Jan. 3, i9=i Fusarium-W lit of Tobacco 527 



host tissue for initial infection. However this may be, it was found 

 that wounding the plant greatly increased the possibilities of infection, 

 and in some of the later experiments the plants were all wounded, 

 usually by pinching or pulling off two basal leaves, in addition to the 

 "natural" wounding resulting from transplanting. 



Soil temperature. — Four series of experiments were run in the soil 

 temperature control "tanks" during the winters of 191 8-1 9 and 1919-20 

 in a manner similar to that which has been described for the Thielavia 

 rootrot studies (5). Two plants in uninfested soil and two in infested 

 soil were grown at each temperature. The temperatures usually ranged 

 from 15 to 38 C, with intervals of 2° — that is, 12 different temperatures 

 were used. The results in two of the trials were not convincing on 

 account of a low percentage of infection, although the later results were 

 approximated. In one series wilt occurred only at the approximate 

 temperatures of 28 and 32 ° and not at the intermediate temperature 

 used. In the other case, wilt occurred only at 26 to 27 and 30 to 32 . 

 These results can only be said to indicate roughly that the higher soil 

 temperatures are more favorable than the lower temperatures. 



In the third experiment, however, more uniform infection was secured. 

 Signs of disease were first evident at 28 to 29 and 25 to 26 C, and 

 these were soon followed by disease at 26 to 27 , 30 to 31 °, 23 to 24 , 

 and 2i° to 22 . Eighteen days later all plants in the infested soil were 

 dead at all temperatures between 21 to 22 ° and 30 to 31 and also at 

 32 to 33 . One plant was dead and one diseased at 31 to 32 , 34 to 

 35 , 19 to 20 , and 17° to 18 , and two were slightly diseased at 15 to 

 1 6°. The most favorable temperature for infection and progress of the 

 wilt appeared to be between 25 and 30 , but it seemed evident that a 

 wide range of temperature existed within which the disease could occur. 



The surface soil of the pots in the first three experiments was not 

 insulated, though it should be, particularly in diseases of this sort where 

 the parasite is systemic; therefore, it is quite likely that infection may 

 have occurred near the surface where for short periods the temperature 

 varied considerably from those given, particularly at temperatures 

 above 30 C. Difficulties are encountered in controlling soil tempera- 

 tures sufficiently accurately at all points in the soil containers in dealing 

 with a systemic disease, though these difficulties do not play so large 

 a role in cases where a parasite is limited entirely to subterranean parts. 

 In a fourth test, using soil temperatures 3 apart, in which special attempts 

 were made to keep the temperature at the surface of the soil constant 

 by means of glass covers and shading of the jars in the tanks, wilt 

 occurred first and most abundantly at 30 to 31 °, and no wilting occurred 

 at 1 3 to 14° or at 35 to 36 . We feel confident in concluding from 

 the results of these experiments that the optimum temperature for the 

 disease lies between 2 8° and 31 — that is, the Fusarium-wilt organism is 



