252 



Journal of Agricultural Research voi. xxii. No. 5 



Table X. — Effect of high temperature, following infection, in inhibiting the further 

 development of smut. The plants were from a series of pot cultures started at 25° C. 

 and held there until incipient infection had occurred, then transferred for the period 

 indicated to a high temperature, J0° to 33°, and finally brought back to the original 25°. 



Length of exposure to temperature of 30° to 33°. 



None (25° throughout) 



2 days 



4 days 



9 days 



14 days 



Percentage 

 smutted. 



94 

 45 

 37 

 33 

 17 



It is to be noted that in the experiments just reviewed potted plants 

 were used. The entire pot, thus including roots and tops of the experi- 

 mental plants, was exposed to the stated temperature condition. 



The question then arose whether the results secured were due entirely 

 to the effect of high air temperature upon the fungus or to an indirect 

 effect of the changed conditions upon the metabolism of the host. In 

 order to throw some light upon this point two experiments were con- 

 ducted in which seedlings were grown in infected soil at three constant 

 soil temperatures and each of two air temperatures, 25° and 30° to 33° C. 

 The results (Table XI) at the lower air temperature (25°) coincided 

 closely with those previously secured at air temperatures of 13° to 25°, 

 inasmuch as abundant infection occmred when the soil temperature was 

 25°, while complete inhibition was attained at 30°. It is, however, 

 significant to note, in comparison with the results in Table X, that, with 

 the soil temperature held at 20° or 24°, the 30° to 33° air temperatm-e 

 did not greatly check the development of the disease. It appears, then, 

 that roots as well as tops must be exposed to the inhibitive higher tem- 

 perature, 30° to 33°, in order fully to check the parasite after incipient 

 infection has taken place. This suggests that the inhibitory effect may 

 be due in part at least to the influence of the environmental conditions 

 upon the metabolism of the host and not entirely to a direct effect upon 

 the fungus itself. 



Table XI. — Effect of different combinations of soil and air temperature upon onion smut 



infection 



Air temperature. 



2S 



30 to 33 . 



Soil 

 tempera- 

 ture. 



20 

 25 

 30 

 20 

 24 

 30 



Experiment i. 



Number 

 of plants. 



23 

 68 



42 



Op 



73 

 24 



Percentage 

 smutted. 



100 

 97 



86 

 o 



Experiment 2. 



Number 

 of plants. 



8S 

 49 

 22 



41 

 .■50 

 30 



Percentage 

 smutted. 



95 

 92 



o 

 46 

 60 



o 



1 stand reduced by damping-o£f fimgi. 



