CHAPTER VII 
MOSAIC DISEASES 
In previous chapters an account has been given of two 
big groups of diseases caused by fungi and bacteria 
respectively. Considerable progress has been made in 
the study of these, but there is a third large group of 
diseases the cause and nature of which, although studied 
by investigators in all parts of the world, remains largely 
unknown. This is the group of Mosaic Diseases, or 
Virus Diseases, as they have latterly been named. 
Of profound interest, and of great and perhaps in- 
creasing economic importance, the solution of their 
main problems would probably constitute one of the 
most important events in the study of plant diseases of 
modern times. 
The name “ Mosaic Disease” (Mosaikkrankheit) was 
first applied by Mayer (33) in 1886 to a disease of the 
tobacco in which the leaves assumed a mottled, dis- 
torted appearance. To-day the name is applied to several 
diseases the cause of which is unknown, but which all 
possess two very definite characters. The first is a 
mottling of the plant, in which light yellow patches 
alternate with others of a dark green colour. Also 
the various parts of the plant may be distorted, giving 
rise to blistered, irregular leaves and flowers. The 
second character is the highly infectious nature of the 
plant juices, even after filtration through porcelain 
vessels, 
Practically one hundred different species belonging 
to widely separated genera are known to be susceptible 
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