In this article we will discuss about the classification of plant diseases.
(i) Diseases classified in relation to their occurrence:
Study of plant diseases in relation to their occurrence (interaction of populations of plants, pathogens and environment) is known as epidemiology. There may be infectious disease and contagious disease. A disease which spreads slowly and is incited by a transmissible pathogen is referred as infectious disease, and that which spreads rapidly is a contagious disease.
Considering from the point of view of occurrence the diseases are often classified as:
Endemic, epidemic or epiphytotic, and sporadic.
A disease which constantly occurs from year to year in moderate to severe form is an endemic disease. Greeneria fruit rot of grape (Vitis vinifera) is an endemic disease in India caused by Greeneria uvicola. But an epidemic disease or epiphytotic disease is the one whose incidence is periodical in wide areas spreading very fast. The term epiphytotic is rather used in case of plant disease.
The epiphytotic disease is usually very responsive to variations of environmental conditions which actually control its incidence. Like endemic disease, the pathogen may be more or less constantly present but to ensure disease development environmental conditions must be favourable.
The Epiphytotic Disease incidence is controlled by:
(a) Availability of susceptible host,
(b) Virulence of pathogen,
(c) Rapidity of dissemination of inoculum and host infection,
(d) Rate of production of inoculum; and
(e) Micro- and macro-climatic conditions.
According to their occurrence in crops, the epiphytotic diseases may be of two kinds; the diseases ordinarily endemic but assume epidemic proportions from time to time, and those completely newly introduced in the locality—the exotic diseases.
Wart disease of potato caused by Synchytrium endobioticum is an exotic disease in India. In the former kind, the disease incidence depends mainly on the variations of the climatic conditions. Whereas, the latter is resulted either from a new parasite having been introduced from another country, or from the passage of a parasite of some pre-existing plant of a place to a newly introduced host.
Some of the important epiphytotic outbreaks are:
The Irish famine of 1945 and 1946 due to the destruction of the potato crop by the late blight of potato disease; the destruction of vineyards in England in 1845 and in France in 1848, by downy mildew disease; the devastation of the flourishing coffee plantation of Ceylon caused by coffee rust disease; and the destruction of sugarcane industry of the West Indies in 1895 by the rind disease.
Similar situations arose in the United States where almost the entire chestnut trees were completely killed by chestnut blight disease, so also the fast spreading of the Dutch elm disease in the wiping out of elm trees from the country.
The sporadic disease is rather an epiphytotic disease except that its occurrence is at very irregular intervals and in lesser areas. In a sporadic disease, pathogen affects only a few plants in a large population of host plants and others remain unaffected.
(ii) Diseases classified based on the nature of dispersal medium:
Diseases are often classified as: soil-borne disease, seed-borne disease, wind-borne disease, etc. Here importance is given only on the dispersal medium of the pathogen and nothing else.
(iii) Diseases classified according to parts of host affected:
The classification of plant diseases based on parts of the host affected may be outlined as:
(a) Foliage disease
(b) Stem disease
(c) Boot disease
(d) Vascular disease.
Such a classification neither does clearly indicate the nature of the causal agency nor the effect produced on the host tissues and does not have much practical value.
(iv) Diseases classified on the basis of hosts affected:
The classification of plant diseases on the basis of hosts affected is simply a matter of convenience. There is no fundamental principle involved in this system of classification.
Some of these diseases are:
Vegetable diseases or Diseases of vegetable, Fruit disease, Cereal diseases, Timber diseases, Diseases of ornamental plants, Diseases of shade trees, etc.
(v) Diseases classified on the basis of symptoms:
On the basis of symptoms, plant diseases are classified into three categories:
(a) Diseases resulting in the death of the affected tissues-necrotic
(b) Diseases resulting in the slowing down or stunting of the growth of the plant or plant parts-atrophic and
(c) Diseases resulting in the overgrowth or over development of the plants or organs affected-hypertrophic.
According to symptoms diseases are often designated as blight disease, mildew disease, rust disease, smut disease, rots, cankers, etc. This system of classification gives an idea about the nature of damage done to the host and as such has some practical value.
(vi) Diseases classified on the nature of causal agencies:
The diseases of plants may be attributed to one or more of the following causal agencies:
(c) Slime molds.
(e) Parasitic angiosperms (Fig. e.g., Viscum album, Dendrophthae sp., Phoradendror. sp., Orobanche sp., Cuscuta spp., and Striga spp.
(f) Algae (Cephaleuros spp.)
(a) Soil-moisture disbalance.
(b) Nutritional disorders.
(c) Optimal temperature disbalance.
(d) Light intensity disbalance.
(e) Gas, smoke and other air pollutants.
(f) Careless spraying of chemicals.
The first group comprises of parasitic causal agencies (with the exception of Virus, may be) and the disease produced is the parasitic disease. Whereas, the second group consists of non-parasitic causal agencies and the corresponding disease produced is the non-parasitic disease.
The first group includes a great host of living organisms which live within or are attached to the host plants, affecting their health and frequently causing death.
The extent of their parasitism is extremely variable, which again is dependent upon the nature of host and parasite and their interaction. Some parasites destroy their hosts in very short periods of time; while others live upon their hosts for long period, even for many years, before death results.
Still other parasites never cause the death of the host plant, but do reduce its vitality, growth, or fruit production. Some parasites do not require a great amount of food from the host plant but grow in such a manner as to interfere with the performance of its ordinary functions.
Again some of which are of little importance in themselves, open the way for the attacks of organisms which cause other and more destructive diseases.
A parasitic causal agency may also produce conditions suitable for the growth of causal organisms which are in no way the cause of the disease in question. The presence of two or more organisms in or on a diseased plant thus frequently prevents a satisfactory diagnosis and treatment.
Diseases resulting from the second or non-parasitic group of causal agencies are frequently very difficult to diagnose. Often they are of little importance in themselves, but make a plant so weak as to make it possible for one of the specific organisms of the first group to attack it.
Plant diseases are classified with a view to:
(i) Get a clear idea about the cause or causes of the disease,
(ii) Follow the pattern of disease development, and
(iii) Determine the extent of damage done to the host.
These facts are needed to diagnose a disease and to recommend control measures of the disease concerned. Of the various ways of classification of plant diseases stated above, only the one classified on the nature of causal agencies gives adequate information about the cause of the disease development and related facts.
Hence such a classification has the most practical value and is often considered as modern system of classification of plant diseases. Next best is the one based on the nature of symptoms which is also very useful to diagnose a disease.
A. Non-Parasitic Diseases:
Diseases caused by non-parasitic causal agencies are known as non-parasitic diseases or physiological diseases, also known as deficiency diseases. They are diseases having specific symptoms. Although the symptoms and effects of certain diseases are well-known, the factor or combination of factors producing them is still questionable.
Until early part of 1900 the field of non-parasitic diseases did not receive much attention.
It is now well-recognized that much loss is also brought about through the effect of non-parasitic causal agencies, they are precisely soil-moisture disbalance; nutritional disorders; high or low temperature; change in intensity and quality of light; atmospheric impurities like poisonous gases; certain disagreeable chemicals (fungicides and insecticides) of unusual doses, etc.
I. Soil-Moisture Dis-Balance:
The general condition of water imbalance in diseased plants is associated with a derangement in its absorption, transport, and/or transpiration and is often accompanied by a disturbance to the various other basic processes, such as carbohydrate and nitrogen metabolism, respiration, and mineral uptake.
It is reasonable to presume that the primary effect of a disease is on the water status of the cells, which reflects in the various derangements in metabolism functions, leading to a progressive degeneration of vital activities, culminating in death. It will not be exaggeration to say that the fire of plant life burns in water.
Different plants react differently with regard to soil-moisture variations. The most apparent effect of water deficiency is loss of form in herbaceous plant tissues due to loss of turgour exhibited by wilting, die-back, etc. Continued shortage of water eventually results in cessation of growth and decrease in the production of auxins.
The physical structure of the soil not only affects its water retaining capacity and aeration but, if sufficiently hard and compact, can retard the growth of roots and so result in stunted and weak plants.
Some of the symptoms due to soil-moisture dis balance are:
This is the most common symptom caused by deficiency of water supply. Wilting is recognizable from drooping of leaves and stem tips. There may be physiological and pathological wilting. The former condition is seen due to interference of water balance caused by irregularity of any of the physiological processes connected with water circulation in the body, such as absorption, conduction and transpiration.
This deficiency can be easily corrected by rectifying the hindrance of absorption mechanism existing in the soil. Whereas, pathological wilting is caused by pathogenic agencies.
This symptom would naturally arise not only from the interference with water uptake consequent to the damage to subterranean portions of the plant, but as a result of the presence of metabolites of certain infecting parasitic fungi which act as wilt toxins.
The disease also arises out of the dysfunctions of root system resulting from injury and foot rot incited by some fungi. Water shortage may also be caused by the dys-functioning of conductive elements (rupturing of xylem elements), brought about by the growth of hyphae and masses of bacteria which also obstruct water flow through the xylem (Fig. 343)
The dying backward from the tip of twigs or branches of trees and shrubs during the growing season is the symptom of plants and their branches caused by definite interference with water supply, either by tissue disintegration or by inducing gum formation in the xylem region.
This is a symptom that is exhibited by the sudden death and browning of large indefinite areas of leaves and fruits (a) due to inadequate supply of water, and (b) due to a sudden and rapid loss of water from the leaves which cannot be replaced by the roots, since the soil moisture is low.
The affected leaves show dead brownish areas on their margins or similar areas between the veins, but the leaves remain alive and do not drop so that the tree is little injured (Fig. 344B & G). The symptom is most pronounced on the side of the tree facing the wind.
Blast is the sudden death of young buds, inflorescences, or young fruits due to water shortage.
The drying and dying of leaves specially of grasses, maize and tobacco, for want of water is firming.
(vi) Blossom-end rot:
Water-soaked spots appear at the junction of the floral parts increasing rapidly ultimately forming depressions turning dark sunken areas which are very suitable for the infection of parasites. This is the blossom-end rot. This happens due to excessive loss of water from the plant body.
An excess of water is unfavourable to trees, since it stimulates the excessive development of tender, succulent tissue, which is more readily invaded by parasitic fungi and more susceptible to extremes of heat and cold.
Roots must get oxygen necessary for their development from the soil air, and, since most trees cannot obtain sufficient oxygen from soil saturated with water which results in the death of the roots by asphyxiation. Bark becomes very soft and spongy and several times thicker than normal, such a condition is commonly known as dropsy.
II. Nutritional Disorders:
Abnormalities of one kind or the other appear in plants due to dither pronounced shortage of supply or in excess of certain essential elements which really upsets the normal nutritive processes. These abnormalities are known as mineral-deficiency or mineral-excess diseases.
These diseases are expressed by some characteristic symptoms like chlorosis, yellowing, defoliation, resetting, dark spots, etc. Here only the most common and widely prevalent and comparatively easily recognizable ones are considered.
Unsuitable chemical composition or pH of the soil can be damaging to plants. Some plants can withstand considerable alkalinity or acidity, but most grow best around neutrality or slight acidity. An unfavourable pH results in poor growth and sometimes death.
It also has indirect effects on the availability of mineral elements in the soil to plant roots, as in calcareous soils where it reduces the availability of iron and manganese (lime induced chlorosis). It is sometimes possible to correct high acidity by addition of lime and, less easily, high alkalinity by addition of sulphur, although this may have phytotoxic effects.
The salts responsible for the unfavourable pH can sometimes be washed out by irrigation.
Faulty mineral nutrition can predispose plants to attack by some pathogens. Potato blight caused by Phytophthora infestans is said to be more severe on plants growing in magnesium deficient soil, and excess of nitrogen results in ‘soft’ plants which tend to be more susceptible to pathogens and pests.
(i) Iron-deficiency diseases:
Chlorosis is a most common iron-deficiency disease symptom. It is exhibited by a very acute yellowing (Fig. 344D) of green parts and usually happens when the soil becomes alkaline due to conversion of available ferrous salt to unavailable ferric salts. It is not always due to an absolute deficiency of iron in the soil.
The chief causes of chlorosis are usually due to presence of excess of lime—lime-induced chlorosis or of manganese—manganese-induced chlorosis in the soil.
The lime-induced chlorosis is rather common in vineyards, orchards and in clovers. Whereas, the manganese-induced chlorosis, comparatively less common, is best known in pineapple cultivation as ‘yellows’. This is very well-known pineapple disease.
(ii) Boron-deficiency diseases:
Boron-deficiency, in general, affects storage organs of plants such as, roots, tubers, fruits, and similar other organs. The diseases are recognizable by various symptoms of which more common ones are, heart and dry rot caused by distortion and necrosis of fleshy tissue.
The chief anatomical response to boron-deficiency is visible in the meristematic regions of certain plants leading to hyperplasia, hypertrophy and abnormal differentiation of parts.
Most common boron-deficiency disease in sugar-beet is heart and dry rot. Besides this, the young unfolded leaves fail to develop normally and ultimately become necrotic and fall off. Similar is the condition of the dormant buds in the axils of older leaves. Disturbance of normal functioning and development of vascular tissue takes place.
In apples due to boron-deficiency light brown spots of dead tissue appear in the fruits, and in trees rosette and die back. In case of cruciferous plants particularly in cauliflower, leaves become deformed, brown-coloured, sometimes discolouration takes place.
In comparison with this, cabbage is less susceptible to boron-deficiency. In cabbage, discolouration of head and break down in the core take place.
(iii) Potassium-deficiency diseases:
Symptoms of potassium-deficiency diseases are very clearly exhibited by the leaves and can be recognised from mottle and chlorosis of leaves particularly near the margins. In some cases the entire leaf turns brown, dies, and becomes brittle. Necrotic spots may also occur in the inner portion of the leaf blade.
Often the first symptom of potassium-deficiency is exhibited by dark- brown colour of the leaflets which is followed by yellowing or bronzing.
(iv) Phosphorus-deficiency disease:
A deficiency of phosphorus in many plants results in abnormally high quantities of anthocyanin’s producing purpling of leaves.
III. High or low temperature effects:
Every growing plant has its own temperature range of normal growth. Any variation either above or below of which results in the irregularities, since temperature affects almost every function of life. These irregularities in growth are expressed by the various symptoms of the diseases induced by high or low temperature.
The nature of the symptoms is also dependent on the nature of plants, their respective parts affected and the extent of variations of temperature.
Here only few are discussed:
(i) High-temperature effects:
Growing plants when subjected to temperatures much higher the optimum are liable to sustain certain injury of cell degeneration which is considered as symptom of the disease induced by high-temperature. The symptom may be visible both in aerial as well as subterranean parts.
Most common high soil temperature disease of potato tuber is the blackheart of potato expressed by blackened centres (Fig. 344A). High temperature above 90°F in transit may also cause blackheart.
Similar symptom may also develop when potatoes are put in poorly ventilated storage places. Specially the tubers at the bottom of deep piles are very commonly liable to face shortage of oxygen developing blackheart. Another common high-temperature injury is sunscald of vegetables, particularly of tomato.
To start with, a water-soaked appearance of tissue is visible which is followed by rapid desiccation leading to the formation of sunken areas which ultimately turn grey in green fruits and yellow in ripe ones.
Premature casting of leaves may occur in trees -when subjected to high-temperature. Leaves of the inner crown are first affected, because they are more sensitive to heat, this is known as heat of defoliation. Again bark when suddenly exposed to the summer sun by removal of neighbouring foliages becomes overheated and dries out.
Such heating and drying out of the bark, resulting in open wounds, occurs on certain young trees with smooth, thin bark, known as sunscald also as bark scorch. Sometimes leaves and twigs of Douglas fir are covered with white sugar. Sugar is exuded in solution directly from the leaf tips during the hot, dry, cloudless days of summer—sugar exudation.
The water then evaporates leaving the sugar encrusted on the leaves. The effect of sugar exudation is rather negligible.
(ii) Low-temperature effects:
Potato tubers when stored in cold storage are often liable to freezing injury of temperature range of 0°C. to 5°C. when stored for a long period of time. The freezing injury is seldom externally visible. The freezing injury results in necrosis of cells. Necrosis may be ring necrosis; net necrosis and blotch type of necrosis.
The ring necrosis of discoloured tissue occurs in the peripheral region, net necrosis of blackened small areas are rather scattered throughout, and blotch type usually comprises irregular areas of various sizes and colour- opaque grey to black in the tubers.
IV. Effects of Change in Intensity and Quality of Light:
Both reduced or increased intensities and any appreciable change in the quality of light result in the development of various symptoms of the disease.
Reduced light intensity promotes succulent growth along with developing internodes longer than usual and turning weak in physical structure and retards chlorophyll formation as a result of which normal green colour does not develop resulting in the etiolation which is rather a very common symptom.
The change in quality of light for example, when the atmosphere is clear short waves reach the earth with greater intensity inducing sunscald of leaves and fruits of vegetables.
V. Effects of Atmospheric Impurities:
Gas, smoke and other air pollutants of the atmosphere cause serious damage to the plant parts.
Orchards and shade trees are commonly injured by unfavourable conditions of the air or soil arising from industrial processes. Forest trees escape most of these adverse factors. Death or injury- is often by the escape of artificial illuminating gas into the soil or by the escape of steam over-heating the soil, encountered in young succulent plants which are sensitive to some of the constituents of illuminating gas.
Forest trees are often affected by smoke. The smoke injury is caused by the gases arising from the incomplete combustion of coal, from the smelting of ores containing sulphur, sulphur dioxide frequently causes severe damage, the symptoms consist of a variety of forms from yellowing and reddening of tissues of leaves to defoliation and stunting.
Three stages of smoke injury are recognized:
(i) The acute stage due to high concentration of smoke in the air causes rapid discolouration of the foliage followed by defoliation and in extreme cases by the death of the plant,
(ii) The chronic stage due to small quantities of gas present in the atmosphere produces unhealthy condition of the plant expressed in stunted growth; and
(iii) The invisible stage, due to the action of gas in very dilute quantity, is characterized by reduction in growth.
The gaconfines its effect to the chlorophyll-bering or green parts of plants, woody tissues are not directly affected. In addition to direct injury to foliage indirect injury is claimed because of soil changes, such as the acidity of the smoke depleting the soil of calcium carbonate and thus reducing the number and hampering the activity of nitrifying bacteria.
Sometimes gas injuries occur in certain fruits and vegetables when they are fumigated in storage and transit, since fumigation becomes necessary to protect them against certain diseases during storage and as a quarantine requirement. The symptoms are usually discolouration of pairs, appearance of sunken brown spots, etc.
Again the effects of air pollutants on plants can be seen around the major population and industrial centres. Plants affected by air pollutants generally respond according to the principal toxicant present. Burning, bronzing, silvering, and growth abnormalities and necrosis are common symptoms. Sulphurdioxide causes black tip or mango necrosis.
VI. Effects of Fungicides and Insecticides:
Certain chemicals—fungicides and insecticides when sprayed on plants as a part of plant protection measure, if sufficient care is not taken during use, cause injuries particularly to the leaves.
The injuries are in the nature of discolouration of the affected parts, abnormal growth leading to unusually rapid development of certain parts of the leaf induced by the careless spraying of pesticide 2, 4—D. Resetting in apples may be caused by parathion; burning, spotting discolouration by improper application of Bordeaux mixture sprays.
B. Parasitic Diseases:
Diseases induced by parasitic causal agencies are the parasitic diseases. The development of a parasitic disease induced by parasitic causal agencies particularly by fungi, bacteria, slime molds and viruses has a sequence of events following in cyclic order and as such comprises of disease cycle. The sequence of events of the disease cycle is greatly conditionable.
But the disease cycle basically consists of two phases— the dormant phase and the active phase.
During dormant phase the causal agency (pathogen) remains in dormant condition and overcomes the unfavourable conditions in the perennating organs. These perennating organs may be spores of varied forms, resting vegetative cells, resting mycelium, virus particles, etc.
The perennating organs may remain in or on propagating organs of the host, in diseased plant debris, in soil, in collateral hosts, in insect body, etc. With the advent of favourable environmental conditions the pathogen becomes active and there starts the active phase and the perennating organs produce primary inoculum.
When disease occurs, its quality, its severity and its epidemic potential or how fast it will advance is very much dependent on the interactions among host, pathogen and physical environment.
The process starts with pre-inoculation (pre-penetration) period when the inoculum is produced which is again controlled by pre-inoculation environment such as, condition of the source of inoculum (perennating organs) and its microclimate.
After successful development of inoculum, will follow the dispersal of inoculum. The inoculum arrives the host tissue after either being dispersed by some agency or directly coming in contact without the help of any medium.
The density, viability, and potential infectivity of the inoculum are important for successful infection. It is meaningless unless the inoculum has found susceptible hosts and can penetrate the host and establish infection.
This is the most vital link in the entire process where environmental dependencies are quite strong since most pathogens have an obligate free moisture requirement. This process is also temperature dependent and so the interaction of time span and temperature of moisture on host surfaces become important.
The situation may be further complicated by light effects, by the change that can take place in the infection court and by the age and physiology of hosts.
Precisely speaking, it is the microclimate of the infection court that plays a very important role in the establishment of host infection. The details of development of the disease and the duration of the disease cycle are often extremely variable.
In some case the inoculum (secondary inoculum) produced by the primary infection is dispersed by various agencies and on coming in contact with the same host, produces secondary infection and secondary disease cycle as against the primary disease cycle produced by the primary inoculum developed by the perennating organs of the pathogen. Whereas in others, the secondary disease cycle may be absent.
The general pattern of disease cycle is presented in Figure 345.
Some of the parasitic diseases are as follow:
Some parasitic angiosperms have a wide and apparently unselective host range but in others there seems to be some degree of host specialization. Viscum album is a semi-parasite on trees. It can adversely affect growth and yield as well as bring about premature death of severely infected trees. Dendrophthae sp. and Phoradendron sp. also can cause extensive damage to various trees.
Orobanche sp. damages tobacco crop. Cuscuta spp. can cause severe damage to leguminous plants. Striga spp. are partial root parasites of sugarcane, cereals, maize and millets.
There are large number of parasitic algae of which special mention may be made of the genus Cephaleuros. Different species of Cephaleuros grow as parasites on various angiosperms causing severe damage to leaves and shoots. C. virescens forms the red rust of tea plants causing economic loss. C. mycoidea attacks tea, citrus, and other plants causing red rust disease.
This alga is sometimes epiphytic and sometimes parasitic on the leaves of the tea plant in India, but is usually parasitic on the stems.
In Sudan C. mycoidea has been found to attack coffee, citrus and guava plants. Cephaleuros causes leaf spotting and shedding and the photosynthetic activity of the foliage is reduced. In case of severe infection, branches and twigs may be girdled inducing die back symptom. This disease is spread by air-borne sporangia which produce zoospores which invade the leaf or stem through stomata.
Large number of insects, mites and nematodes feed on various plants, thereby causing injury and symptoms which can resemble those caused by other pathogens. The damaged tissue may subsequently be colonized by fungi and bacteria, so that it is sometimes difficult to find out the primary cause of symptoms.
Many wound parasites enter the injured plants through the injuries caused by insects, mites and nematodes.
Insects damage plants by chewing, sucking, by injecting phytotoxic saliva besides behaving as vectors and inoculators of pathogens, especially viruses.
The injected phytotoxic saliva may diffuse into the tissues beyond the actual point of feeding and cause symptoms similar to those caused by fungi, bacteria and viruses. Such insects are known as toxicogenic insects. Insects also play a great role as disseminators of inoculum. Mites are frequently present on diseased plant tissue and it is sometimes difficult to assess their significance.
Nematodes attack and sometimes kill host plants. More often their attack results in weakening, reduced growth and diminished yield, and it may also enable other more destructive pathogens to enter the host plant.
When feeding on roots, nematodes can cause several types of injury:
Necrotic lesions, induce formation of hypertrophic cells, and suppression of cell division of apical meristem of roots. Nematodes which attack the aerial parts of plants may cause discolouration, neorosis, blotches, spots, distortion, and galls on leaf, stem and seed. Buds, growing points or flower primoidia are attacked by some nematodes, resulting in abnormal growth of the affected plant.