INFLUENCE OF TEMPERATURE ON INFECTION 109 



In some cases seed disinfection does not require exposure to ex- 

 cessive temperatures. Edgerton (1915) found that 30° to 31° C 

 is maximum for the growth in culture of Colletotrichinn linde- 

 muthianum and has been able to produce in Louisiana, during 

 summer, anthracnose-free bean seed from a crop planted with 

 infected seed. 



INFLUENCE OF TEMPERATURE ON INFECTION 



The severity of certain soil-borne diseases, especially those 

 caused by Fusarium, Verticillium, Rhizoctonia, Sclerotinia, and 

 Thielaviopsis, is known to be correlated with temperature. Data 

 bearing on this matter have been amassed from the use of soil- 

 temperature tanks equipped with thermostatic controls. Plans 

 for the construction and operation of this type of apparatus are 

 described by Jones, Johnson, and Dickson (1926). Their account 

 should be carefully read to obtain an appreciation of the problems 

 relating to the influence of temperature in the development of 

 plant diseases and to the construction and operation of ecostats. 

 These workers conclude that disease is the resultant of the "inter- 

 action of the plastic host and a plastic parasite under the play of 

 variable environment." Temperature, as a variable, modifies the 

 metabolic activity not only of the host but also of the parasite, 

 and it may happen that such temperatures as approximate opti- 

 mum for the one may exercise an adverse influence upon the other. 



By means of soil-temperature tanks Gilman (1916) determined 

 that symptoms of cabbage yellows, caused by Fusarium conglu- 

 tinans, are absent at maintained soil temperatures between 12° and 

 16° C, but that characteristic symptoms appear within the range 

 17° to 22° C. When this organism is grown in culture, its opti- 

 mum, indicated by a daily increase in the diameter of colonies, 

 approximates 25° C. 



Johnson and Hartman (1919), also using soil-temperature tanks, 

 found that soil temperatures of 17° to 23° C are most favorable for 

 the development of tobacco-root rot. The disease gradually di- 

 minished in severity above 26° C and was absent at 29° to 30° C. 

 As they indicate, account must be taken in experimentation of 

 such other factors as soil moisture, soil reaction, supplv of nu- 

 trients in the soil, and amount of infestation, none of which can be 

 isolated and evaluated completely. The sum total of all these 



