CHAPTER IV 
SOME REPRESENTATIVE VIRUS. DISEASES 
In this chapter short descriptions are given of a number of virus 
diseases of plants and animals. They are chosen as being as repre- 
sentative as possible of the wide variety of symptoms which infection 
with viruses calls forth. 
In plants, the first virus disease to be described was tobacco mosaic 
and the name was given because of the fancied resemblance of the 
leaf-mottling to a mosaic pattern. This term is now used to describe 
a large number of virus diseases where the main characteristic is a 
mottling of the leaves (Plate Il). 
In tobacco plants infected with the tobacco mosaic virus, the first 
sien of infection is an intensification or “picking out’ of the veins of 
the youngest leaves; this phenomenon, known as “clearing of the 
veins,” is a common initial symptom of many mosaic diseases. After a 
few days the vein-clearing disappears and a mottling of the leaves 
develops; the mottling varies somewhat in appearance according to 
environmental conditions but consists usually of dark green areas, 
sometimes in the neighbourhood of the vein, upon a lighter green 
background. Frequently large blisters of green tissue are raised or 
sunken yellow areas may develop together with a certain amount of 
malformation of the leaves. 
Included in the general category of mosaic diseases is a different 
type of response to virus infection which is usually known as a ringspot 
disease. There are various types of ringspot patterns; the commonest 
consists of numbers of concentric rings with a spot in the centre 
(Plate III). Sometimes the rings are single, at other times three 
or four concentric rings occur at a time; in some diseases the number 
may reach six or seven. The type of ring may differ also, but not as 
a rule in the same disease. The rings may be narrow and indented; 
these are necrotic and cause the death of the cells. Alternatively, 
rings of a lighter green colour, chlorotic, upon a darker background, 
may develop. There is no central spot to this type of ring. 
There are variations in the symptoms of ringspot diseases. Some- 
times instead of rings, complicated ring and line, or oakleaf, patterns 
may develop in the leaves. There is no very convincing explanation 
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