MOSAIC DISEASES 149 
susceptible host the disease spreads rapidly through the 
plant, and juice taken from all parts is found to be in- 
fective if introduced into a healthy plant. It has been 
shown also that the disease is very easily spread, it being 
sufficient to transfer a minute quantity of juice from an 
infected plant to the wounded tissues of a healthy one to 
render it diseased. The infection has been transmitted 
by such delicate operations as cutting the leaf hairs of a 
diseased plant with a pair of scissors and then cutting 
those of a healthy plant. 
Under conditions of cultivation the disease may be 
spread by such processes as defoliation, picking, pruning, 
“stopping,” and tying. Transmission by means of 
insects has been proved to take place in many instances 
and is now an accepted fact. In this respect aphides are 
_ especially dangerous, and experiments conducted by the 
author indicate that the white fly (Alewrodes vapora- 
riorum) is an important factor in the spread of mosaic 
disease of the tomato under glass. 
Experiments with tobacco mosaic were conducted by 
Allard to determine if the disease is transmitted in the 
soil, and his results lead him to conclude that infection 
does not take place in this way, but there is evidence that 
the active virus may be carried over the winter in the 
roots of susceptible perennials. 
The importance of seed transmission has attracted 
much attention to this phase of the problem, but except 
for isolated cases it has not been proved to occur. Dick- 
son (17) claims that seed transmission occurs in the case 
of the pea (Pisum sativum), certain clovers (Trifolium 
pratense and 7’. hybridum), and the sweet clover (Melilotus 
alba). Doolittle has found also that it takes place in the 
cucumber. No records of seed transmission are known for 
the petunia, potato, tobacco, and tomato. 
Cross Inoculations 
The question of the host range of any disease is 
always important to its control, and experiments with 
