It may be concluded from the foregoing experiments that a high per- 

 centage of infection may be expected up to 25° C, above which there is 

 a rather abrupt reduction, leading to complete inhibition at 29°. There 

 appears to be no lower limit of temperature for infection within the range 

 where onion seeds will germinate and normal growth occur. 



After it was clearly shown that no infection v/ould take place at 29° C, 

 the question arose as to how long seedlings must grow at this temperature 

 to become entirely immune. It has been shown that at moderate tem- 

 peratures the plant becomes immune in about 20 days, or at about the 

 time when the cotyledon has reached its maximum growth. To deter- 

 mine whether or not this condition is altered when the plants are grown 

 at 29°., several pots each of the naturally and the artificially inoculated 

 soil were started off at this high soil temperature. Pots were then 

 transferred from time to time to a lower temperature favorable for 

 infection (15° to 20°) where they were held for about three weeks before 

 they were examined for signs of the disease. The results of these experi- 

 ments are summarized in Table IX. 



It is quite evident that the amount of infection was markedly reduced 

 by an exposure of 15 to 18 days at 29° C. Complete inhibition of infection 

 by even more protracted exposure to this high temperature was not 

 attained. However, where infection did occur there was usually not 

 more than one lesion per plant, which in the majority of cases was so 

 situated that subsequent infection of newly forming leaves would be 

 impossible. It is thus quite certain that where seedlings develop at 

 about 29° for the first 20 days the amount of damage from smut will 

 be negligible, especially in an area where the amount of inoculum is 

 slight. 



