38 DISEASES OF GLASSHOUSE PLANTS 
Verticillium wilt, which below 16°C. and above 24°C. 
is exceedingly slow. Plants badly wilted at a temperature 
of 20°C. recovered when transferred to a temperature 
of 25°C., and cultural methods based on these facts 
were devised for controlling the disease. Recently 
Johnson (26) has demonstrated that Mosaic disease of the 
tobacco is most virulent between 28°-30°C., and that 
above 36°-37°C. mosaic plants send out new leaves 
showing no symptoms of the disease, and the chlorotic 
leaves may recover their normal colour. 
The value of such work will be appreciated by all 
growers of glasshouse plants, who are more favourably 
situated for controlling the temperature to which their 
crops are exposed than those who cultivate outside 
crops. 
Water 
(a) Humidity of the Atmosphere.—In a similar manner 
the presence of moisture in the air is related to the 
incidence of disease by reason of its effect upon the plant 
and the pathogen, both separately and combined. 
As is well known, different plants do not require the 
same amount of atmospheric moisture for their maximum 
development. Thus the atmosphere of a cucumber 
house must contain a higher percentage of humidity 
than that of a tomato house, otherwise the plants will 
suffer. When plants are exposed to an atmospheric 
humidity lower than that normally required they remain 
under-developed and produce small, thin leaves, but are 
generally more resistant to disease. With humidities 
above the normal the rate of transpiration of water from 
the leaves becomes reduced. In consequence, the plants 
become gorged with water, the tissues become tender, 
and sappy growths are produced, which are very sus- 
ceptible to disease. Abundant moisture has an important | 
effect upon the fungus parasites, for it increases spore 
