The following points highlight the six main symptoms of viral diseases. The symptoms are: 1. Mosaic Spotting 2. Ring Spotting 3. Chlorosis 4. Distortion 5. Necrosis 6. Breaking of Blossoms.
Symptom # 1. Mosaic Spotting:
In this case usually the leaves, sometimes the blossoms and other parts of the host plants show spots or patches of light green, yellow or even white. These patches or chlorotic areas may be circular or irregular. They also vary in size.
Symptom # 2. Ring Spotting.
This is a symptom which appears in localized spots. Sometimes circular chlorotic areas appear on the surface of leaves. These are called the chlorotic ring spots. Usually necrosis appears in rings alternating with normal green. The rings are concentric to a central spot. This symptom is called necrotic ring spot.
Symptom # 3. Chlorosis:
It is uniform yellowing of leaves subsequent to infection. It occurs when young growing leaves are infected. It is not a very common symptom.
Symptom # 4. Distortion:
It is a common symptom of virus diseases. It may be in the form of rolling and crinkling of leaves caused by retarded growth of veins. Sometimes there is production of a cluster of thin wiry shoots. It is called witches broom.
Symptom # 5. Necrosis:
It is the death of cells in localized regions. It appears in various forms. Usually it is restricted to small areas on the leaves or streaks on the stem. In the former case the chlorotic tissue may break down and necrotic spots appear.
The necrotic or dead tissue is usually demarcated from the living tissue by a distinct dark brown border. In top necrosis there is rapid killing of a terminal bud or a branch or even entire top of the plant. Sometimes it extends over large areas finally resulting in the death of the whole plant.
Symptom # 6. Breaking of Blossoms:
It is another symptom of virus diseases. Attractively variegated leaves of some ornamental shrubs such as Abutilon striatum are the result of virus infection.
Stunting and Premature Defoliation:
The virus diseases in general are hypoplastic and cause under-development of the host. The host plant remains stunted with shorter internodes, smaller leaves and fruits and general reduction in size. The virus infected plants are more often stunted than killed. Cabbage mosaic causes premature defoliation.
