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Journal of Agricultural Research voi.xxn.Ncs 



Table IX. — Effect oj different soil temperatures upon onion smut infection. In all 

 cases except the fifth, tenth, and eleventh, the pots -were held for the stated period at 20° 

 C, where infection was inhibited, then transferred to 15° to 20°, a temperature favor- 

 able for infection. In the fifth and tenth, where the continuous temperature was high, 

 note that practically no smut developed; in the eleventh, where the soil temperature was 

 continuously low, note that practically all the plants were smutted; in the remaining 

 series, where the plants were transferred from the higher temperature (29°) to the lower 

 (75° to 20°) after varying periods, note that long exposures at the higher temperature 

 tended to reduce the amount of infection. 



It is interesting to note also in this connection that continued expo- 

 sure of onion roots to a temperature of 29° C. led to the gradual slowing 

 up of groM^th. With the transfer of the pots to the lower temperatui-e 

 (15° to 20°) both root and top development were greatly stimulated. 

 In attempting to correlate these results one must keep in mind the 

 fact that in nattire the temperature conditions under which the onions 

 develop are much different from those in the experimental pots. 

 Whereas in the pots the soil temperature is uniform throughout their 

 depth, in the natural soil there is a gradual decrease in temperature 

 at progressively greater depths. The temperature of the upper inch runs 

 extremely high during the summer months because of direct exposure 

 to the sun's rays, and this is the area critical for infection by onion smut. 

 The young roots, on the other hand, as they develop progressively 

 reach strata of lower temperature, which are more favorable for their 

 growth. 



