Introduction to Plant Diseases
Agricultural production of the world sustains annual loss of about 20 to 30% on an average due to plant diseases in different crops and in different countries. Occasionally, the losses rise even to 100% in the most favorable circumstances or when no control measures are undertaken in case of some important diseases. Plant diseases is one of the major bottlenecks in Agricultural Production particularly in irrigated crops, in monoculture cultivations and in certain widely grown rainfed crops as well.
To avoid such losses, therefore, it is necessary to know exactly what is a `plant disease’? Under given optimum conditions for plant growth viz. The soil type, climate and nutrition, the crops are usually affected by two undesired factors i.e. diseases and/or pests. Most of the farmers even today are not able to identify the difference between these two factors causing considerable losses to a particular crop and the cause is considered as some `disease' even if actually it is some `insect pest'. However, if one wants to adopt some control measures, it is absolutely essential to know the difference between the two i.e. a `Disease' and a `Pest' for the symptoms developed and losses caused by the two are different, their life cycles are different and consequently the control measures to be adopted are also entirely different. Further, although `pesticide' is a term commonly used for chemicals employed for control of insect pests as well as of plant diseases, the chemicals actually effective against the two are totally different.
When an insect pest affects any crop, by critical examination of the affected plant specimen with an ordinary hand lense, several minute insects like aphids, jassid hoppers, flies or small larvae, etc. can be seen moving and feeding on the plant parts. In case of a plant disease, however, no such minute insects are seen on the affected plant parts but only the symptoms like leaf spots, blightening of leaves or growing shoots, writing of entire plant, root rot or fruit rot, curling and dwarfing, etc. are seen to have developed.
Insect pests cause losses to plants by two ways. Some insects eat away the plant parts while others only suck the plant sap. The former are called `biting' or `chewing' insects while the latter are called as `sucking' insects. In case of diseases on the other hand, the micro-organisms responsible for causing the most diseases utilize the plant food available in different parts viz. Leaves, flowers, fruits, stems, roots, etc. and grow inside or on the surface of these plant parts under favorable conditions and develop different kinds of symptoms on the affected parts. Neverthless, in certain cases besides such involvement of any kind of micro-organism, other unfavourable factors relating to soil, climate or nutrition, etc. and sometimes certain insects also develop symptoms resembling to those caused by different micro-organisms.
Two types of factors cause plant diseases viz.-
Parasitic causes - i.e. different kinds of microorganisms.
Non-Parasitic causes - i.e. unfavorable conditions relating to soil, climate, nutrition, water, sunlight, harmful chemicals, etc.
Parasitic Causes: These include different types of micro-organisms. Infection and development of micro-organisms inside or on the plant surface, however, necessarily require suitable climate and a susceptible host plant variety. In majority of the plant diseases following types of micro-organisms are encountered as the causal agents -
Fungi: - (fig.1) :- In the rainy season, when high humidity prevails for longer duration’s leather goods, fruits and vegetables, bread or other organic matter left in unventilated places usually develop white, gray, yellow or bluish green powdery growth on their surface. This growth consists of different kinds of living micro-organisms called Fungi (singular - fungus) and thousands of such fungal species exists in nature. Many of them survive and grow on dead decaying organic matter of plant or animal origin; while some others grow only on living tissues of plants or animals. The latter, therefore, are called `parasites' while the former are termed as `saprophytes.' The parasitic fungi are thus responsible for causing different diseases to plants and animals. (fig.1.A) Fungal body consists of minute threadlike structures called `mycelium' which may be unicellular (fig.1.B) or it may be multicellular (fig. 1.C). Under favorable conditions, they grow very fast and in a few days produce innumerable microscopic seedlike reproductive structures called `spores.' Wind, rain or irrigation water and insects, etc easily carry these spores from infected plants to healthy plants and thus are responsible for rapid spread of the disease. Under unfavorable conditions of climate (temperature and humidity) and food the fungi develop different types of resting structures which remain in dormant condition and start their activity afresh as soon as favorable conditions prevail.
The lifecycle of fungi, thus, consists mainly of two phases viz.:-
Resting stage - In many fungi, this stage is in the form of different types of spores developed by sexual methods or it may be in the form of thick-walled mycelium or its thick-walled individual cells. These resting or dormant structures are highly resistant to adverse conditions like very high or low temperatures, lack of food and water, etc. for varying periods ranging from few days to some years.
Active Stage: - As soon as the conditions of climate and food become favorable the resting structures restart their ability by producing fresh mycelium which causes the `primary infection' of plant diseases, the fungal mycelium growing fast inside or on the host plant tissues. Further spread of the causal fungus and the `secondary spread' of the disease occurs mainly by innumerable spores rapidly developed by asexual methods and carried by wind, water, insects, implements, etc. from diseased plants to healthy ones. Most of the severe plant diseases of different crops are caused mainly by different kinds or `species' of fungi and are responsible for great economic losses. The common symptoms of such diseases can be short-listed as leafspots, mildews, blights, rots and wilts, etc.
Bacteria - (fig.2)
Viruses :-
Viruses are not mentioned as true microorganisms because of their characters in between living and non-living entities and their sub-microscopic size. They are therefore, treated as ultra-microscopic `particles'. They also do not ingest any food and do not grow in size and number like true microorganisms. They survive only inside the living individual cells of plants or animal tissues. When once they come in contact with the cell sap or contents of the cells, several such `particles' are naturally developed from the cell sap itself only due to the presence or contact of the original particle(s). Due to this character, the viruses are treated as ` contagious'. Although, their body is composed of organic matter it lacks in certain criteria to donate the characters of a true living organisms. Viruses are mainly carried from diseased plants to healthy ones by different kinds of insects such as aphids, jassid hoppers, flies and mites, etc. Besides these agents some viruses are also carried by nematodes or by direct contact through wounds and injuries. When a viral disease infects once a plant, it never recovers for the viruses are practically immune to any chemical treatment. The symptoms of viral diseases are characteristic and they include chlorosis, mosaic, curling, dwarfing and sometimes leaf spots or specks. The affected plants usually do not die immediately but remain nearly unproductive.
Bacteria (singular-bacterium) are microscopic unicellular organisms with their shapes varying from round to oval or short rods (fig.2) They do not produce reproductive structures like fungi but the bacterial cells themselves divide from one to two, from two to four and so on. The dispersion commonly occurs by water, insects, seeds and implements, etc. Bacteria usually are not carried by wind. Plant parasitic bacteria have no specialized resting stage but under unfavorable conditions some of the several bacterial cells remain dormant inside the affected parts or even in the dead and dried plant tissues for some days or months and start their activity when conditions become favorable. As compared to fungal plant diseases severe plant diseases caused by bacteria are less.
The common symptoms developed by bacterial plant diseases are leafspots, blights, fruits and vegetables, rots, cankers and wilts.
Nematodes - (fig.3) :
Nematodes are microscopic wormlike organisms reproducing sexually by production of eggs. The males and females differ in their shape as shown in figure. These organisms live either freely in soil or inside plant roots. Plant parasitic nematodes damage the plants by feeding on the roots either externally or internally. The nematodes living freely in the soil may be stationary or may travel short distances. Water, soil and implements, etc. are the factors responsible for spread of nematodes from one place to another. Some nematodes enter the plant roots and develop knots of different sizes on the roots while some other kinds of nematodes remain outside the roots and feed on the roots externally. The root injuries caused by such external feeders serve as avenues for entry of parasitic fungi and bacteria, which cause further damage to the plants. The plants infected by nematodes loose vigor and turgidity, appear sick or unhealthy, lack in productivity and may die eventually.
Non-Parasitic causes:-
The disease symptoms resembling to those produced by different microorganisms or viruses are sometimes developed by other factors also. Such factors include -
Unfavorable soil conditions- illdrained soil with high salt contents.
Imbalanced nutrients- high or low proportion of macro and/or micronutrients.
Chemicals or Hormones:- Some chemicals used for control of pests or diseases may prove to be phytotoxic to plants. Chemicals or hormones used in excess proportions may also prove harmful.
Water- salty, with high or low pH, polluted, inadequate or excess water develops undesired symptoms on the plants.
Climate- Very high or low temperature, fog, hailstorm etc.
Sunlight- usually inadequate.
Insects- some kinds of sucking insects also produce disease like symptoms.
Due to non-parasitic diseases the plants become dull, remain dwarfed, the leaf margins may burn, the leaves may be blighted or develop clorotic or dead, dried spots of varying sizes on their surface, sometimes the leaves may become thick and rough and productivity of such plants is significantly reduced in terms of quantity as well as quality. In severe cases the plants may succumb to these undesired factors. Neverthless as no parasitic micro-organism are involved, There is no possibility of spread of these diseases and the trouble remains as localized one. Such diseases can be controlled if the cause is exactly known and remedial measures are possible and feasible.
Ag.
Technologies
(Disease Management)
