over relatively a large area and within a relatively short time, the phenomenon is called an epidemic. An epidemic has been defined as “any increase of disease in a population”. A plant disease epidemic (or epiphytotic) implies the development and rapid spread of a disease on a particular kind of crop plant cultivated over a large area, a large field, a valley, a section of a country, the whole country, or even a part of a continent. The study of epidemics and the factors that influence them is called epidemiology. Plant disease epidemics are referred to also epiphytotic.

Plant disease epidemics develop as a result of the timely combination of the same components that result in plant disease: susceptible host plants, a virulent pathogen, and favourable environmental conditions over fairly a long period of time. Besides, human activities may also help to initiate and develop epidemics. When susceptible host plants and virulent pathogens are juxtaposed, the duration of all favourable environmental conditions is prolonged, and with no human intervention, the disease assumes an epidemic state.

Structure of an Epidemic

Epidemic develops as a consequence of the interactions of the populations of their two components, host and pathogen, as influenced by environmental and human interference over time. The interactions of the host and pathogen produce the third component, disease. Each of the three primary components (host, pathogen, disease) consists of subcomponents. These for the host are, for example, annual/perennial/tree; its growth stages i.e. seedling, tillering, blossoming; propagation by seed/vegetative; resistant/ susceptible.

Subcomponents of the pathogen are pathogenicity (biotroph, necrotroph, toxins, penetration mode); virulence (varietal specialization/race); sporulation (kind and amount of inoculum); dispersal (wind, water, vector) and survival (duration, form). Subcomponents of the disease include infection (no. of lesions, systemic); pathogenesis (incubation period); lesion formation (size, rate, toxins), infectiousness; spread (infection gradient in plant population); multiplication (length of reproduction cycle, duration, number of cycles per season) and survival (longevity in months/years).

Factors That Affect the Development of Epidemic

Hosts, pathogens, environments, and human activities are the factors affecting the development of an epidemic.

[I] Host factors

These include external as well as internal factors of host plants. These are as follows.

1. Levels of genetic resistance or susceptibility. Host plants with high levels of (vertical) resistance do not allow a pathogen to become established unless a new race of pathogens develops. Plants with lower levels of (horizontal) resistance become infected.

2. Degree of genetic uniformity of plants. When genetically uniform host plants, particularly in terms of genes for disease resistance, are grown over a large area, there is much possibility of the appearance of a new race of pathogens to attack them leading to an epidemic. This is the reason for the highest rates of epidemics generally on vegetatively propagated crops and the next highest in self-pollinated crops, whereas the lowest in cross-pollinated crops.

The type of crop and age of plants are also important in epidemic development. In annual crops like corn, wheat, vegetables, cotton, tobacco, etc, epidemics generally develop many moves rapidly in a few weeks) then in the perennial woody fruit and forest trees (slow epiphytotic, some taking several years). In pear decline, Dutch elm disease, and chestnut blight, epidemics are very slow.

Plants’ susceptibility to disease also changes with age and generally three trends of disease progression could be seen. In some, such as Pythium damping off and root rots, downy mildew, leaf curl of peach, rusts, systemic smuts, bacterial blights and viral infections, the hosts (or their parts) are susceptible only during the growth period, becoming resistant when adult.

BSc 2nd Year Effect of Environment on Disease Development notes
Change of susceptibility of plant parts with age.

With several diseases like rusts and viral infections plant parts are resistant while still very young, become more susceptible later in their growth, and then become resistant again before they are fully expanded. In other diseases like blossoms or fruit infections by Botrytis, Penicillium, Monilinia, and Glomerella and in all post-harvest diseases, plant parts i.e. fruits are resistant during growth and early adult period but become susceptible near ripening.

Still, in diseases like the late blight of potato, early blight of potato and tomato, etc., there is a stage of juvenile susceptibility during the growth period of a plant, followed by a period of relative resistance in the early adult period and then susceptibility after maturity.

[II] Pathogen factors

These include the following:

1. Virulence level. Virulent pathogens infect the host rapidly and produce large amounts of inoculum at faster rates.

2. Inoculum density. The greater the amount of inoculum near the host (within fields), the more inoculum reaches the plants, and at an earlier time.

3. Reproduction pattern of the pathogen. Pathogens, such as most fungi, bacteria, and viruses, have short reproduction cycles and thus can produce many generations in a single growing season. These are polycyclic pathogens, that usually cause rusts, mildews, and leaf spots and are responsible for sudden catastrophic disease epidemics. Such pathogens are high birth rate pathogens, causing rapid epidemics as the diseases spread in a compound interest manner (compound interest diseases).

Some soil fungi, such as Fusarium spp., Verticillium spp, and most nematodes usually have one too few (2-4) reproductive cycles per season. Limitation in their dispersal limits their potential to cause sudden epidemics in a single season. They often cause more localized, slower epidemics. There are pathogens like smuts and several short-cycled rusts that lack a repeating spore.

They require an entire year to complete their life cycle (monocyclic pathogens). In such cases, inoculum builds up from one year to the next, and epidemics develop over several years. Similarly, epidemics caused by pathogens that require more than one year to complete a reproduction cycle are slow to develop (slow epidemic). They are low death rate pathogens and cause simple interest diseases. Examples – are cedar- apple rust (2 years), and white pine blister rust (3-6 years).

4. Ecology of the pathogen. As in most fungi and parasitic phanerogams, the inoculum is produced on the surfaces of aerial parts of plants. This can be dispersed easily to longer distances and can cause widespread epidemics. Other pathogens like vascular pathogens (fungi as well as bacteria), mycoplasmas, viruses, etc. reproduce inside the plants.

The inoculum in such cases can spread only with the help of vectors. Thus epidemic development is possible only when there are abundant and active vectors. In soil-borne fungi, bacteria, and nematodes, which produce inocula on plant debris lying in soil, inoculum dispersal is very slow. (BSc 2nd Year Effect of Environment on Disease Development Notes Study Material)

5. Mode of the spread of the pathogen. Inocula of rusts, mildews, and leaf spot fungi are spores that can be easily dispersed by air and strong winds over a long distance. They cause the most frequent and most widespread epidemics. The next important group is those whose inocula are carried by airborne vectors. There are many viruses.

Wind-blown rain pathogens, such as anthracnose and apple scab fungi and most bacteria, cause every year severe but localized epidemics within a field, a growing belt, etc. (BSc 2nd Year Effect of Environment on Disease Development Notes Study Material)

Seed and soil-borne pathogens suffer from several limitations and hardly cause sudden or widespread epidemics but may cause local, slow-spreading diseases. (BSc 2nd Year Effect of Environment on Disease Development Notes Study Material)

[III] Environmental factors

The concurrent presence in the same area of susceptible plants and virulent pathogens does not always guarantee numerous infections and the development of an epidemic. (BSc 2nd Year Effect of Environment on Disease Development Notes Study Material)

That’s how the environment has a controlling influence on the development of epidemics. The environment may affect each component of the epidemic i.e. host as well as a pathogen, vectors, etc.

The most important environmental factors that affect disease epidemics are moisture, temperature, and the activities of humans in terms of cultural practices and control measures. (BSc 2nd Year Effect of Environment on Disease Development Notes Study Material)

Development of an Epidemic

For a disease to spread over a large area and develop into an epidemic, the right combinations of environmental factors must occur and spread constantly or repeatedly at frequent intervals over a large area.

An epidemic can occur in a garden, a greenhouse, or a small field, but an epidemic generally implies the development and rapid spread of a disease on a particular kind of crop plant cultivated over a large area (a large field, a valley, a part of country, the whole country or even part of a continent). (BSc 2nd Year Effect of Environment on Disease Development Notes Study Material)

Therefore, the first component of a plant disease epidemic is a large area planted to one, more or less genetically uniform crop plant, with the plants and the fields being close together. (BSc 2nd Year Effect of Environment on Disease Development Notes Study Material)

The second component of an epidemic is the presence or appearance of a virulent pathogen at some point among or near the cultivated host plants i.e. cohabitations of host plants and pathogens. Such cohabitation may occur daily. But an epidemic will develop only when the combination and progression of the right environmental conditions exist. The third component of an epidemic is this combination of environmental factors. (BSc 2nd Year Effect of Environment on Disease Development Notes Study Material)

Epidemics develop only when the combination and progression of the right sets of environmental conditions i.e. moisture, temperature, and wind or insect vector, coincide with the susceptible stages of the plant and with the production, spread, inoculation, penetration, infection, and reproduction of the pathogen. (BSc 2nd Year Effect of Environment on Disease Development Notes Study Material)

Lastly and the most important fourth component of an epidemic is that in each new location, the same set of favourable moisture, temperature, and wind or vectors must be repeated so that new infections, reproduction, and dispersal of pathogens must occur as quickly as possible. (BSc 2nd Year Effect of Environment on Disease Development Notes Study Material)

These conditions must be repeated several times within each location. It is these repeated several infections that would result in the more or less complete destruction of almost every plant within the area of an epidemic. Fortunately, these conditions do not occur very often over very large areas, so plant epidemics are relatively rare. (BSc 2nd Year Effect of Environment on Disease Development Notes Study Material)

Briefly, we may say that the following conditions favour the epiphytotic: (i) virulent pathogen with short sporulation (latent) period. (ii) rapid spore germination and infection. (iii) susceptible plant at the susceptible stage of growth. (iv) intensive monoculture of susceptible plants over large areas, and (v) environmental conditions favourable to the pathogen at all points in the infections cycle i.e. sporulation and subsequent liberation, dissemination, and deposition of spores in variable conditions, germination, infection, and growth in host tissues.

Weather during the intercrop period as well as during crop season can be important in seasonal carry-over and it may affect the initial outbreak and subsequent spread of the pathogens. (BSc 2nd Year Effect of Environment on Disease Development Notes Study Material)

Modeling and Computer Simulation of Epidemics

In recent years attempts have been made to develop models of potential epidemics of some common and severe diseases. Various components and subcomponents of the disease are taken into account in quantitative terms to construct a model.

The availability of computers has allowed us to write programmes that allow the simulation of epidemics of several plant diseases. EPIDEM was the first computer simulation programme written in 1969 for early blight epidemics of potato and tomato caused by Alternaria solani. This resulted from modelling each stage of the life cycle of the fungus.

Subsequently, such programmes were written for Mycosphaerella blight of chrysanthemums (MYCOS), southern corn leaf blight caused by Helminthosporium maydis (EPICORN), and apple scab caused by Venturia inaequalis (EPIVEN). EPIDEMIC was also written for stripe rust of wheat. In computer simulation, the data describing the various subcomponents of the epidemic and control methods at specific points are given to the computer. (BSc 2nd Year Effect of Environment on Disease Development Notes Study Material)

This is the success of modelling and computer simulation that we could be able to forecast plant disease epidemics. A forecast is extremely useful to farmers in the practical management of crop diseases.

BSc 2nd Year Effect of Environment on Disease Development Notes Study Material

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