DISEASED CONDITION OF PLANTS 37 
as 75°C., but usually few will grow at temperatures 
above 40°C. The spores of certain bacteria are highly 
resistant to high temperatures, and in some cases have 
not been killed by boiling for two or three hours. 
Comparatively little information is available con- 
cerning the resistance of fungus spores to high tempera- 
tures, but it would appear that some resting spores 
and sclerotial bodies are capable of withstanding high 
temperatures. 
The optimum temperature for growth of many fungi 
does not coincide with that for spore production, and it 
is important that this should be taken into account 
when studying plant diseases. Thus, at a certain 
temperature the fungus will progress most rapidly 
through the plant tissues, and at another temperature 
most readily produce spores and increase the rate of 
spread from plant to plant. 
Plant and Pathogen Combined.—While the temperature 
relation of many fungi in pure culture has been studied, 
it is only within the last decade or so that any research 
has been carried out on the relation of temperature to 
the process of infection. This is largely the result of 
investigations by the United States Department of 
Agriculture and by Professor L. R. Jones and his col- 
leagues at the University of Wisconsin. 
The temperature of the soil has been shown to be a 
limiting factor in certain diseases, among which may be 
mentioned the Fusarium wilt of tomatoes. Edgerton (20) 
has shown that infection takes place and the disease 
_ develops most rapidly if the soil temperature is main- 
tained around 29° C. Very little infection occurs if the 
temperature is much below this for any length of time. 
Investigations upon the Verticillium wilt of tomatoes 
carried out at Cheshunt Experimental Station illustrate 
the importance of air and soil temperatures in con- 
ditioning the progress of disease (10). It was shown that 
temperatures between 16° C. and 24° C., with an optimum 
of 21°-22°C. are favourable to the rapid spread of the 
