BSc 2nd Year Effect of Environment on Disease Development Notes Study Material
BSc 2nd Year Plant Pathology Effect of Environment on Disease Development Notes Study Material: BSc is a three-year program in most of the 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 gurujistudy.com you can easily get all these study material and notes for free. Here in this post, we are happy to provide you BSc 2nd Year Plant Pathology Effect of Environment on Disease Development Notes Study Material.
Effect of Environment on Disease Development Notes Study Material
The mere presence of pathogen and its susceptible host in the field does not necessarily mean that disease will develop. For instance, keeping in view the environmental conditions as these prevail in our country, most diseases develop well during winter rather than summer months of the year. Although, all pathogens, all perennial 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 fertilised with nitrogen usually are more severely attacked by some pathogens than are less fertilised 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 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: susceptible plant, infective (virulent) pathogen, and favourable environment. Although plant susceptibility and pathogen virulence remain essentially unchanged in the same plant for atleast several days, weeks or months, the environmental conditions may change more or less suddenly and in various degrees. Such changes influence development of disease in progress, or the initiation of new diseases, more or less drastically. Of course, a change in any environmental factor may favour the host or the pathogen or both, or it may be more favourable to one than is to the other, and the expression of disease will be affected accordingly.
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 favourable temperature pathogens become active and if other conditions are favourable they can cause the disease. Pathogens differ for 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), the bacterial wilt of solanaceous plants (Pseudomonas solanacearum) are more prevalent to 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 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 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 much poorly and is so much weaker, than the pathogen that even weakened pathogen can cause maximum disease development. In root rots of wheat and corn Gibberella zeae) the maximum disease development on wheat occurs at temperature above the optimum for development of both the pathogen and the wheat, but on corn, it occurs at temperature below the optimum for the pathogen and for com. This is because wheat grows best at low temperature whereas corn grows best at high temperature.
Temperature, in combination with sunlight may determine seasonal appearance of symptoms in various viral diseases. Viruses causing yellows or leaf-rolls are most severe in summer whereas those causing mosaics or ringspots during spring or winter.
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 penetration of host by germ tube. Moisture also activates the bacterial, fungal and nematodal pathogens. As splashing rain and running water, moisture also plays important role in 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 atmosphere only during the spore germination, and become independent once they can obtain nutrients and water from the host. Some, 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 a 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 mildews can germinate, penetrate and cause infection when there is 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 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, common scab of potato (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.
Wind affects disease development mainly through its effect on 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 the 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 we surfaces. Wind also injures plant surfaces, that become susceptible to viruses, fungi and bacteria.
Light is relatively less important under natural conditions. It affects host where reduced light causes etiolation of plant parts, which increases their susceptibility to some unspecialised pathogens – Fusarium, Botrytis. Reduced light however decreases 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 completely absent at pH 7.8. But, 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.(Effect of Environment on Disease Development Notes)
[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 as nitrogen results in the production of young, succulent growth and may also prolong the vegetative growth period and delay maturity, that make plants more susceptible to pathogens which prefer to attack such tissues. Lack of nitrogen makes plant weaker, slow growing and faster ageing and may make them susceptible to pathogens that attack weak tissues. Thus high nitrogen fertilisation 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 solanceous plant to Pseudomonas solanacearum wilt, of sugar beets to Sclerotium rolfsii, of most seedlings to Pythium damping off. Perhaps the form of nitrogen (ammonium or nitrate) rather than the amount is also important in decrease 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 as phosphorus, potassium, calcium and also micronutrients also have similar effects on disease development. In general, plants receiving a 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.(Effect of Environment on Disease Development Notes)
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 pathogen, 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 crop. There is only one generation of the pathogen in the life of crop. The number of infected plants may increase as the season progresses but these represent new infections from primary source of inocula, rather than spread from one plant to another. Such pathogens include soil borne fungi attacking roots and seed, 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 pathogen are produced which are disseminated and infect other plants in which it, in turn, produces spores which are disseminated and infect further plants, and so on. There are several generations of pathogen in the life of crop. Many destructive diseases as potato late blight and black stem rust of wheat belong to this category.(Effect of Environment on Disease Development Notes)
Epiphytotics are epidemics of plant diseases. Pandemic are destructive epiphytotics 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 epiphytotics or rapid epiphytotics.
1. Slow epiphytotics. 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.(Effect of Environment on Disease Development Notes)
2. Rapid epiphytotics. 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 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 of the factors that influence them is called epidemiology. Plant disease epidemics are referred to also epiphytotics.(Effect of Environment on Disease Development Notes)
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 pathogen are juxtaposed, the duration of all favourable environmental condition is prolonged, 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 host and pathogen produce the third component, disease. Each of the three primary components (host, pathogen, disease) consists of subcomponents. These for 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 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 Development of Epidemic
Host, pathogen, environment, and human activities are the factors affecting 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 pathogen 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 large area, there is much possibility of the appearance of a new race of pathogen to attack them leading to an epidemic. This is the reason for the highest rates of epidemics generally on vegetatively propagated crops and next highest in self-pollinated crops, whereas the lowest in cross- pollinated crops.
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 much move rapidly in few weeks) than in the perenmial woody fruit and forest trees (slow epiphytotics, some taking several years). In pear decline, Dutch elm disease, chestnut blight, epidemics are very slow.
Plants susceptibility to disease also changes with age and generally three trends of disease progress could be seen. In some, such as Pythium damping off and root rots, downy mildews, 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.
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 late blight of potato, carly blight of potato and tomato etc., there is a stage of juvenile susceptibility during the growth period of 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 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), more inoculum reaches the plants, and at an earlier time.
3. Reproduction pattern of 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 compound interest manner (compound interest diseases).
Some soil fungi, as Fusarium spp., Verticillium spp and most nematodes usually have one to 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 localised, slower epidemics. There are pathogens like smuts and several short-cycled rusts that lack a repeating spore. They require the entire year to complete 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 – cedar- apple rust (2 years), white pine blister rust (3-6 years).
4. Ecology of pathogen. As in most fungi and parasitic phanerogams, inoculum is produced on the surfaces of aerial parts of plant. 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.
5. Mode of spread of pathogen. Inocula of rusts, mildews, leaf spot fungi are spores that can be easily dispersed by air and strong winds over long distance. They cause most frequent and most widespread epidemics. The next important group is of those whose inocula are carried by airborne vectors. These are many viruses. Wind-blown rain pathogens, such as anthracnose and apple scab fungi and most bacteria, cause every year severe but localised epidemics within a field, a growing belt etc. Seed and soil borne pathogens suffer from several limitations and hardly cause sudden or widespread epidemics but may cause local, slow-spreading diseases.
[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. That’s how the environment has a controlling influence on development of epidemics. The environment may affect each component of the epidemic i.e. host as well as 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.
Development of an Epidemic
For a disease to spread over 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 green house or a small field, but 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). 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. 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 right environmental conditions exist. The third component of epidemic is this combination of environmental factors.
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. 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 pathogen must occur as quickly as possible. These conditions must be repeated several times within each location. It is these repeated several infections that would result in 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 that plant epidemics are relatively rare.
Briefly we may say that following conditions favour the epiphytotics: (i) virulent pathogen with short sporulation (latent) period. (ii) rapid spore germination and infection. (iii) susceptible plant at 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 condition, 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.
Modelling 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 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.
This is the success of modelling and computer simulation that we could be able to forecast plant disease epidemics. 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