MOSAIC DISEASES 151 
the tomato the development of the symptoms is slow 
during the spring, when low temperatures prevail and 
plant growth is retarded, but rapid during the higher 
summer temperatures, when the plant is growing quickly. 
Indeed, as noted before (page 143) the character of 
mosaic symptoms at these two periods may vary 
considerably. 
Similar facts have been proved for the tobacco, where 
the incubation period for the disease is shortest at 
temperatures between 28° C. and 30° C. The optimum 
temperature for cucumber mosaic has proved to be 30° C., 
infection failing below 20° C. 
Light may also have an effect upon the development 
of symptoms, as shown by the researches of Lodewijks 
(29) and Chapman (15). The latter showed that when 
tobacco plants inoculated with the mosaic virus were 
exposed to blue light they developed slight symptoms of 
the disease, although the expressed juice was infectious. 
Dickson (17) has shown that this effect is due to the 
modified conditions of growth occurring in plants exposed 
to this light. 
The Control of the Disease 
Mosaic disease appears to be attaining serious pro- 
portions, and its control becomes of great economic 
importance. 
Four main lines of control may be considered, as 
follows : 
(1) The determination and elimination of infection 
centres. 
(2) The determination and elimination of agents by © 
which the disease is transmitted. 
(3) The determination of the environmental conditions 
which will induce a high resistance in the host plant. 
(4) The breeding of immune varieties. 
The Determination and Elimination of Infection Centres. 
—This involves a knowledge of all host plants of the 
