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In this article we will discuss about the external and internal symptoms of virus infection in plants.
External Symptoms of Virus Infection:
These are further divided into two main categories:
(a) Primary symptoms and
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(b) Systemic symptoms.
(a) Primary Symptoms:
These are the initial symptoms and are the result of local reaction at the spot of inoculation. These symptoms appear in the form of local lesions and clearing of veins. The local lesions appear as a result of death of the cells of small area at the inoculation point. In vein clearing, the veins of the young leave become conspicuous due to clearing or chlorosis of tissue in or adjacent to it.
(b) Systemic Symptoms:
In this most parts or whole of the plant is involved.
The chief systemic symptoms are:
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(i) Chlorosis:
It is characterized by the uneven distribution of chlorophyll in yellow and green patches on the leaf. These patches are irregularly distributed among normal green tissues and make a mosaic pattern. This is the most common symptom and is produced by various viruses e.g., mosaic of cucurbits, mosaic of potato, mosaic of sugarcane and Tobacco mosaic (Fig. 1) etc.
(ii) Yellows:
In this symptom uniform chlorosis of the leaves take place e.g., Rice yellows.
(iii) Necrosis (death of cells):
In this type of symptom, the infected part of the plant, group of cells collapse, become brown and die. It appears in various forms. Some viruses affect the tissue at point of inoculation by causing a localized breakdown, it is known as local necrosis. Some-times necrosis involves parenchyma and veins of leaves and is called as streak. A rapid killing of bud or branch of entire top of the plants is known as top necrosis e.g., Tobacco necrosis, Tomato streak etc.
(iv) Ring Spotting:
On infected leaves, this symptom appears in localized spots. These spots consist of various types of chlorosis and necrosis. The spots may be circular chlorotic areas and are called chlorotic ring spots. In other cases, the necrosis may appear in rings alternating with normal green areas. Such spots are called necrotic ring spot, for example, Tobacco ring spot disease.
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(v) Distortion:
It is a common symptom of virus diseases. This symptom is characterized by the alteration in the symmetry of leaf arrangement, crinkling of edges of the leaf, leaf rolling and leaf resetting, e.g., leaf roll of potato, leaf curl of papaya, leaf curl of tomato etc. Sometimes the leaves become reduced in size, at the internodes and there is production of a cluster of distorted shoots. It is called witches broom.
(vi) Stunting:
This symptom is characterized by the reduction of the size of the leaves, fruits, petioles and internodes e.g., Bean yellow mosaic, Bunchy top of banana etc.
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(vii) Breaking and Greening of Blossoms:
This symptom is characterized by the attractive variegation in flower colour. This is called breaking of flower colour e.g., Tulip, Abutilon etc. Sometimes petals become green due to virus infection and it is called virescence.
(viii) Entations:
Hair-like out-growth appears on the leaves and stems etc. These out-growths are known as entations for example, Dolichos entation mosaic.
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(ix) Production of Outgrowths:
Various types of abnormal growths like tumors, swelling and hills appear on infected parts for example, Fiji disease of sugarcane, Tobacco leaf curl disease, etc.
(x) Root Symptoms:
Infected plants show the drying of lateral roots, over production of tumors and galls in roots e.g., wound tumor disease of Pea.
Internal Symptoms of Virus Infection:
These are of two types:
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(a) Histological and
(b) Cytological
(a) Histological Symptoms:
(i) Hypoplasia:
Infected plants show reduced growth and differentiation.
(ii) Hyperplasia:
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Infected plants show excessive growth and abnormal development of tissues due to an increase in the number of cells.
(iii) Necrosis:
Death of the cells or tissues takes place and some other histological changes can also be seen. Phloem cells degenerate or die, callose deposition occurs on the phloem sieve plates. Tyloses are formed in the xylem elements. The xylem elements develop characteristic signified strands which are known as endo-cellular cordons.
(b) Cytological Symptoms:
The main cytological symptom of virus infection is the development of intracellular inclusion bodies, which are of two main types (a) crystalline and (b) amoeba like amorphous bodies. The latter are also known as X bodies. The exact nature of these bodies is not known.
These bodies are common in the epidermal cells of leaves and stems. They are also present in roots, flowers and in most tissues, except the phloem sieve element. The bodies have been reported in plants infected with TMV, Tobacco-ring spot, Turnip yellow mosaic, Potato virus and Hyosyamus mosaic virus etc.
Synergistic Effect:
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In many cases symptoms shown by the infected plants are due to synergistic or combined action of two or more viruses. For example, Rugose mosaic of potato is due to infection by two viruses namely potato virus X and potato virus Y.
